探索Android开源框架 - 4. Glide使用及源码解析

参考

使用

1. 添加依赖

implementation 'com.github.bumptech.glide:glide:4.12.0'
annotationProcessor 'com.github.bumptech.glide:compiler:4.12.0'
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2. 基础使用

1. 1. 
      
2. 加载图片
   1. 
      

Glide.with(this)
    .load(imgUrl)
    .into(imageView1)
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2. 2. 
      
3. 取消加载
   2. 
      

Glide.with(this).clear(imageView1)
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• • 
    
• 实际上，当 Glide.with() 中传入的 Activity 或 Fragment 实例销毁时，Glide 会自动取消加载并回收资源；
  • 
    

3. 进阶操作

1. 1. 
      
2. 更多的可选配置
   1. 
      

Glide.with(this)//上下文对象Context
    .load(imgUrl)//图片地址
    .placeholder(R.mipmap.ic_launcher)//加载前的占位图
    .error(R.mipmap.ic_launcher)//加载失败的占位图
    .override(150,100)//指定图片尺寸(一般不用设置，Glide会自动根据ImageView的大小来决定图片的大小)
    .skipMemoryCache(true)//跳过内存缓存
    .diskCacheStrategy(DiskCacheStrategy.AUTOMATIC)//根据图片资源智能地选择使用哪一种缓存策略（默认选项）
    .diskCacheStrategy(DiskCacheStrategy.ALL)//既缓存原始图片，也缓存转换过的图片
    .diskCacheStrategy(DiskCacheStrategy.DATA)//只缓存原始图片
    .diskCacheStrategy(DiskCacheStrategy.RESOURCE)//只缓存转换过的图片
    .diskCacheStrategy(DiskCacheStrategy.NONE)//禁用磁盘缓存
    .priority(Priority.HIGH)//优先级
    .thumbnail(
        Glide.with(this).load(imgUrl).override(20)
    ) //设置缩略图
    .thumbnail(0.25f)//压缩原图为缩略图
    .into(ivGlide2)
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• • 
    
• 更多的可选配置可以参考​​官方文档​​

2. 2. 
      
3. 通过 RequestOptions 共享配置
   2. 
      

val options = RequestOptions()
    .placeholder(R.mipmap.ic_launcher)
    .error(R.mipmap.ic_launcher)
Glide.with(this)
    .load(imgUrl)
    .apply(options)
    .into(imageView1)
Glide.with(this)
    .load(imgUrl)
    .apply(options)
    .into(imageView2)
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源码解析

• • 
    
• 源码方面主要从最基本的三个方法入手：with，load，into
  • 
    

1. with

• • 
    
• Glide.with源码如下，根据不同上下文对象有多个重载方法，首先通过getRetriever方法获取RequestManagerRetriever对象，再通过get方法获取RequestManager实例
  • 
    

public static RequestManager with(@NonNull Context context) {
    return getRetriever(context).get(context);
}

public static RequestManager with(@NonNull Activity activity) {
    return getRetriever(activity).get(activity);
}

public static RequestManager with(@NonNull FragmentActivity activity) {
    return getRetriever(activity).get(activity);
}

public static RequestManager with(@NonNull Fragment fragment) {
    return getRetriever(fragment.getContext()).get(fragment);
}

public static RequestManager with(@NonNull android.app.Fragment fragment) {
    return getRetriever(fragment.getActivity()).get(fragment);
}

public static RequestManager with(@NonNull View view) {
    return getRetriever(view.getContext()).get(view);
}
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• • 
    
• getRetriever方法中通过Glide.get通过懒汉式+双重校验锁获取Glide单例,再通过getter方法获取requestManagerRetriever属性；
  • 
    

private static RequestManagerRetriever getRetriever(@Nullable Context context) {
    Preconditions.checkNotNull(
        context,
        "You cannot start a load on a not yet attached View or a Fragment where getActivity() "
            + "returns null (which usually occurs when getActivity() is called before the Fragment "
            + "is attached or after the Fragment is destroyed).");
    return Glide.get(context).getRequestManagerRetriever();
}
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获取Glide实例

public static Glide get(@NonNull Context context) {
    if (glide == null) {
      GeneratedAppGlideModule annotationGeneratedModule =
          getAnnotationGeneratedGlideModules(context.getApplicationContext());
      synchronized (Glide.class) {
        if (glide == null) {
          checkAndInitializeGlide(context, annotationGeneratedModule);
        }
      }
    }
    return glide;
}
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• • 
    
• 真正创建并初始化Glide实例实在initializeGlide方法, 调用GlideBuilder.build方法通过建造者模式创建Glide实例
  • 
    

private static void checkAndInitializeGlide(
      @NonNull Context context, @Nullable GeneratedAppGlideModule generatedAppGlideModule) {
    if (isInitializing) {
      throw new IllegalStateException(
          "You cannot call Glide.get() in registerComponents(),"
              + " use the provided Glide instance instead");
    }
    isInitializing = true;
    initializeGlide(context, generatedAppGlideModule);
    isInitializing = false;
}

private static void initializeGlide(
      @NonNull Context context, @Nullable GeneratedAppGlideModule generatedAppGlideModule) {
    initializeGlide(context, new GlideBuilder(), generatedAppGlideModule);
}

private static void initializeGlide(
      @NonNull Context context,
      @NonNull GlideBuilder builder,
      @Nullable GeneratedAppGlideModule annotationGeneratedModule) {
    Context applicationContext = context.getApplicationContext();
    ...
    Glide glide = builder.build(applicationContext);
    ...
    Glide.glide = glide;
}
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• • 
    
• GlideBuilder.build方法代码如下
  • 
    

Glide build(@NonNull Context context) {
    //获取图片的线程池
    if (sourceExecutor == null) {
      sourceExecutor = GlideExecutor.newSourceExecutor();
    }
    //磁盘缓存的线程池
    if (diskCacheExecutor == null) {
      diskCacheExecutor = GlideExecutor.newDiskCacheExecutor();
    }
    //动画的线程池
    if (animationExecutor == null) {
      animationExecutor = GlideExecutor.newAnimationExecutor();
    }
    //创建内存大小控制器
    if (memorySizeCalculator == null) {
      memorySizeCalculator = new MemorySizeCalculator.Builder(context).build();
    }
    //创建网络监听的工厂
    if (connectivityMonitorFactory == null) {
      connectivityMonitorFactory = new DefaultConnectivityMonitorFactory();
    }

    if (bitmapPool == null) {
      int size = memorySizeCalculator.getBitmapPoolSize();
      if (size > 0) {
        // 使用缓存，则创建bitmap对象池
        bitmapPool = new LruBitmapPool(size);
      } else {
        // 不使用缓存
        bitmapPool = new BitmapPoolAdapter();
      }
    }
    //创建对象数组缓存池
    if (arrayPool == null) {
      arrayPool = new LruArrayPool(memorySizeCalculator.getArrayPoolSizeInBytes());
    }
    //创建内存缓存
    if (memoryCache == null) {
      memoryCache = new LruResourceCache(memorySizeCalculator.getMemoryCacheSize());
    }
    //创建硬盘缓存工厂
    if (diskCacheFactory == null) {
      diskCacheFactory = new InternalCacheDiskCacheFactory(context);
    }
    //创建引擎类
    if (engine == null) {
      engine =
          new Engine(
              memoryCache,
              diskCacheFactory,
              diskCacheExecutor,
              sourceExecutor,
              GlideExecutor.newUnlimitedSourceExecutor(),
              animationExecutor,
              isActiveResourceRetentionAllowed);
    }

    if (defaultRequestListeners == null) {
      defaultRequestListeners = Collections.emptyList();
    } else {
      defaultRequestListeners = Collections.unmodifiableList(defaultRequestListeners);
    }

    GlideExperiments experiments = glideExperimentsBuilder.build();
    //创建请求管理类，其构造方法中会实例化一个主线程的handler对象，用于线程的切换
    RequestManagerRetriever requestManagerRetriever =
        new RequestManagerRetriever(requestManagerFactory, experiments);

    return new Glide(
        context,
        engine,
        memoryCache,
        bitmapPool,
        arrayPool,
        requestManagerRetriever,
        connectivityMonitorFactory,
        logLevel,
        defaultRequestOptionsFactory,
        defaultTransitionOptions,
        defaultRequestListeners,
        experiments);
}
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• • 
    
• 上面代码中RequestManagerRetriever的构造方法如下
  • 
    

public RequestManagerRetriever(
      @Nullable RequestManagerFactory factory, GlideExperiments experiments) {
    this.factory = factory != null ? factory : DEFAULT_FACTORY;
    handler = new Handler(Looper.getMainLooper(), this /* Callback */);
    frameWaiter = buildFrameWaiter(experiments);
}
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RequestManagerRetriever.get

• • 
    
• 上面通过glide单例获取RequestManagerRetriever实例，下面再调用其get方法获取RequestManager实例，也是有多个重载方法
  • 
    

public RequestManager get(@NonNull Context context) {
    //context判空及实际类型判断
    if (context == null) {
      throw new IllegalArgumentException("You cannot start a load on a null Context");
    } else if (Util.isOnMainThread() && !(context instanceof Application)) {
      if (context instanceof FragmentActivity) {
        return get((FragmentActivity) context);
      } else if (context instanceof Activity) {
        return get((Activity) context);
      } else if (context instanceof ContextWrapper
          && ((ContextWrapper) context).getBaseContext().getApplicationContext() != null) {
        return get(((ContextWrapper) context).getBaseContext());
      }
    }
    return getApplicationManager(context);
}

public RequestManager get(@NonNull FragmentActivity activity) {
    if (Util.isOnBackgroundThread()) {
      return get(activity.getApplicationContext());
    } else {
      ...
      FragmentManager fm = activity.getSupportFragmentManager();
      return supportFragmentGet(activity, fm, /*parentHint=*/ null, isActivityVisible(activity));
    }
}

public RequestManager get(@NonNull Fragment fragment) {
    ...
    if (Util.isOnBackgroundThread()) {
      return get(fragment.getContext().getApplicationContext());
    } else {
      ...
      FragmentManager fm = fragment.getChildFragmentManager();
      return supportFragmentGet(fragment.getContext(), fm, fragment, fragment.isVisible());
    }
}

public RequestManager get(@NonNull Activity activity) {
    if (Util.isOnBackgroundThread()) {
      return get(activity.getApplicationContext());
    } else if (activity instanceof FragmentActivity) {
      return get((FragmentActivity) activity);
    } else {
      ...
      android.app.FragmentManager fm = activity.getFragmentManager();
      return fragmentGet(activity, fm, /*parentHint=*/ null, isActivityVisible(activity));
    }
}

 public RequestManager get(@NonNull View view) {
    ...
    Activity activity = findActivity(view.getContext());
    if (activity == null) {
      return get(view.getContext().getApplicationContext());
    }
    if (activity instanceof FragmentActivity) {
      Fragment fragment = findSupportFragment(view, (FragmentActivity) activity);
      return fragment != null ? get(fragment) : get((FragmentActivity) activity);
    }
    android.app.Fragment fragment = findFragment(view, activity);
    if (fragment == null) {
      return get(activity);
    }
    return get(fragment);
}
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• • 
    
• 最终会调用supportFragmentGet或fragmentGet获取requestManager实例,其中会创建一个Fragment，那么我们就知道了glide是通过在内部维护一个fragment来管理生命周期的
  • 
    

private RequestManager supportFragmentGet(
      @NonNull Context context,
      @NonNull FragmentManager fm,
      @Nullable Fragment parentHint,
      boolean isParentVisible) {
    SupportRequestManagerFragment current = getSupportRequestManagerFragment(fm, parentHint);
    RequestManager requestManager = current.getRequestManager();
    if (requestManager == null) {
      Glide glide = Glide.get(context);
      requestManager =
          factory.build(
              glide, current.getGlideLifecycle(), current.getRequestManagerTreeNode(), context);
      if (isParentVisible) {
        requestManager.onStart();
      }
      current.setRequestManager(requestManager);
    }
    return requestManager;
}

private RequestManager fragmentGet(
      @NonNull Context context,
      @NonNull android.app.FragmentManager fm,
      @Nullable android.app.Fragment parentHint,
      boolean isParentVisible) {
    RequestManagerFragment current = getRequestManagerFragment(fm, parentHint);
    RequestManager requestManager = current.getRequestManager();
    if (requestManager == null) {
      Glide glide = Glide.get(context);
      requestManager =
          factory.build(
              glide, current.getGlideLifecycle(), current.getRequestManagerTreeNode(), context);
      if (isParentVisible) {
        requestManager.onStart();
      }
      current.setRequestManager(requestManager);
    }
    return requestManager;
}

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2. load

• • 
    
• 此时的load其实已经是RequestManager.load方法，也是有多个重载方法，可以传入不同的图片源
  • 
    

public RequestBuilder<Drawable> load(@Nullable Bitmap bitmap) {
    return asDrawable().load(bitmap);
}
public RequestBuilder<Drawable> load(@Nullable Drawable drawable) {
    return asDrawable().load(drawable);
}
public RequestBuilder<Drawable> load(@Nullable String string) {
    return asDrawable().load(string);
}
public RequestBuilder<Drawable> load(@Nullable Uri uri) {
    return asDrawable().load(uri);
}
public RequestBuilder<Drawable> load(@Nullable File file) {
    return asDrawable().load(file);
}
public RequestBuilder<Drawable> load(@RawRes @DrawableRes @Nullable Integer resourceId) {
    return asDrawable().load(resourceId);
}
public RequestBuilder<Drawable> load(@Nullable URL url) {
    return asDrawable().load(url);
}
public RequestBuilder<Drawable> load(@Nullable byte[] model) {
    return asDrawable().load(model);
}
public RequestBuilder<Drawable> load(@Nullable Object model) {
    return asDrawable().load(model);
}
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• • 
    
• 上面代码中都会先调用asDrawable()，那么我们先来看看他是个啥
  • 
    

asDrawable()

• • 
    
• asDrawable代码如下，其中只是调用as方法并传入Drawable.class, 再看看as方法，原来是创建一个图片类型为Drawable类型的RequestBuilder请求
  • 
    

public RequestBuilder<Drawable> asDrawable() {
    return as(Drawable.class);
}

public <ResourceType> RequestBuilder<ResourceType> as(
      @NonNull Class<ResourceType> resourceClass) {
    return new RequestBuilder<>(glide, this, resourceClass, context);
}
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• • 
    
• 而且我们发现还要相似的asBitmap，asGif
  • 
    

public RequestBuilder<GifDrawable> asGif() {
    return as(GifDrawable.class).apply(DECODE_TYPE_GIF);
}

public RequestBuilder<Bitmap> asBitmap() {
    return as(Bitmap.class).apply(DECODE_TYPE_BITMAP);
}
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• • 
    
• 现在我们知道了RequestManager.load是通过获取RequestBuilder实例，再调用其load方法
  • 
    

RequestBuilder.load

• • 
    
• RequestBuilder.load也是有多个重载方法,但其中都是调用loadGeneric方法
  • 
    

public RequestBuilder<TranscodeType> load(@Nullable String string) {
    return loadGeneric(string);
}
public RequestBuilder<TranscodeType> load(@Nullable Uri uri) {
    return loadGeneric(uri);
}
public RequestBuilder<TranscodeType> load(@Nullable File file) {
    return loadGeneric(file);
}
public RequestBuilder<TranscodeType> load(@Nullable Drawable drawable) {
    return loadGeneric(drawable).apply(diskCacheStrategyOf(DiskCacheStrategy.NONE));
}
public RequestBuilder<TranscodeType> load(@Nullable Bitmap bitmap) {
    return loadGeneric(bitmap).apply(diskCacheStrategyOf(DiskCacheStrategy.NONE));
}
public RequestBuilder<TranscodeType> load(@RawRes @DrawableRes @Nullable Integer resourceId) {
    return loadGeneric(resourceId).apply(signatureOf(AndroidResourceSignature.obtain(context)));
}
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• • 
    
• loadGeneric代码如下,主要是将图片源保存起来，并设置是否设置了图片源的标识isModelSet = true,最后返回了RequestBuilder自身this
  • 
    

private RequestBuilder<TranscodeType> loadGeneric(@Nullable Object model) {
    if (isAutoCloneEnabled()) {
      return clone().loadGeneric(model);
    }
    this.model = model;
    isModelSet = true;
    return selfOrThrowIfLocked();
}

protected final T selfOrThrowIfLocked() {
    if (isLocked) {
      throw new IllegalStateException("You cannot modify locked T, consider clone()");
    }
    return self();
}

private T self() {
    return (T) this;
}
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3. into

• • 
    
• 接下来是通过RequestBuilder.into方法设置目标view
  • 
    

public ViewTarget<ImageView, TranscodeType> into(@NonNull ImageView view) {
    Util.assertMainThread();//断言是在主线程调用的
    Preconditions.checkNotNull(view);//非空判断
    BaseRequestOptions<?> requestOptions = this;
    if (!requestOptions.isTransformationSet()
        && requestOptions.isTransformationAllowed()
        && view.getScaleType() != null) {
      //熟悉的ScaleType们
      switch (view.getScaleType()) {
        case CENTER_CROP:
          requestOptions = requestOptions.clone().optionalCenterCrop();
          break;
        case CENTER_INSIDE:
          requestOptions = requestOptions.clone().optionalCenterInside();
          break;
        case FIT_CENTER:
        case FIT_START:
        case FIT_END:
          requestOptions = requestOptions.clone().optionalFitCenter();
          break;
        case FIT_XY:
          requestOptions = requestOptions.clone().optionalCenterInside();
          break;
        case CENTER:
        case MATRIX:
        default:
          // Do nothing.
      }
    }

    return into(
        glideContext.buildImageViewTarget(view, transcodeClass),
        /*targetListener=*/ null,
        requestOptions,
        Executors.mainThreadExecutor());
}
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• • 
    
• 最终是将view通过buildImageViewTarget方法转为target,transcodeClass为Bitmap.class或Drawable.class
  • 
    

public <X> ViewTarget<ImageView, X> buildImageViewTarget(
      @NonNull ImageView imageView, @NonNull Class<X> transcodeClass) {
    return imageViewTargetFactory.buildTarget(imageView, transcodeClass);
}

public class ImageViewTargetFactory {
  @NonNull
  @SuppressWarnings("unchecked")
  public <Z> ViewTarget<ImageView, Z> buildTarget(
      @NonNull ImageView view, @NonNull Class<Z> clazz) {
    if (Bitmap.class.equals(clazz)) {
      return (ViewTarget<ImageView, Z>) new BitmapImageViewTarget(view);
    } else if (Drawable.class.isAssignableFrom(clazz)) {
      return (ViewTarget<ImageView, Z>) new DrawableImageViewTarget(view);
    } else {
      throw new IllegalArgumentException(
          "Unhandled class: " + clazz + ", try .as*(Class).transcode(ResourceTranscoder)");
    }
  }
}
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• • 
    
• 入参为target的into方法中，有四个入参，分别为target，监听,配置信息,和线程池，其代码如下，
  • 
    

其中通过buildRequest创建一个请求

private <Y extends Target<TranscodeType>> Y into(
      @NonNull Y target,
      @Nullable RequestListener<TranscodeType> targetListener,
      BaseRequestOptions<?> options,
      Executor callbackExecutor) {
    Preconditions.checkNotNull(target);
    if (!isModelSet) {
      throw new IllegalArgumentException("You must call #load() before calling #into()");
    }
    //创建request对象
    Request request = buildRequest(target, targetListener, options, callbackExecutor);

    Request previous = target.getRequest();
    if (request.isEquivalentTo(previous)
        && !isSkipMemoryCacheWithCompletePreviousRequest(options, previous)) {
      if (!Preconditions.checkNotNull(previous).isRunning()) {
        previous.begin();
      }
      return target;
    }
    //将target原先所关联的请求断开
    requestManager.clear(target);
    //并设置为新的请求，
    target.setRequest(request);
    //然后通过requestManager.track()进行一些列的加载和处理显示操作
    requestManager.track(target, request);

    return target;
}

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requestManager.track

• • 
    
• track方法代码如下
  • 
    

synchronized void track(@NonNull Target<?> target, @NonNull Request request) {
    targetTracker.track(target);
    requestTracker.runRequest(request);
}

//将请求保存起来
public void track(@NonNull Target<?> target) {
    targets.add(target);
}

//如果当前不是暂停状态则调用begin方法处理请求，如果是暂停状态则加入到pendingRequests集合中
public void runRequest(@NonNull Request request) {
    requests.add(request);
    if (!isPaused) {
      request.begin();
    } else {
      request.clear();
      if (Log.isLoggable(TAG, Log.VERBOSE)) {
        Log.v(TAG, "Paused, delaying request");
      }
      pendingRequests.add(request);
    }
}
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SingleRequest.begin

• • 
    
• 而通过跟踪buildRequest方法，发现到最后是调用SingleRequest.obtain创建Request实例，也就是Request的实现类是SingleRequest，那么来看一下SingleRequest的begin方法
  • 
    

public void begin() {
    synchronized (requestLock) {
      assertNotCallingCallbacks();
      stateVerifier.throwIfRecycled();
      startTime = LogTime.getLogTime();
      ...
      status = Status.WAITING_FOR_SIZE;
      if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
        //将目标view的宽高传入onSizeReady
        onSizeReady(overrideWidth, overrideHeight);
      } else {
        target.getSize(this);
      }

      if ((status == Status.RUNNING || status == Status.WAITING_FOR_SIZE)
          && canNotifyStatusChanged()) {
        target.onLoadStarted(getPlaceholderDrawable());
      }
      if (IS_VERBOSE_LOGGABLE) {
        logV("finished run method in " + LogTime.getElapsedMillis(startTime));
      }
    }
}
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• • 
    
• 通过上面代码可以看出begin中最关键的一行是调用onSizeReady方法，并传入目标view的宽高，onSizeReady代码如下
  • 
    

public void onSizeReady(int width, int height) {
    stateVerifier.throwIfRecycled();
    synchronized (requestLock) {
      ...
      if (status != Status.WAITING_FOR_SIZE) {
        return;
      }
      //修改status为running
      status = Status.RUNNING;

      float sizeMultiplier = requestOptions.getSizeMultiplier();
      this.width = maybeApplySizeMultiplier(width, sizeMultiplier);
      this.height = maybeApplySizeMultiplier(height, sizeMultiplier);

      //调用Engine.load对请求进行处理
      loadStatus =
          engine.load(
              glideContext,
              model,
              requestOptions.getSignature(),
              this.width,
              this.height,
              requestOptions.getResourceClass(),
              transcodeClass,
              priority,
              requestOptions.getDiskCacheStrategy(),
              requestOptions.getTransformations(),
              requestOptions.isTransformationRequired(),
              requestOptions.isScaleOnlyOrNoTransform(),
              requestOptions.getOptions(),
              requestOptions.isMemoryCacheable(),
              requestOptions.getUseUnlimitedSourceGeneratorsPool(),
              requestOptions.getUseAnimationPool(),
              requestOptions.getOnlyRetrieveFromCache(),
              this,
              callbackExecutor);

      if (status != Status.RUNNING) {
        loadStatus = null;
      }
    }
}
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Engine.load

• • 
    
• 可以看到上面onSizeReady主要是调用了Engine.load()对请求进行处理,其代码如下，
  • 
    

public <R> LoadStatus load(
      GlideContext glideContext,
      Object model,
      Key signature,
      int width,
      int height,
      Class<?> resourceClass,
      Class<R> transcodeClass,
      Priority priority,
      DiskCacheStrategy diskCacheStrategy,
      Map<Class<?>, Transformation<?>> transformations,
      boolean isTransformationRequired,
      boolean isScaleOnlyOrNoTransform,
      Options options,
      boolean isMemoryCacheable,
      boolean useUnlimitedSourceExecutorPool,
      boolean useAnimationPool,
      boolean onlyRetrieveFromCache,
      ResourceCallback cb,
      Executor callbackExecutor) {
    long startTime = VERBOSE_IS_LOGGABLE ? LogTime.getLogTime() : 0;

    EngineKey key =
        keyFactory.buildKey(
            model,
            signature,
            width,
            height,
            transformations,
            resourceClass,
            transcodeClass,
            options);

    EngineResource<?> memoryResource;
    synchronized (this) {
      //获取缓存中的图片资源
      memoryResource = loadFromMemory(key, isMemoryCacheable, startTime);

      if (memoryResource == null) {
        //缓存中没有则等待或新开一个job获取图片资源
        return waitForExistingOrStartNewJob(
            glideContext,
            model,
            signature,
            width,
            height,
            resourceClass,
            transcodeClass,
            priority,
            diskCacheStrategy,
            transformations,
            isTransformationRequired,
            isScaleOnlyOrNoTransform,
            options,
            isMemoryCacheable,
            useUnlimitedSourceExecutorPool,
            useAnimationPool,
            onlyRetrieveFromCache,
            cb,
            callbackExecutor,
            key,
            startTime);
      }
    }

    // Avoid calling back while holding the engine lock, doing so makes it easier for callers to
    // deadlock.
    cb.onResourceReady(
        memoryResource, DataSource.MEMORY_CACHE, /* isLoadedFromAlternateCacheKey= */ false);
    return null;
}
复制代码

• • 
    
• Engine.load中主要是先通过loadFromMemory获取缓存中的图片资源，取不到则调用waitForExistingOrStartNewJob
  • 
    

Engine.loadFromMemory

• • 
    
• loadFromMemory代码如下
  • 
    

private EngineResource<?> loadFromMemory(
      EngineKey key, boolean isMemoryCacheable, long startTime) {

    //判断是否允许内存缓存
    if (!isMemoryCacheable) {
      return null;
    }
    //从ActiveResources中取
    EngineResource<?> active = loadFromActiveResources(key);
    if (active != null) {
      return active;
    }
    //从lruCache中取
    EngineResource<?> cached = loadFromCache(key);
    if (cached != null) {
      return cached;
    }

    return null;
}
复制代码

loadFromActiveResources

• • 
    
• loadFromActiveResources代码如下，其中ActiveResources维护了一个value为弱引用的HashMap<Key, ResourceWeakReference>, 调用其get方法获取图片资源，如果取到则调用acquire对 对象的引用数量 进行加一操作，内存缓存是通过计数散列算法来进行相应的操作的，如果对象的计数树为0，则说明暂无其他对象引用此资源，那么此资源可以被释放了
  • 
    

private EngineResource<?> loadFromActiveResources(Key key) {
    EngineResource<?> active = activeResources.get(key);
    if (active != null) {
      active.acquire();
    }
    return active;
}

private final ActiveResources activeResources;

final class ActiveResources {
    private final boolean isActiveResourceRetentionAllowed;
    private final Executor monitorClearedResourcesExecutor;
    @VisibleForTesting final Map<Key, ResourceWeakReference> activeEngineResources = new HashMap<>();
    private final ReferenceQueue<EngineResource<?>> resourceReferenceQueue = new ReferenceQueue<>();
    private ResourceListener listener;
    private volatile boolean isShutdown;
    @Nullable private volatile DequeuedResourceCallback cb;
    ...
    synchronized void activate(Key key, EngineResource<?> resource) {
        ResourceWeakReference toPut =
            new ResourceWeakReference(
                key, resource, resourceReferenceQueue, isActiveResourceRetentionAllowed);

        ResourceWeakReference removed = activeEngineResources.put(key, toPut);
        if (removed != null) {
          removed.reset();
        }
   }
   synchronized EngineResource<?> get(Key key) {
       ResourceWeakReference activeRef = activeEngineResources.get(key);
       if (activeRef == null) {
         return null;
       }

       EngineResource<?> active = activeRef.get();
       if (active == null) {
         cleanupActiveReference(activeRef);
       }
       return active;
   }
}
class EngineResource<Z> implements Resource<Z> {
    ...
    private int acquired;
    private boolean isRecycled;
    ...
    synchronized void acquire() {
        if (isRecycled) {
          throw new IllegalStateException("Cannot acquire a recycled resource");
        }
        ++acquired;
    }
}
复制代码

loadFromCache

• • 
    
• 继续回到loadFromMemory，如果ActiveResources中没有取到，则调用loadFromCache尝试从lruCache中取，loadFromCache代码如下，从lruCache中取图片资源，如果取到则调用acquire对 对象的引用数量 进行加一操作，并调用activeResources.activate添加到activeResources中
  • 
    

private EngineResource<?> loadFromCache(Key key) {
    EngineResource<?> cached = getEngineResourceFromCache(key);
    if (cached != null) {
      cached.acquire();
      activeResources.activate(key, cached);
    }
    return cached;
}

private EngineResource<?> getEngineResourceFromCache(Key key) {
    // 对于Lru的数据获取是通过删除的方式来获取
    Resource<?> cached = cache.remove(key);

    final EngineResource<?> result;
    if (cached == null) {
      result = null;
    } else if (cached instanceof EngineResource) {
      // Save an object allocation if we've cached an EngineResource (the typical case).
      result = (EngineResource<?>) cached;
    } else {
      result =
          new EngineResource<>(
              cached, /*isMemoryCacheable=*/ true, /*isRecyclable=*/ true, key, /*listener=*/ this);
    }
    return result;
}
复制代码

• • 
    
• 上面代码中的cache就是前面GlideBuilder.build中初始化的memoryCache = new LruResourceCache(memorySizeCalculator.getMemoryCacheSize());
  • 
    

LruResourceCache

• • 
    
• LruResourceCache继承了LruCache，并实现MemoryCache接口，LruCache内部利用了LinkedHashMap accessOrder 等于true时会对内容按照访问的先后顺序排序，会把最近访问的元素放到双链表的尾部这样一个特性实现的，代码如下
  • 
    

public class LruResourceCache extends LruCache<Key, Resource<?>> implements MemoryCache {
    ...
}

public class LruCache<T, Y> {
    private final Map<T, Entry<Y>> cache = new LinkedHashMap<>(100, 0.75f, true);
    private final long initialMaxSize;
    private long maxSize;
    private long currentSize;
    ...
    public synchronized Y remove(@NonNull T key) {
        Entry<Y> entry = cache.remove(key);
        if (entry == null) {
          return null;
        }
        currentSize -= entry.size;
        return entry.value;
    }
    ...
}
public class LinkedHashMap<K,V> extends HashMap<K,V> implements Map<K,V>{
    public LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder) {
            super(initialCapacity, loadFactor);
            this.accessOrder = accessOrder;
    }
    ...
}
复制代码

waitForExistingOrStartNewJob

• • 
    
• 那么回到Engine.load，loadFromMemory取不到, 则调用waitForExistingOrStartNewJob,代码如下
  • 
    

final class Jobs {
  private final Map<Key, EngineJob<?>> jobs = new HashMap<>();
  private final Map<Key, EngineJob<?>> onlyCacheJobs = new HashMap<>();
  ...
}
private <R> LoadStatus waitForExistingOrStartNewJob(...) {
    //先从jobs中取EngineJob
    EngineJob<?> current = jobs.get(key, onlyRetrieveFromCache);
    if (current != null) {
      current.addCallback(cb, callbackExecutor);
      //取到则直接创建一个LoadStatus返回
      return new LoadStatus(cb, current);
    }

    //取不到则通过engineJobFactory.build创建一个EngineJob
    EngineJob<R> engineJob =
        engineJobFactory.build(
            key,
            isMemoryCacheable,
            useUnlimitedSourceExecutorPool,
            useAnimationPool,
            onlyRetrieveFromCache);

    DecodeJob<R> decodeJob =
        decodeJobFactory.build(
            glideContext,
            model,
            key,
            signature,
            width,
            height,
            resourceClass,
            transcodeClass,
            priority,
            diskCacheStrategy,
            transformations,
            isTransformationRequired,
            isScaleOnlyOrNoTransform,
            onlyRetrieveFromCache,
            options,
            engineJob);

    //将创建的engineJob加入到jobs
    jobs.put(key, engineJob);

    engineJob.addCallback(cb, callbackExecutor);
    //调用start开启线程获取图片
    engineJob.start(decodeJob);

    return new LoadStatus(cb, engineJob);
}
复制代码

engineJob.start

• • 
    
• 上面最重要的就是engineJob.start这一行，开启线程获取图片，start方法代码如下
  • 
    

public synchronized void start(DecodeJob<R> decodeJob) {
    this.decodeJob = decodeJob;
    GlideExecutor executor =
        //判断是否需要磁盘缓存，是则使用磁盘线程池，否则使用内存缓存线程池
        decodeJob.willDecodeFromCache() ? diskCacheExecutor : getActiveSourceExecutor();
    //执行job
    executor.execute(decodeJob);
}
复制代码

• • 
    
• 从GlideBuilder.build中可以找到diskCacheExecutor的初始化
  • 
    

diskCacheExecutor = GlideExecutor.newDiskCacheExecutor();

public final class GlideExecutor implements ExecutorService {
    public static GlideExecutor newDiskCacheExecutor() {
        return newDiskCacheBuilder().build();
    }
    public static GlideExecutor.Builder newDiskCacheBuilder() {
        return new GlideExecutor.Builder(/*preventNetworkOperations=*/ true)
            .setThreadCount(DEFAULT_DISK_CACHE_EXECUTOR_THREADS)
            .setName(DEFAULT_DISK_CACHE_EXECUTOR_NAME);
    }
    GlideExecutor(ExecutorService delegate) {
        this.delegate = delegate;
    }
    public void execute(@NonNull Runnable command) {
        delegate.execute(command);
    }
    public static final class Builder {
        ...
        public GlideExecutor build() {
              if (TextUtils.isEmpty(name)) {
                throw new IllegalArgumentException(
                    "Name must be non-null and non-empty, but given: " + name);
              }
              ThreadPoolExecutor executor =
                  new ThreadPoolExecutor(
                      corePoolSize,
                      maximumPoolSize,
                      /*keepAliveTime=*/ threadTimeoutMillis,
                      TimeUnit.MILLISECONDS,
                      new PriorityBlockingQueue<Runnable>(),
                      new DefaultThreadFactory(name, uncaughtThrowableStrategy, preventNetworkOperations));

              if (threadTimeoutMillis != NO_THREAD_TIMEOUT) {
                executor.allowCoreThreadTimeOut(true);
              }

              return new GlideExecutor(executor);
          }
    }
}
复制代码

• • 
    
• 不管是磁盘缓存还是内存最终都会调用DecodeJob(继承Runnable)的run，接着会调用runWrapped
  • 
    

class DecodeJob<R>
    implements DataFetcherGenerator.FetcherReadyCallback,
        Runnable, Comparable<DecodeJob<?>>, Poolable {

    public void run() {
        ...
        try {
          if (isCancelled) {
            notifyFailed();
            return;
          }

          runWrapped();
        } catch (
        ...
    }

    private void runWrapped() {
        switch (runReason) {
          case INITIALIZE:
            //调用getNextStage后返回Stage.RESOURCE_CACHE
            stage = getNextStage(Stage.INITIALIZE);
            currentGenerator = getNextGenerator();
            runGenerators();
            break;
          case SWITCH_TO_SOURCE_SERVICE:
            runGenerators();
            break;
          case DECODE_DATA:
            decodeFromRetrievedData();
            break;
          default:
            throw new IllegalStateException("Unrecognized run reason: " + runReason);
        }
    }
}

//getNextState中会根据磁盘缓存的策略判断来指定Stage值
private Stage getNextStage(Stage current) {
    switch (current) {
      case INITIALIZE:
        return diskCacheStrategy.decodeCachedResource()
            ? Stage.RESOURCE_CACHE
            : getNextStage(Stage.RESOURCE_CACHE);
      case RESOURCE_CACHE:
        return diskCacheStrategy.decodeCachedData()
            ? Stage.DATA_CACHE
            : getNextStage(Stage.DATA_CACHE);
      case DATA_CACHE:
        // Skip loading from source if the user opted to only retrieve the resource from cache.
        return onlyRetrieveFromCache ? Stage.FINISHED : Stage.SOURCE;
      case SOURCE:
      case FINISHED:
        return Stage.FINISHED;
      default:
        throw new IllegalArgumentException("Unrecognized stage: " + current);
    }
}
复制代码

• • 
    
• runReason在初始化DecodeJob的时候设置初始为INITIALIZE,所以会调用getNextGenerator，且getNextStage后stage为RESOURCE_CACHE，所以最终返回ResourceCacheGenerator
  • 
    

private DataFetcherGenerator getNextGenerator() {
    switch (stage) {
      case RESOURCE_CACHE:
        return new ResourceCacheGenerator(decodeHelper, this);
      case DATA_CACHE:
        return new DataCacheGenerator(decodeHelper, this);
      case SOURCE:
        return new SourceGenerator(decodeHelper, this);
      case FINISHED:
        return null;
      default:
        throw new IllegalStateException("Unrecognized stage: " + stage);
    }
}
复制代码

runGenerators

• • 
    
• DecodeJob.runWrapped最后会调用runGenerators
  • 
    

private void runGenerators() {
    ...
    while (!isCancelled
        && currentGenerator != null
        && !(isStarted = currentGenerator.startNext())) {
      stage = getNextStage(stage);
      currentGenerator = getNextGenerator();

      if (stage == Stage.SOURCE) {
        reschedule();
        return;
      }
    }
    ...
}
复制代码

• • 
    
• 其中while中的currentGenerator就是上面几个状态对应的ResourceCacheGenerator，DataCacheGenerator，SourceGenerator； 前两个都是缓存的让我们来看一下SourceGenerator的startNext；
  • 
    

SourceGenerator.startNext

• • 
    
• runGenerators中最关键的一步是调用了currentGenerator.startNext
  • 
    

public boolean startNext() {
    ...
    while (!started && hasNextModelLoader()) {
      loadData = helper.getLoadData().get(loadDataListIndex++);
      if (loadData != null
          && (helper.getDiskCacheStrategy().isDataCacheable(loadData.fetcher.getDataSource())
              || helper.hasLoadPath(loadData.fetcher.getDataClass()))) {
        started = true;
        startNextLoad(loadData);
      }
    }
    return started;
}

private void startNextLoad(final LoadData<?> toStart) {
    //这里只需要关注loadData.fetcher.loadData方法
    loadData.fetcher.loadData(
        helper.getPriority(),
        new DataCallback<Object>() {
          @Override
          public void onDataReady(@Nullable Object data) {
            if (isCurrentRequest(toStart)) {
              onDataReadyInternal(toStart, data);
            }
          }

          @Override
          public void onLoadFailed(@NonNull Exception e) {
            if (isCurrentRequest(toStart)) {
              onLoadFailedInternal(toStart, e);
            }
          }
        });
}
复制代码

• • 
    
• 上面代码中helper.getLoadData代码如下
  • 
    

List<LoadData<?>> getLoadData() {
    if (!isLoadDataSet) {
      isLoadDataSet = true;
      loadData.clear();
      List<ModelLoader<Object, ?>> modelLoaders = glideContext.getRegistry().getModelLoaders(model);
      //noinspection ForLoopReplaceableByForEach to improve perf
      for (int i = 0, size = modelLoaders.size(); i < size; i++) {
        ModelLoader<Object, ?> modelLoader = modelLoaders.get(i);
        LoadData<?> current = modelLoader.buildLoadData(model, width, height, options);
        if (current != null) {
          loadData.add(current);
        }
      }
    }
    return loadData;
}
复制代码

• • 
    
• 其中buildLoadData是在HttpGlideUrlLoader中实现的
  • 
    

HttpGlideUrlLoader.buildLoadData

public LoadData<InputStream> buildLoadData(
      @NonNull GlideUrl model, int width, int height, @NonNull Options options) {
    // GlideUrls memoize parsed URLs so caching them saves a few object instantiations and time
    // spent parsing urls.
    GlideUrl url = model;
    if (modelCache != null) {
      url = modelCache.get(model, 0, 0);
      if (url == null) {
        modelCache.put(model, 0, 0, model);
        url = model;
      }
    }
    int timeout = options.get(TIMEOUT);
    return new LoadData<>(url, new HttpUrlFetcher(url, timeout));
}
复制代码

• • 
    
• 而SourceGenerator.startNext的startNextLoad中，调用了loadData.fetcher.loadData方法，而从上面代码最后一行我们知道fetcher的实际类型就是HttpUrlFetcher，那么我们看看其loadData方法的实现
  • 
    

HttpUrlFetcher.loadData

public void loadData(
      @NonNull Priority priority, @NonNull DataCallback<? super InputStream> callback) {
    long startTime = LogTime.getLogTime();
    try {
      InputStream result = loadDataWithRedirects(glideUrl.toURL(), 0, null, glideUrl.getHeaders());
      callback.onDataReady(result);
    } catch (IOException e) {
      if (Log.isLoggable(TAG, Log.DEBUG)) {
        Log.d(TAG, "Failed to load data for url", e);
      }
      callback.onLoadFailed(e);
    } finally {
      if (Log.isLoggable(TAG, Log.VERBOSE)) {
        Log.v(TAG, "Finished http url fetcher fetch in " + LogTime.getElapsedMillis(startTime));
      }
    }
}
复制代码

• • 
    
• 可以看到其中调用了loadDataWithRedirects，代码如下：
  • 
    

private InputStream loadDataWithRedirects(
      URL url, int redirects, URL lastUrl, Map<String, String> headers) throws HttpException {
    ...

    urlConnection = buildAndConfigureConnection(url, headers);

    try {
      // Connect explicitly to avoid errors in decoders if connection fails.
      urlConnection.connect();
      // Set the stream so that it's closed in cleanup to avoid resource leaks. See #2352.
      stream = urlConnection.getInputStream();
    } catch (IOException e) {
      throw new HttpException(
          "Failed to connect or obtain data", getHttpStatusCodeOrInvalid(urlConnection), e);
    }

    if (isCancelled) {
      return null;
    }

    final int statusCode = getHttpStatusCodeOrInvalid(urlConnection);
    ...
}
复制代码

• • 
    
• 上面代码中我们看到了熟悉的urlConnection，那么看一下buildAndConfigureConnection方法的实现吧
  • 
    

private HttpURLConnection buildAndConfigureConnection(URL url, Map<String, String> headers)
      throws HttpException {
    HttpURLConnection urlConnection;
    try {
      urlConnection = connectionFactory.build(url);
    } catch (IOException e) {
      throw new HttpException("URL.openConnection threw", /*statusCode=*/ 0, e);
    }
    for (Map.Entry<String, String> headerEntry : headers.entrySet()) {
      urlConnection.addRequestProperty(headerEntry.getKey(), headerEntry.getValue());
    }
    urlConnection.setConnectTimeout(timeout);
    urlConnection.setReadTimeout(timeout);
    urlConnection.setUseCaches(false);
    urlConnection.setDoInput(true);
    // Stop the urlConnection instance of HttpUrlConnection from following redirects so that
    // redirects will be handled by recursive calls to this method, loadDataWithRedirects.
    urlConnection.setInstanceFollowRedirects(false);
    return urlConnection;
}
复制代码

• • 
    
• 到此我们终于看到了网络请求相关的代码，原来Glide使用的HttpURLConnection进行网络请求
  • 
    

SourceGenerator.onDataReadyInternal

• • 
    
• 而从上面SourceGenerator.startNextNext中的loadData.fetcher.loadData的入参DataCallback的onDataReady我们知道，如果请求成功，会将资源通过onDataReady传递回去，其中调用了onDataReadyInternal，代码如下
  • 
    

void onDataReadyInternal(LoadData<?> loadData, Object data) {
    //获取磁盘加载策略
    DiskCacheStrategy diskCacheStrategy = helper.getDiskCacheStrategy();
    //判断是否缓存原始数据
    if (data != null && diskCacheStrategy.isDataCacheable(loadData.fetcher.getDataSource())) {
      dataToCache = data;
      // We might be being called back on someone else's thread. Before doing anything, we should
      // reschedule to get back onto Glide's thread.
      cb.reschedule();
    } else {
      //cb为DecodeJob
      cb.onDataFetcherReady(
          loadData.sourceKey,
          data,
          loadData.fetcher,
          loadData.fetcher.getDataSource(),
          originalKey);
    }
}
复制代码

• • 
    
• 上面的cb实际为DecodeJob，那么来看一下DecodeJob.onDataFetcherReady
  • 
    

public void onDataFetcherReady(
      Key sourceKey, Object data, DataFetcher<?> fetcher, DataSource dataSource, Key attemptedKey) {
    this.currentSourceKey = sourceKey;
    this.currentData = data;
    this.currentFetcher = fetcher;
    this.currentDataSource = dataSource;
    this.currentAttemptingKey = attemptedKey;
    this.isLoadingFromAlternateCacheKey = sourceKey != decodeHelper.getCacheKeys().get(0);

    if (Thread.currentThread() != currentThread) {
      runReason = RunReason.DECODE_DATA;
      callback.reschedule(this);
    } else {
      GlideTrace.beginSection("DecodeJob.decodeFromRetrievedData");
      try {
        decodeFromRetrievedData();
      } finally {
        GlideTrace.endSection();
      }
    }
}
复制代码

• • 
    
• 上面代码中又调用了decodeFromRetrievedData，代码如下
  • 
    

private void decodeFromRetrievedData() {
    if (Log.isLoggable(TAG, Log.VERBOSE)) {
      logWithTimeAndKey(
          "Retrieved data",
          startFetchTime,
          "data: "
              + currentData
              + ", cache key: "
              + currentSourceKey
              + ", fetcher: "
              + currentFetcher);
    }
    Resource<R> resource = null;
    try {
      //解析图片
      resource = decodeFromData(currentFetcher, currentData, currentDataSource);
    } catch (GlideException e) {
      e.setLoggingDetails(currentAttemptingKey, currentDataSource);
      throwables.add(e);
    }
    if (resource != null) {
      //通知图片获取完毕
      notifyEncodeAndRelease(resource, currentDataSource, isLoadingFromAlternateCacheKey);
    } else {
      runGenerators();
    }
}
复制代码

• • 
    
• notifyEncodeAndRelease代码如下
  • 
    

private void notifyEncodeAndRelease(
      Resource<R> resource, DataSource dataSource, boolean isLoadedFromAlternateCacheKey) {
    if (resource instanceof Initializable) {
      ((Initializable) resource).initialize();
    }

    Resource<R> result = resource;
    LockedResource<R> lockedResource = null;
    if (deferredEncodeManager.hasResourceToEncode()) {
      lockedResource = LockedResource.obtain(resource);
      result = lockedResource;
    }

    notifyComplete(result, dataSource, isLoadedFromAlternateCacheKey);

    stage = Stage.ENCODE;
    try {
      if (deferredEncodeManager.hasResourceToEncode()) {
        deferredEncodeManager.encode(diskCacheProvider, options);
      }
    } finally {
      if (lockedResource != null) {
        lockedResource.unlock();
      }
    }
    // Call onEncodeComplete outside the finally block so that it's not called if the encode process
    // throws.
    onEncodeComplete();
}

参考

• ​​Glide源码​​
• ​​Glide官方中文文档​​
• ​​Android图片加载框架最全解析（二），从源码的角度理解Glide的执行流程​​
• ​​Glide原理之史上最全的​​
• ​​Glide源码浅析（五）缓存策略，加载策略​​

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