Glide****的生命周期的实现主要是通过创建一个****Fragment进行实现的

在****Glide.with(context)****这段代码中****Glide****会创建一个****RequestManager****类。该类是实现生命周期的关键方法
public class RequestManager implements LifecycleListener {
private final Context context;
private final Lifecycle lifecycle;
private final RequestManagerTreeNode treeNode;
private final RequestTracker requestTracker;
private final Glide glide;
private final OptionsApplier optionsApplier;
private DefaultOptions options;
下面是获取RequestManager的方法：
RequestManager supportFragmentGet(Context context, FragmentManager fm) {
SupportRequestManagerFragment current = getSupportRequestManagerFragment(fm);
RequestManager requestManager = current.getRequestManager();
if (requestManager == null) {
requestManager = new RequestManager(context, current.getLifecycle(), current.getRequestManagerTreeNode());
current.setRequestManager(requestManager);
}
return requestManager;
}
其中创建一个了一个SupportRequestManagerFragment的对象，我们来看这个类：
public class RequestManagerFragment extends Fragment {
private final ActivityFragmentLifecycle lifecycle;
private final RequestManagerTreeNode requestManagerTreeNode = new FragmentRequestManagerTreeNode();
private RequestManager requestManager;
private final HashSet childRequestManagerFragments
= new HashSet();
private RequestManagerFragment rootRequestManagerFragment;
其中的ActivityFragmentLifecycle实现了Lifecycle，根据Lifecycle的注释：
A {@link com.bumptech.glide.manager.Lifecycle} implementation for tracking and notifying listeners of {@link android.app.Fragment} and {@link android.app.Activity} lifecycle events.
可知，这个接口用于实现Activity或者Fragment的生命周期的回调
RequestManagerFragment
onAttach()
@Override
public void onAttach(Activity activity) {
super.onAttach(activity);
rootRequestManagerFragment = RequestManagerRetriever.get()
.getRequestManagerFragment(getActivity().getFragmentManager());
if (rootRequestManagerFragment != this) {
rootRequestManagerFragment.addChildRequestManagerFragment(this);
}
}
先获取到root，如果当前获取的root不等于当前的管理的Fragment，则添加到当前管理类中的一个set结合中(实际上，RequestManagerFragment既充当了一个Fragment来组织生命周期的回调，同样也是一个管理者的角色，他管理了依附于不同fragmentActivty和Activity的fragment)。
那么当该fragment在销毁的时候，会执行到onDetach()放方法，直接将自己销毁：
@Override
public void onDetach() {
super.onDetach();
if (rootRequestManagerFragment != null) {
rootRequestManagerFragment.removeChildRequestManagerFragment(this);
rootRequestManagerFragment = null;
}
}
值得注意的是，Glide会在actiivty进行重新创建的时候，可能因为内存不够而无法创建时，回调onLowMemory:
@Override
public void onLowMemory() {
super.onLowMemory();
// If an activity is re-created, onLowMemory may be called before a manager is ever set.
// See #329.
if (requestManager != null) {
requestManager.onLowMemory();
}
}
lifecycle的注入
SupportRequestManagerFragment getSupportRequestManagerFragment(final FragmentManager fm) {
SupportRequestManagerFragment current = (SupportRequestManagerFragment) fm.findFragmentByTag(
FRAGMENT_TAG);
if (current == null) {
current = pendingSupportRequestManagerFragments.get(fm);
if (current == null) {
current = new SupportRequestManagerFragment();
pendingSupportRequestManagerFragments.put(fm, current);
fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
handler.obtainMessage(ID_REMOVE_SUPPORT_FRAGMENT_MANAGER, fm).sendToTarget();
}
}
return current;
}
我们知道，在每次创建SupportRequestManagerFragment的时候，均需要一个fm作为参数，而该fm会根据一个TAG获取fragment,这样，不同的activity中的Imageview在创Glide请求的时候，会创建不同的SupportRequestManagerFragment对象，并且，我们看到SupportRequestManagerFragment的构造方法：
public SupportRequestManagerFragment() {
this(new ActivityFragmentLifecycle());
}
// For testing only.
@SuppressLint("ValidFragment")
public SupportRequestManagerFragment(ActivityFragmentLifecycle lifecycle) {
this.lifecycle = lifecycle;
}
可以看见，每个不用的activity中创建的fragment会拥有自己的生命周期。
接下来，我们看上面RequestManagerFragment中的其他的生命周期的回调是在何种时候进行回调的。
1、经过前文的分析，我们知道，Glide.with(context)会创建一个RequestManager对象，该对象
requestManager = new RequestManager(context, current.getLifecycle(), current.getRequestManagerTreeNode());
中已经set进去了管理生命周期的RequestManagerfragment的lifecycle的对象。
2、那么我们就继续朝着api的调用方向：load(string)继续向下看
public DrawableTypeRequest load(String string) {
return (DrawableTypeRequest) fromString().load(string);
}
首先创建一个DrawableTypeRequest对象，该对象，会被RequestManager注入Lifecycle：
return optionsApplier.apply(
new DrawableTypeRequest(modelClass, streamModelLoader, fileDescriptorModelLoader, context,
glide, requestTracker, lifecycle, optionsApplier));
这里很厉害的一个地方就是：optionsApplier.apply会把optionsApplier自己注入进去，这样，
public > Y into(Y target) {
...
Request request = buildRequest(target);
/** setTag **/
target.setRequest(request);
lifecycle.addListener(target);
requestTracker.runRequest(request);
return target;
}
中的lifecycle即为上面requestManager传递过去。
3、RequestTracker
该类是真正起到保证fragment的生命周期和进行图片资源请求的生命周期的桥梁。
从RequestManager中的其他生命周期代码可知：
onStart()
/**
* Lifecycle callback that registers for connectivity events (if the android.permission.ACCESS_NETWORK_STATE
* permission is present) and restarts failed or paused requests.
*/
@Override
public void onStart() {
// onStart might not be called because this object may be created after the fragment/activity's onStart method.
resumeRequests();
}
其中追踪resumeRequests()是：
/**
* Restarts any loads that have not yet completed.
*
* @see #isPaused()
* @see #pauseRequests()
*/
public void resumeRequests() {
Util.assertMainThread();
requestTracker.resumeRequests();
}
最终的执行者是requestTracker，调用该段代码会将
isPaused = false;
这样，在接下来的进行请求或者重新请求时，会依据这个标志位进行判断。
其他的生命周期也是这样子的，就不一一说了。
****最后，说明如何进行参数****requestTracker的传递的
即为上面：
创建一个DrawableTypeRequest对象，该对象，会被RequestManager注入Lifecycle，
return optionsApplier.apply(
new DrawableTypeRequest(modelClass, streamModelLoader, fileDescriptorModelLoader, context,
glide, requestTracker, lifecycle, optionsApplier));

同时，requestTracker也会被传入进去，这样，所有的都已经串通了，Glide的生命周期就是这样了。