一般人比较少去研究android源码，但是阅读源码不仅能够提高个人代码水平，还能锻炼自己的逻辑思维能力，对于常见的代码，我们必须要知道原理是怎么回事，不一定要能写出来，毕竟这个是谷歌那么多人弄出来的，网上也有很多关于setContentView的源码分析，但是很多都是activity的基础上分析，现在我们android开发基本都是使用AppCompatActivity，所以今天就一起分析其中的过程，为什么setContentView就能显示页面。
首先看下我们经常使用setContentView的代码。
protected voidonCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(setLayout());
}
我们先来看下 AppCompatActivity 中 setContentView 中做了什么
@Override
public void setContentView(@LayoutRes int layoutResID) {
getDelegate().setContentView(layoutResID);
}
@Override
public void setContentView(View view) {
getDelegate().setContentView(view);
}
@Override
public void setContentView(View view, ViewGroup.LayoutParams params) {
getDelegate().setContentView(view, params);
}这里有三个重载的方法，明显看到了使用的是代理模式，（代理模式其实就是，实现代理的对象能够访问对象本身的资源），
getDelegate() 这个代理方法是干嘛用的呢，到底给我们返回什么东西。进去看下
/**
* @return The {@link AppCompatDelegate} being used by this Activity.
*/
@NonNull
public AppCompatDelegate getDelegate() {
if (mDelegate == null) {
mDelegate = AppCompatDelegate.create(this, this);
}
return mDelegate;
} 字面意思，创建代理对象，并且返回代理对象。那么这个代理对象是如何创立的？
AppCompatDelegate.create(this, this); 这个方法，传入一个上下文，还有一个接口回调
public static AppCompatDelegate create(Activity activity, AppCompatCallback callback) {
return create(activity, activity.getWindow(), callback);
} 继续走。。。
private static AppCompatDelegate create(Context context, Window window,
AppCompatCallback callback) {
final int sdk = Build.VERSION.SDK_INT;
if (BuildCompat.isAtLeastN()) {
return new AppCompatDelegateImplN(context, window, callback);
} else if (sdk >= 23) {
return new AppCompatDelegateImplV23(context, window, callback);
} else if (sdk >= 14) {
return new AppCompatDelegateImplV14(context, window, callback);
} else if (sdk >= 11) {
return new AppCompatDelegateImplV11(context, window, callback);
} else {
return new AppCompatDelegateImplV9(context, window, callback);
}
} 原来这里是根据sdk版本返回不同的代理对象。
我们知道返回的对象最后是调用了
getDelegate().setContentView(view); 那我们肯定能够确认，这些对象的类里边肯定是有setContentView方法，我们找找。
class AppCompatDelegateImplN extends AppCompatDelegateImplV23 这里面没有，继续找，最终我们在他们的父类V9里面找到了这个方法。
@Override
public void setContentView(View v) {//这里很重要，暂时先放着不动
ensureSubDecor();
ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
contentParent.removeAllViews();
contentParent.addView(v);
mOriginalWindowCallback.onContentChanged();
}
@Override
public void setContentView(int resId) {
ensureSubDecor();
ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
contentParent.removeAllViews();
LayoutInflater.from(mContext).inflate(resId, contentParent);
mOriginalWindowCallback.onContentChanged();
}
@Override
public void setContentView(View v, ViewGroup.LayoutParams lp) {
ensureSubDecor();
ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
contentParent.removeAllViews();
contentParent.addView(v, lp);
mOriginalWindowCallback.onContentChanged();
}
原来是在这里设置了布局。对应三个方法都调用了ensureSubDecor(),然后根据id找到contentparent，先移除里面的内容，然后再添加我们的布局进去，然后通知界面发生改变。
接下来重点看下这个，你到底确认了什么东西
ensureSubDecor(); mSubDecor 这个对象是什么？接下来我们看他的代码
private void ensureSubDecor() {
if (!mSubDecorInstalled) {//原来在这里创建啊
mSubDecor = createSubDecor();
// If a title was set before we installed the decor, propagate it now
CharSequence title = getTitle();
if (!TextUtils.isEmpty(title)) {
onTitleChanged(title);
}
applyFixedSizeWindow();
onSubDecorInstalled(mSubDecor);
//标记位，判断到底创建了没，避免重复创建
mSubDecorInstalled = true;
// Invalidate if the panel menu hasn't been created before this.
// Panel menu invalidation is deferred avoiding application onCreateOptionsMenu
// being called in the middle of onCreate or similar.
// A pending invalidation will typically be resolved before the posted message
// would run normally in order to satisfy instance state restoration.
PanelFeatureState st = getPanelState(FEATURE_OPTIONS_PANEL, false);
if (!isDestroyed() && (st == null || st.menu == null)) {
invalidatePanelMenu(FEATURE_SUPPORT_ACTION_BAR);
}
}
} 原来是这里创建了一个subdector，这个玩意还是个viewgroup，以为后面需要这个强转成viewgroup并且找id
private ViewGroup createSubDecor() {
TypedArray a = mContext.obtainStyledAttributes(R.styleable.AppCompatTheme);
if (!a.hasValue(R.styleable.AppCompatTheme_windowActionBar)) {
a.recycle();//这里有个异常，记得么，当我们使用ativity集成appcompactActivity的时候，主题不能随便用的，必须使用appCompact的主题，否则会报错
throw new IllegalStateException(
"You need to use a Theme.AppCompat theme (or descendant) with this activity.");
}
if (a.getBoolean(R.styleable.AppCompatTheme_windowNoTitle, false)) {//默认设置没有标题
requestWindowFeature(Window.FEATURE_NO_TITLE);
} else if (a.getBoolean(R.styleable.AppCompatTheme_windowActionBar, false)) {
// Don't allow an action bar if there is no title.
requestWindowFeature(FEATURE_SUPPORT_ACTION_BAR);
}
if (a.getBoolean(R.styleable.AppCompatTheme_windowActionBarOverlay, false)) {
requestWindowFeature(FEATURE_SUPPORT_ACTION_BAR_OVERLAY);
}
if (a.getBoolean(R.styleable.AppCompatTheme_windowActionModeOverlay, false)) {
requestWindowFeature(FEATURE_ACTION_MODE_OVERLAY);
}
mIsFloating = a.getBoolean(R.styleable.AppCompatTheme_android_windowIsFloating, false);
a.recycle();
//前面都是些设置主题相关的，到了下面又是一个非常关键的地方，我们的窗口对象获取了一个什么view// Now let's make sure that the Window has installed its decor by retrieving it
mWindow.getDecorView();
final LayoutInflater inflater = LayoutInflater.from(mContext);
ViewGroup subDecor = null;
if (!mWindowNoTitle) {//上面的已经说了，默认的是没有title的，所以下面的方法不会走
if (mIsFloating) {
// If we're floating, inflate the dialog title decor
subDecor = (ViewGroup) inflater.inflate(
R.layout.abc_dialog_title_material, null);
// Floating windows can never have an action bar, reset the flags
mHasActionBar = mOverlayActionBar = false;
} else if (mHasActionBar) {
/**
* This needs some explanation. As we can not use the android:theme attribute
* pre-L, we emulate it by manually creating a LayoutInflater using a
* ContextThemeWrapper pointing to actionBarTheme.
*/
TypedValue outValue = new TypedValue();
mContext.getTheme().resolveAttribute(R.attr.actionBarTheme, outValue, true);
Context themedContext;
if (outValue.resourceId != 0) {
themedContext = new ContextThemeWrapper(mContext, outValue.resourceId);
} else {
themedContext = mContext;
}
// Now inflate the view using the themed context and set it as the content view
subDecor = (ViewGroup) LayoutInflater.from(themedContext)
.inflate(R.layout.abc_screen_toolbar, null);
mDecorContentParent = (DecorContentParent) subDecor
.findViewById(R.id.decor_content_parent);
mDecorContentParent.setWindowCallback(getWindowCallback());
/**
* Propagate features to DecorContentParent
*/
if (mOverlayActionBar) {
mDecorContentParent.initFeature(FEATURE_SUPPORT_ACTION_BAR_OVERLAY);
}
if (mFeatureProgress) {
mDecorContentParent.initFeature(Window.FEATURE_PROGRESS);
}
if (mFeatureIndeterminateProgress) {
mDecorContentParent.initFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
}
}
} else {//一般都是走这里的，原来subdector就是一个布局文件，这个布局还是个viewgroup，并且根据这个group还能找id
if (mOverlayActionMode) {
subDecor = (ViewGroup) inflater.inflate(
R.layout.abc_screen_simple_overlay_action_mode, null);
} else {
subDecor = (ViewGroup) inflater.inflate(R.layout.abc_screen_simple, null);
}
if (Build.VERSION.SDK_INT >= 21) {
// If we're running on L or above, we can rely on ViewCompat's
// setOnApplyWindowInsetsListener
ViewCompat.setOnApplyWindowInsetsListener(subDecor,
new OnApplyWindowInsetsListener() {
@Override
public WindowInsetsCompat onApplyWindowInsets(View v,
WindowInsetsCompat insets) {
final int top = insets.getSystemWindowInsetTop();
final int newTop = updateStatusGuard(top);
if (top != newTop) {
insets = insets.replaceSystemWindowInsets(
insets.getSystemWindowInsetLeft(),
newTop,
insets.getSystemWindowInsetRight(),
insets.getSystemWindowInsetBottom());
}
// Now apply the insets on our view
return ViewCompat.onApplyWindowInsets(v, insets);
}
});
} else {
// Else, we need to use our own FitWindowsViewGroup handling
((FitWindowsViewGroup) subDecor).setOnFitSystemWindowsListener(
new FitWindowsViewGroup.OnFitSystemWindowsListener() {
@Override
public void onFitSystemWindows(Rect insets) {
insets.top = updateStatusGuard(insets.top);
}
});
}
}
if (subDecor == null) {
throw new IllegalArgumentException(
"AppCompat does not support the current theme features: { "
+ "windowActionBar: " + mHasActionBar
+ ", windowActionBarOverlay: "+ mOverlayActionBar
+ ", android:windowIsFloating: " + mIsFloating
+ ", windowActionModeOverlay: " + mOverlayActionMode
+ ", windowNoTitle: " + mWindowNoTitle
+ " }");
}
if (mDecorContentParent == null) {
mTitleView = (TextView) subDecor.findViewById(R.id.title);
}
// Make the decor optionally fit system windows, like the window's decor
ViewUtils.makeOptionalFitsSystemWindows(subDecor);
final ContentFrameLayout contentView = (ContentFrameLayout) subDecor.findViewById(
R.id.action_bar_activity_content);
final ViewGroup windowContentView = (ViewGroup) mWindow.findViewById(android.R.id.content);
if (windowContentView != null) {
// There might be Views already added to the Window's content view so we need to
// migrate them to our content view
while (windowContentView.getChildCount() > 0) {
final View child = windowContentView.getChildAt(0);
windowContentView.removeViewAt(0);
contentView.addView(child);
}
// Change our content FrameLayout to use the android.R.id.content id.
// Useful for fragments.
windowContentView.setId(View.NO_ID);
contentView.setId(android.R.id.content);
// The decorContent may have a foreground drawable set (windowContentOverlay).
// Remove this as we handle it ourselves
if (windowContentView instanceof FrameLayout) {
((FrameLayout) windowContentView).setForeground(null);
}
}
// Now set the Window's content view with the decor//这里将subdector放入window里面
mWindow.setContentView(subDecor);
contentView.setAttachListener(new ContentFrameLayout.OnAttachListener() {
@Override
public void onAttachedFromWindow() {}
@Override
public void onDetachedFromWindow() {
dismissPopups();
}
});
return subDecor;
}
这段代码的核心思想就是加载布局，获取subDector，然后把这个view放入到window中
大家还记得下面这段代码么，这是设置去掉标题栏，全屏的方法，而且这个还必须设置到setcontengview前面，就是在这里，只有设置了没有标题，在这里才会判断有没有，否则的话，就是默认有标题栏的，就会加载布局，后设置了肯定是无效的
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
requestWindowFeature(Window.FEATURE_NO_TITLE);
getWindow().setFlags(WindowManager.LayoutParams.FILL_PARENT
, WindowManager.LayoutParams.FILL_PARENT);
setContentView(R.layout.activity_test);
}
接下来看下我们得到subDector的布局到底是什么样子的
<android.support.v7.internal.widget.FitWindowsLinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/action_bar_root"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:fitsSystemWindows="true">
<android.support.v7.internal.widget.ViewStubCompat
android:id="@+id/action_mode_bar_stub"
android:inflatedId="@+id/action_mode_bar"
android:layout="@layout/abc_action_mode_bar"
android:layout_width="match_parent"
android:layout_height="wrap_content" />
<include layout="@layout/abc_screen_content_include" />
</android.support.v7.internal.widget.FitWindowsLinearLayout>
abc_screen_content_include.xml
<merge xmlns:android="http://schemas.android.com/apk/res/android">
<android.support.v7.internal.widget.ContentFrameLayout
android:id="@id/action_bar_activity_content"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:foregroundGravity="fill_horizontal|top"
android:foreground="?android:attr/windowContentOverlay" />
</merge>
原来，这个布局就是个FitWindowsLinearLayout，用一张网上的图片，整个布局是这样子的
接下来我们继续看
mWindow.getDecorView();mWindow.setContentView(subDecor); 这两个方法是干啥的？
点进去发现
getDecorView() 是一个抽象方法，这个方法是在Window这个类里边，因此我们必须找到这个类的实现类，到了这里，我也不是很明白怎么回事，因为找了半天没找到他的实现类，万能的百度出场了，百度下发现这个类的实现类是phonewindow，好了，其他的不追了，我们看下phonewindow这个方法都干了些啥
public class PhoneWindow extends Window
implements MenuBuilder.Callback {
@Override
public final View getDecorView() {
if (mDecor == null || mForceDecorInstall) {
installDecor();
}
return mDecor;
}
private void installDecor() {
//mDecor是DecorView,第一次mDecor=null，所以调用generateDecor
if (mDecor == null) {
mDecor = generateDecor();
...
}
//第一次mContentParent也等于null
if (mContentParent == null) {
//可以看到把DecorView传入进去了
mContentParent = generateLayout(mDecor);
}
}
}
在generateDecor()做了什么？其实返回了一个DecorView对象。
protected DecorView generateDecor(int featureId) {
// System process doesn't have application context and in that case we need to directly use
// the context we have. Otherwise we want the application context, so we don't cling to the
// activity.
Context context;
if (mUseDecorContext) {
Context applicationContext = getContext().getApplicationContext();
if (applicationContext == null) {
context = getContext();
} else {
context = new DecorContext(applicationContext, getContext().getResources());
if (mTheme != -1) {
context.setTheme(mTheme);
}
}
} else {
context = getContext();
}
return new DecorView(context, featureId, this, getAttributes());
}
DecorView是啥呢？
publicclassDecorView extends FrameLayoutimplements RootViewSurfaceTaker,WindowCallbacks
原来继承FrameLayout
protected ViewGroup generateLayout(DecorView decor) {
TypedArray a = getWindowStyle();
//设置一堆标志位...
...
if (!mForcedStatusBarColor) {
//获取主题状态栏的颜色
mStatusBarColor = a.getColor(R.styleable.Window_statusBarColor, 0xFF000000);
}
if (!mForcedNavigationBarColor) {
//获取底部NavigationBar颜色
mNavigationBarColor = a.getColor(R.styleable.Window_navigationBarColor, 0xFF000000);
}
//获取主题一些资源
...
// Inflate the window decor.
int layoutResource;
int features = getLocalFeatures();
// System.out.println("Features: 0x" + Integer.toHexString(features));
if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
...我们设置不同的主题以及样式，会采用不同的布局文件...
} else {
//记住这个布局，之后我们会来验证下布局的结构
layoutResource = R.layout.screen_simple;
// System.out.println("Simple!");
}
//要开始更改mDecor啦~
mDecor.startChanging();
//注意，此时把screen_simple放到了DecorView里面
mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
//这里的ID_ANDROID_CONTENT就是R.id.content;
ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
if (contentParent == null) {
throw new RuntimeException("Window couldn't find content container view");
}
...
//这里的getContainer()返回的是个Window类，也就是父Window，一般为空
if (getContainer() == null) {
final Drawable background;
if (mBackgroundResource != 0) {
background = getContext().getDrawable(mBackgroundResource);
} else {
background = mBackgroundDrawable;
}
//设置背景
mDecor.setWindowBackground(background);
final Drawable frame;
if (mFrameResource != 0) {
frame = getContext().getDrawable(mFrameResource);
} else {
frame = null;
}
mDecor.setWindowFrame(frame);
mDecor.setElevation(mElevation);
mDecor.setClipToOutline(mClipToOutline);
if (mTitle != null) {
setTitle(mTitle);
}
if (mTitleColor == 0) {
mTitleColor = mTextColor;
}
setTitleColor(mTitleColor);
}
mDecor.finishChanging();
return contentParent;
}可以看到根据不同主题属性使用的不同的布局，然后返回了这个布局 contentParent 。
我们来看看这个screen_simple.xml布局是什么样子的
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true"
android:orientation="vertical">
<ViewStub android:id="@+id/action_mode_bar_stub"
android:inflatedId="@+id/action_mode_bar"
android:layout="@layout/action_mode_bar"
android:layout_width="match_parent"
android:layout_height="wrap_content" />
<FrameLayout
android:id="@android:id/content"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:foregroundInsidePadding="false"
android:foregroundGravity="fill_horizontal|top"
android:foreground="?android:attr/windowContentOverlay" />
</LinearLayout>咦，这个布局结构跟 subDecor 好相似啊。。
好了，到目前为止我们知道了，当我们调用 mWindow.getDecorView(); 的时候里面创建DecorView，然后又根据不同主题属性添加不同布局放到DecorView下，然后找到这个布局的 R.id.content ，也就是 mContentParent 。ok，搞清楚 mWindow.getDecorView(); 之后，我们在来看看 mWindow.setContentView(subDecor); （注意：此时把subDecor传入进去）
@Override
public void setContentView(View view, ViewGroup.LayoutParams params) {
//在mWindow.getDecorView()已经创建了mContentParent
if (mContentParent == null) {
installDecor();
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}
//是否有transitions动画。没有，进入else
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
view.setLayoutParams(params);
final Scene newScene = new Scene(mContentParent, view);
transitionTo(newScene);
} else {
//重要！！将这个subDecor也就是FitWindowsLinearLayout添加到这个mContentParent里面了
//mContentParent是FrameLayout，在之前设置的View.NO_ID
mContentParent.addView(view, params);
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
mContentParentExplicitlySet = true;
}当调用了 mWindow.getDecorView(); 创建了DecorView以及 mContentParent ,并且把 subDecor 放到了 mContentParent 里面。我们再来回头看看 AppCompatDelegateImplV9 ，还记得它吗？当我们在 AppCompatActivity 的 setContentView 的时候会去调用 AppCompatDelegateImplV9 的 setContentView
AppCompatDelegateImplV9.java
@Override
public void setContentView(View v) {
//此时DecorView和subDecor都创建好了
ensureSubDecor();
//还记得调用createSubDecor的时候把原本是R.id.content的windowContentView设置成了NO_ID
//并且将contentView也就是ContentFrameLayout设置成了R.id.content吗？
//也就是说此时的contentParent就是ContentFrameLayout
ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
contentParent.removeAllViews();
//将我的布局放到contentParent里面
contentParent.addView(v);
mOriginalWindowCallback.onContentChanged();
}
@Override
public void setContentView(int resId) {
ensureSubDecor();
ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
contentParent.removeAllViews();
//将我们的布局id映射成View并且放到contentParent下
LayoutInflater.from(mContext).inflate(resId, contentParent);
mOriginalWindowCallback.onContentChanged();
}
@Override
public void setContentView(View v, ViewGroup.LayoutParams lp) {
ensureSubDecor();
ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
contentParent.removeAllViews();
contentParent.addView(v, lp);
mOriginalWindowCallback.onContentChanged();
}
最后，网络的资源非常充足，感谢给我提供资源的小伙伴们
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