Android中Window添加View的底层原理
最后更新于：2022-04-01 14:26:37
### 一，WIndow和windowManager
Window是一个抽象类，它的具体实现是PhoneWindow，创建一个window很简单，只需要创建一个windowManager即可，window具体实现在windowManagerService中，windowManager和windowManagerService的交互是一个IPC的过程。
下面是用windowManager的例子:
~~~
mFloatingButton = new Button(this);
mFloatingButton.setText( "window");
mLayoutParams = new WindowManager.LayoutParams(
LayoutParams. WRAP_CONTENT, LayoutParams.WRAP_CONTENT, 0, 0,
PixelFormat. TRANSPARENT);
mLayoutParams. flags = LayoutParams.FLAG_NOT_TOUCH_MODAL
| LayoutParams. FLAG_NOT_FOCUSABLE
| LayoutParams. FLAG_SHOW_WHEN_LOCKED;
mLayoutParams. type = LayoutParams. TYPE_SYSTEM_ERROR;
mLayoutParams. gravity = Gravity. LEFT | Gravity. TOP;
mLayoutParams. x = 100;
mLayoutParams. y = 300;
mFloatingButton.setOnTouchListener( this);
mWindowManager.addView( mFloatingButton, mLayoutParams);
~~~
flags和type两个属性很重要，下面对一些属性进行介绍，首先是flags：
FLAG_NOT_TOUCH_MODAL表示不需要获取焦点，也不需要接收各种输入，最终事件直接传递给下层具有焦点的window。
FLAG_NOT_FOCUSABLE：在此window外的区域单击事件传递到底层window中。当前的区域则自己处理，这个一般都要设置，很重要。
FLAG_SHOW_WHEN_LOCKED ：开启可以让window显示在锁屏界面上。
再来看下type这个参数：
window有三种类型：应用window，子window，系统window。应用类对应一个Activity，子Window不能单独存在，需要附属在父Window上，比如常用的Dialog。系统Window是需要声明权限再创建的window，如toast等。
window有z-ordered属性，层级越大，越在顶层。应用window层级1-99，子window1000-1999，系统2000-2999。这此层级对应着windowManager的type参数。系统层级常用的有两个TYPE_SYSTEM_OVERLAY或者TYPE_SYSTEM_ERROR。比如想用TYPE_SYSTEM_ERROR，只需
mLayoutParams.type = LayoutParams.TYPE_SYSTEM_ERROR。还要添加权限。
有了对window的基本认识之后，我们来看下它底层如何实现加载View的。
### 二，window的创建。
其实Window的创建跟之前我写的一篇博客[LayoutInflater源码分析](http://blog.csdn.net/u014486880/article/details/50707672)有点相似。Window的创建是在Activity创建的attach方法中，通过PolicyManager的makeNewWindow方法。Activity中实现了Window的Callback接口，因此当window状态改变时就会回调Activity方法。如onAttachedToWindow等。PolicyManager的真正实现类是Policy，看下它的代码：
~~~
public Window makeNewWindow(Context context) {
return new PhoneWindow(context);
}
~~~
到此Window创建完成。
下面分析view是如何附属到window上的。看Activity的setContentView方法。
~~~
public void setContentView(int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}
~~~
两部分，设置内容和设置ActionBar。window的具体实现是PhoneWindow，看它的setContent。
~~~
public void setContentView(int layoutResID) {
// Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
// decor, when theme attributes and the like are crystalized. Do not check the feature
// before this happens.
if (mContentParent == null) {
installDecor();
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
getContext());
transitionTo(newScene);
} else {
mLayoutInflater.inflate(layoutResID, mContentParent);
}
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
}
~~~
看到了吧，又是分析它。
这里分三步执行：
1.如果没有DecorView，在installDecor中的generateDecor()创建DecorView。之前就分析过，这次就不再分析它了。
2.将View添加到decorview中的mContentParent中。
3.回调Activity的onContentChanged接口。
经过以上操作，DecorView创建了，但还没有正式添加到Window中。在ActivityResumeActivity中首先会调用Activity的onResume，再调用Activity的makeVisible，makeVisible中真正添加view ，代码如下：
~~~
void makeVisible() {
if (!mWindowAdded) {
ViewManager wm = getWindowManager();
wm.addView(mDecor, getWindow().getAttributes());
mWindowAdded = true;
}
mDecor.setVisibility(View.VISIBLE);
}
~~~
通过上面的addView方法将View添加到Window。
### 三，Window操作View内部机制
## 1.window的添加
一个window对应一个view和一个viewRootImpl,window和view通过ViewRootImpl来建立联系，它并不存在，实体是view。只能通过 windowManager来操作它。
windowManager的实现类是windowManagerImpl。它并没有直接实现三大操作，而是委托给WindowManagerGlobal。addView的实现分为以下几步：
1.检查参数是否合法。
~~~
if (view == null) {
throw new IllegalArgumentException("view must not be null");
}
if (display == null) {
throw new IllegalArgumentException("display must not be null");
}
if (!(params instanceof WindowManager.LayoutParams)) {
throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");
}
final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams)params;
if (parentWindow != null) {
parentWindow.adjustLayoutParamsForSubWindow(wparams);
} else {
// If there's no parent and we're running on L or above (or in the
// system context), assume we want hardware acceleration.
final Context context = view.getContext();
if (context != null
&& context.getApplicationInfo().targetSdkVersion >= Build.VERSION_CODES.LOLLIPOP) {
wparams.flags |= WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED;
}
}
~~~
2.创建ViewRootImpl并将View添加到列表中。
~~~
root = new ViewRootImpl(view.getContext(), display);
view.setLayoutParams(wparams);
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
~~~
3.通过ViewRootImpl来更新界面并完成window的添加过程 。
~~~
root.setView(view, wparams, panelParentView);
~~~
上面的root就是ViewRootImpl，setView中通过requestLayout()来完成异步刷新，看下requestLayout：
~~~
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
~~~
接下来通过WindowSession来完成window添加过程，WindowSession是一个Binder对象，真正的实现类是 Session，window的添加是一次IPC调用。
~~~
try {
mOrigWindowType = mWindowAttributes.type;
mAttachInfo.mRecomputeGlobalAttributes = true;
collectViewAttributes();
res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
getHostVisibility(), mDisplay.getDisplayId(),
mAttachInfo.mContentInsets, mAttachInfo.mStableInsets, mInputChannel);
} catch (RemoteException e) {
mAdded = false;
mView = null;
mAttachInfo.mRootView = null;
mInputChannel = null;
mFallbackEventHandler.setView(null);
unscheduleTraversals();
setAccessibilityFocus(null, null);
throw new RuntimeException("Adding window failed", e);
}
~~~
在Session内部会通过WindowManagerService来实现Window的添加。
~~~
public int addToDisplay(IWindow window, int seq, WindowManager.LayoutParams attrs,
int viewVisibility, int displayId, Rect outContentInsets, Rect outStableInsets,
InputChannel outInputChannel) {
return mService.addWindow(this, window, seq, attrs, viewVisibility, displayId,
outContentInsets, outStableInsets, outInputChannel);
}
~~~
在WindowManagerService内部会为每一个应用保留一个单独的session。
## 2.window的删除
看下WindowManagerGlobal的removeView：
~~~
public void removeView(View view, boolean immediate) {
if (view == null) {
throw new IllegalArgumentException("view must not be null");
}
synchronized (mLock) {
int index = findViewLocked(view, true);
View curView = mRoots.get(index).getView();
removeViewLocked(index, immediate);
if (curView == view) {
return;
}
throw new IllegalStateException("Calling with view " + view
+ " but the ViewAncestor is attached to " + curView);
}
}
~~~
首先调用findViewLocked来查找删除view的索引，这个过程就是建立数组遍历。然后再调用removeViewLocked来做进一步的删除。
~~~
private void removeViewLocked(int index, boolean immediate) {
ViewRootImpl root = mRoots.get(index);
View view = root.getView();
if (view != null) {
InputMethodManager imm = InputMethodManager.getInstance();
if (imm != null) {
imm.windowDismissed(mViews.get(index).getWindowToken());
}
}
boolean deferred = root.die(immediate);
if (view != null) {
view.assignParent(null);
if (deferred) {
mDyingViews.add(view);
}
}
}
~~~
真正删除操作是viewRootImpl来完成的。windowManager提供了两种删除接口，removeViewImmediate，removeView。它们分别表示异步删除和同步删除。具体的删除操作由ViewRootImpl的die来完成。
~~~
boolean die(boolean immediate) {
// Make sure we do execute immediately if we are in the middle of a traversal or the damage
// done by dispatchDetachedFromWindow will cause havoc on return.
if (immediate && !mIsInTraversal) {
doDie();
return false;
}
if (!mIsDrawing) {
destroyHardwareRenderer();
} else {
Log.e(TAG, "Attempting to destroy the window while drawing!\n" +
" window=" + this + ", title=" + mWindowAttributes.getTitle());
}
mHandler.sendEmptyMessage(MSG_DIE);
return true;
}
~~~
由上可知如果是removeViewImmediate，立即调用doDie，如果是removeView，用handler发送消息，ViewRootImpl中的Handler会处理消息并调用doDie。重点看下doDie:
~~~
void doDie() {
checkThread();
if (LOCAL_LOGV) Log.v(TAG, "DIE in " + this + " of " + mSurface);
synchronized (this) {
if (mRemoved) {
return;
}
mRemoved = true;
if (mAdded) {
dispatchDetachedFromWindow();
}
if (mAdded && !mFirst) {
destroyHardwareRenderer();
if (mView != null) {
int viewVisibility = mView.getVisibility();
boolean viewVisibilityChanged = mViewVisibility != viewVisibility;
if (mWindowAttributesChanged || viewVisibilityChanged) {
// If layout params have been changed, first give them
// to the window manager to make sure it has the correct
// animation info.
try {
if ((relayoutWindow(mWindowAttributes, viewVisibility, false)
& WindowManagerGlobal.RELAYOUT_RES_FIRST_TIME) != 0) {
mWindowSession.finishDrawing(mWindow);
}
} catch (RemoteException e) {
}
}
mSurface.release();
}
}
mAdded = false;
}
WindowManagerGlobal.getInstance().doRemoveView(this);
}
~~~
主要做四件事:
1.垃圾回收相关工作，比如清数据，回调等。
2.通过Session的remove方法删除Window,最终调用WindowManagerService的removeWindow
3.调用dispathDetachedFromWindow，在内部会调用onDetachedFromWindow()和onDetachedFromWindowInternal()。当view移除时会调用onDetachedFromWindow，它用于作一些资源回收。
4.通过doRemoveView刷新数据，删除相关数据，如在mRoot，mDyingViews中删除对象等。
~~~
void doRemoveView(ViewRootImpl root) {
synchronized (mLock) {
final int index = mRoots.indexOf(root);
if (index >= 0) {
mRoots.remove(index);
mParams.remove(index);
final View view = mViews.remove(index);
mDyingViews.remove(view);
}
}
if (HardwareRenderer.sTrimForeground && HardwareRenderer.isAvailable()) {
doTrimForeground();
}
}
~~~
## 3.更新window
看下WindowManagerGlobal中的updateViewLayout。
~~~
public void updateViewLayout(View view, ViewGroup.LayoutParams params) {
if (view == null) {
throw new IllegalArgumentException("view must not be null");
}
if (!(params instanceof WindowManager.LayoutParams)) {
throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");
}
final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams)params;
view.setLayoutParams(wparams);
synchronized (mLock) {
int index = findViewLocked(view, true);
ViewRootImpl root = mRoots.get(index);
mParams.remove(index);
mParams.add(index, wparams);
root.setLayoutParams(wparams, false);
}
}
~~~
通过viewRootImpl的setLayoutParams更新viewRootImpl的layoutParams,接着scheduleTraversals对view重新布局，包括测量，布局，重绘，此外它还会通过WindowSession来更新window。这个过程由WindowManagerService实现。这跟上面类似，就不再重复。到此Window底层源码就分析完啦。