Android中Window添加View的底层原理

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<uses-permission andorid:name="Android.permission.SYSTEM_ALERT_WINDOW"/>。
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。

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