Android中Window添加View的底層原理

一，WIndow和windowManager

Window是一個抽象類，它的具體實現是PhoneWindow，創建一個window很簡單，只需要創建一個windowManager即可，window具體實現在windowManagerService中，windowManager和windowManagerService的交互是一個IPC的過程。 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兩個屬性很重要，下面對一些屬性進行介紹，首先是flags： FLAG_NOT_TOUCH_MODAL表示不需要獲取焦點，也不需要接收各種輸入，最終事件直接傳遞給下層具有焦點的window。 FLAG_NOT_FOCUSABLE：在此window外的區域單擊事件傳遞到底層window中。當前的區域則自己處理，這個一般都要設置，很重要。 FLAG_SHOW_WHEN_LOCKED：開啟可以讓window顯示在鎖屏界面上。 再來看下type這個參數： window有三種類型：應用window，子window，系統window。應用類對應一個Activity，子Window不能單獨存在，需要附屬在父Window上，比如常用的Dialog。系統Window是需要聲明權限再創建的window，如toast等。 window有z-ordered屬性，層級越大，越在頂層。應用window層級1-99，子window1000-1999，系統2000-2999。這此層級對應著windowManager的type參數。系統層級常用的有兩個TYPE_SYSTEM_OVERLAY或者TYPE_SYSTEM_ERROR。比如想用TYPE_SYSTEM_ERROR，只需 mLayoutParams.type = LayoutParams.TYPE_SYSTEM_ERROR。還要添加權限。 有了對window的基本認識之後，我們來看下它底層如何實現加載View的。

二，window的創建。

其實Window的創建跟之前我寫的一篇博客LayoutInflater源碼分析有點相似。Window的創建是在Activity創建的attach方法中，通過PolicyManager的makeNewWindow方法。Activity中實現了Window的Callback接口，因此當window狀態改變時就會回調Activity方法。如onAttachedToWindow等。PolicyManager的真正實現類是Policy，看下它的代碼：

 public Window makeNewWindow(Context context) {
        return new PhoneWindow(context);
    }

到此Window創建完成。 下面分析view是如何附屬到window上的。看Activity的setContentView方法。

public void setContentView(int layoutResID) {
        getWindow().setContentView(layoutResID);
        initWindowDecorActionBar();
    }

兩部分，設置內容和設置ActionBar。window的具體實現是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。 這裡分三步執行： 1.如果沒有DecorView，在installDecor中的generateDecor()創建DecorView。之前就分析過，這次就不再分析它了。 2.將View添加到decorview中的mContentParent中。 3.回調Activity的onContentChanged接口。 經過以上操作，DecorView創建了，但還沒有正式添加到Window中。在ActivityResumeActivity中首先會調用Activity的onResume，再調用Activity的makeVisible，makeVisible中真正添加view ，代碼如下：

  void makeVisible() {
        if (!mWindowAdded) {
            ViewManager wm = getWindowManager();
            wm.addView(mDecor, getWindow().getAttributes());
            mWindowAdded = true;
        }
        mDecor.setVisibility(View.VISIBLE);
    }

通過上面的addView方法將View添加到Window。

三，Window操作View內部機制

1.window的添加

一個window對應一個view和一個viewRootImpl,window和view通過ViewRootImpl來建立聯系，它並不存在，實體是view。只能通過windowManager來操作它。 windowManager的實現類是windowManagerImpl。它並沒有直接實現三大操作，而是委托給WindowManagerGlobal。addView的實現分為以下幾步： 1.檢查參數是否合法。

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.創建ViewRootImpl並將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.通過ViewRootImpl來更新界面並完成window的添加過程 。

root.setView(view, wparams, panelParentView);

上面的root就是ViewRootImpl，setView中通過requestLayout()來完成異步刷新，看下requestLayout：

 public void requestLayout() {
        if (!mHandlingLayoutInLayoutRequest) {
            checkThread();
            mLayoutRequested = true;
            scheduleTraversals();
        }
    }

接下來通過WindowSession來完成window添加過程，WindowSession是一個Binder對象，真正的實現類是 Session，window的添加是一次IPC調用。

 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內部會通過WindowManagerService來實現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內部會為每一個應用保留一個單獨的session。

2.window的刪除

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);
        }
    }

首先調用findViewLocked來查找刪除view的索引，這個過程就是建立數組遍歷。然後再調用removeViewLocked來做進一步的刪除。

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);
            }
        }
    }

真正刪除操作是viewRootImpl來完成的。windowManager提供了兩種刪除接口，removeViewImmediate，removeView。它們分別表示異步刪除和同步刪除。具體的刪除操作由ViewRootImpl的die來完成。

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，立即調用doDie，如果是removeView，用handler發送消息，ViewRootImpl中的Handler會處理消息並調用doDie。重點看下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.垃圾回收相關工作，比如清數據，回調等。 2.通過Session的remove方法刪除Window,最終調用WindowManagerService的removeWindow   3.調用dispathDetachedFromWindow，在內部會調用onDetachedFromWindow()和onDetachedFromWindowInternal()。當view移除時會調用onDetachedFromWindow，它用於作一些資源回收。 4.通過doRemoveView刷新數據，刪除相關數據，如在mRoot，mDyingViews中刪除對象等。

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);
        }
    }

通過viewRootImpl的setLayoutParams更新viewRootImpl的layoutParams,接著scheduleTraversals對view重新布局，包括測量，布局，重繪，此外它還會通過WindowSession來更新window。這個過程由WindowManagerService實現。這跟上面類似，就不再重復。到此Window底層源碼就分析完啦。