SurfaceView源码分析

SV继承自View，但是并不是利用View的Draw来进行绘制。而是直接跟SurfaceFlinger进行通信。那么看下具体SV是怎么实现这个功能的，它的用法如何？
还是先从构造方法开始看起。

  public SurfaceView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {super(context, attrs, defStyleAttr, defStyleRes);//Enable callbacks for position changes. mRenderNode.requestPositionUpdates(this);//此方法规定不会走正常的draw方法。setWillNotDraw(true);}

假设我们直接贴一个SurfaceView 到布局中，给SV一定宽高，SV会形成一个"黑洞"区域。如图。

现在来分析下这是如何形成的。
由于SV也是一个View，只是绘制的时候会走自己的绘制机制。对于常规的onMesure和onLayout还是一样会跑的。这个也是SV得以能够进行绘制的前提条件，因为你必须首先有大小，然后有位置，其次才是在具体位置利用画笔（paint）进行绘制的。那么先来看看onMeasure。

@Overrideprotected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {//mRequestedWidth 由setFixedSize设置，默认情况是-1。那么默认是走getDefaultSize.也就是基本的测量方法。int width = mRequestedWidth >= 0? resolveSizeAndState(mRequestedWidth, widthMeasureSpec, 0): getDefaultSize(0, widthMeasureSpec);int height = mRequestedHeight >= 0? resolveSizeAndState(mRequestedHeight, heightMeasureSpec, 0): getDefaultSize(0, heightMeasureSpec);setMeasuredDimension(width, height);}

所以onMeasure可以说平平无奇。
接下来看下布局。发现SV并没有重写这个onLayout方法。但是有重写setFrame方法，这个方法是View中，layout过程调用的。所以布局的时候必然会走到这个setFrame方法。

/** @hide */
@Override
protected boolean setFrame(int left, int top, int right, int bottom) {boolean result = super.setFrame(left, top, right, bottom);updateSurface();return result;
}

这里面的关键方法就是updateSurface方法.

    /** @hide */protected void updateSurface() {//判断是否有长宽。经历了measure之后mHaveFrame为true.if (!mHaveFrame) {return;}//获取View 根。ViewRootImpl viewRoot = getViewRootImpl();//主要看最后一个就是说View的mSurface是否准备完毕。也就是说，必须ViewTree的surface准备完毕才会走到后面的方法。正常流程经历了onResume，surface准备完毕，才会发起requestlayout，才会有布局的。if (viewRoot == null || viewRoot.mSurface == null || !viewRoot.mSurface.isValid()) {return;}// mTranslator 属于一个转换器。   mTranslator = viewRoot.mTranslator;if (mTranslator != null) {mSurface.setCompatibilityTranslator(mTranslator);}int myWidth = mRequestedWidth;if (myWidth <= 0) myWidth = getWidth();int myHeight = mRequestedHeight;if (myHeight <= 0) myHeight = getHeight();final boolean formatChanged = mFormat != mRequestedFormat;final boolean visibleChanged = mVisible != mRequestedVisible;//判断Surface是否正在创建状态，但是没有创建完毕。从这里也可以看到SF的surface和ViewRoot的surfcace并不是同一个。final boolean creating = (mSurfaceControl == null || formatChanged || visibleChanged)&& mRequestedVisible;final boolean sizeChanged = mSurfaceWidth != myWidth || mSurfaceHeight != myHeight;final boolean windowVisibleChanged = mWindowVisibility != mLastWindowVisibility;boolean redrawNeeded = false;if (creating || formatChanged || sizeChanged || visibleChanged || windowVisibleChanged) {//获取sf控件在其整个屏幕上的坐标位置，并且存储在mLocation中。getLocationInWindow(mLocation);try {final boolean visible = mVisible = mRequestedVisible;mWindowSpaceLeft = mLocation[0];mWindowSpaceTop = mLocation[1];mSurfaceWidth = myWidth;mSurfaceHeight = myHeight;mFormat = mRequestedFormat;mLastWindowVisibility = mWindowVisibility;mScreenRect.left = mWindowSpaceLeft;mScreenRect.top = mWindowSpaceTop;mScreenRect.right = mWindowSpaceLeft + getWidth();mScreenRect.bottom = mWindowSpaceTop + getHeight();if (mTranslator != null) {mTranslator.translateRectInAppWindowToScreen(mScreenRect);}final Rect surfaceInsets = getParentSurfaceInsets();mScreenRect.offset(surfaceInsets.left, surfaceInsets.top);if (creating) {//通过ViewRoot的surface创建mSurfaceSession，这个变量用于根SurfaceFlinger进行交互。mSurfaceSession = new SurfaceSession(viewRoot.mSurface);mDeferredDestroySurfaceControl = mSurfaceControl;updateOpaqueFlag();//mSurfaceControl 从字面来看就是Surface的管理类，里面主要都是native方法。mSurfaceControl = new SurfaceControlWithBackground(mSurfaceSession,"SurfaceView - " + viewRoot.getTitle().toString(),mSurfaceWidth, mSurfaceHeight, mFormat,mSurfaceFlags);} else if (mSurfaceControl == null) {return;}boolean realSizeChanged = false;mSurfaceLock.lock();try {mDrawingStopped = !visible;if (DEBUG) Log.i(TAG, System.identityHashCode(this) + " "+ "Cur surface: " + mSurface);//SC开启一次事务。SurfaceControl.openTransaction();try {//mSublayout属于一个type。SV的类型为APPLICATION_MEDIA_SUBLAYERmSurfaceControl.setLayer(mSubLayer);if (mViewVisibility) {mSurfaceControl.show();} else {mSurfaceControl.hide();}//设置Surface的位置。矩阵。if (sizeChanged || creating || !mRtHandlingPositionUpdates) {mSurfaceControl.setPosition(mScreenRect.left, mScreenRect.top);mSurfaceControl.setMatrix(mScreenRect.width() / (float) mSurfaceWidth,0.0f, 0.0f,mScreenRect.height() / (float) mSurfaceHeight);}if (sizeChanged) {mSurfaceControl.setSize(mSurfaceWidth, mSurfaceHeight);}} finally {SurfaceControl.closeTransaction();}if (sizeChanged || creating) {redrawNeeded = true;}mSurfaceFrame.left = 0;mSurfaceFrame.top = 0;if (mTranslator == null) {mSurfaceFrame.right = mSurfaceWidth;mSurfaceFrame.bottom = mSurfaceHeight;} else {float appInvertedScale = mTranslator.applicationInvertedScale;mSurfaceFrame.right = (int) (mSurfaceWidth * appInvertedScale + 0.5f);mSurfaceFrame.bottom = (int) (mSurfaceHeight * appInvertedScale + 0.5f);}final int surfaceWidth = mSurfaceFrame.right;final int surfaceHeight = mSurfaceFrame.bottom;realSizeChanged = mLastSurfaceWidth != surfaceWidth|| mLastSurfaceHeight != surfaceHeight;mLastSurfaceWidth = surfaceWidth;mLastSurfaceHeight = surfaceHeight;} finally {mSurfaceLock.unlock();}try {redrawNeeded |= visible && !mDrawFinished;SurfaceHolder.Callback callbacks[] = null;final boolean surfaceChanged = creating;if (mSurfaceCreated && (surfaceChanged || (!visible && visibleChanged))) {mSurfaceCreated = false;if (mSurface.isValid()) {//需要重建mSurfacecallbacks = getSurfaceCallbacks();for (SurfaceHolder.Callback c : callbacks) {c.surfaceDestroyed(mSurfaceHolder);}if (mSurface.isValid()) {mSurface.forceScopedDisconnect();}}}//如果正在创建，直接copyFromif (creating) {mSurface.copyFrom(mSurfaceControl);}//如果size改变。并且考虑兼容性，需要直接createFromif (sizeChanged && getContext().getApplicationInfo().targetSdkVersion< Build.VERSION_CODES.O) {mSurface.createFrom(mSurfaceControl);}//此时mSurface已经可以用了。可以愉快的进行lockCanvasif (visible && mSurface.isValid()) {if (!mSurfaceCreated && (surfaceChanged || visibleChanged)) {mSurfaceCreated = true;mIsCreating = true;if (callbacks == null) {callbacks = getSurfaceCallbacks();}//此处就是回调callback，我们自定义SV可以在这个方法里面拿到surface。for (SurfaceHolder.Callback c : callbacks) {c.surfaceCreated(mSurfaceHolder);}}//size 改变的情形。if (creating || formatChanged || sizeChanged|| visibleChanged || realSizeChanged) {if (DEBUG) Log.i(TAG, System.identityHashCode(this) + " "+ "surfaceChanged -- format=" + mFormat+ " w=" + myWidth + " h=" + myHeight);if (callbacks == null) {callbacks = getSurfaceCallbacks();}for (SurfaceHolder.Callback c : callbacks) {c.surfaceChanged(mSurfaceHolder, mFormat, myWidth, myHeight);}}//需要重新绘制的情形。if (redrawNeeded) {if (DEBUG) Log.i(TAG, System.identityHashCode(this) + " "+ "surfaceRedrawNeeded");if (callbacks == null) {callbacks = getSurfaceCallbacks();}mPendingReportDraws++;viewRoot.drawPending();SurfaceCallbackHelper sch =new SurfaceCallbackHelper(this::onDrawFinished);sch.dispatchSurfaceRedrawNeededAsync(mSurfaceHolder, callbacks);}}} finally {mIsCreating = false;//如果到这里还是没有创建成功，那么需要释放mSurface。if (mSurfaceControl != null && !mSurfaceCreated) {mSurface.release();// If we are not in the stopped state, then the destruction of the Surface// represents a visual change we need to display, and we should go ahead// and destroy the SurfaceControl. However if we are in the stopped state,// we can just leave the Surface around so it can be a part of animations,// and we let the life-time be tied to the parent surface.if (!mWindowStopped) {mSurfaceControl.destroy();mSurfaceControl = null;}}}} catch (Exception ex) {Log.e(TAG, "Exception configuring surface", ex);}} else {//此处else，是针对surface非初始化创建的情形，也就是surface可利用的情形。getLocationInSurface(mLocation);final boolean positionChanged = mWindowSpaceLeft != mLocation[0]|| mWindowSpaceTop != mLocation[1];final boolean layoutSizeChanged = getWidth() != mScreenRect.width()|| getHeight() != mScreenRect.height();if (positionChanged || layoutSizeChanged) { // Only the position has changedmWindowSpaceLeft = mLocation[0];mWindowSpaceTop = mLocation[1];// For our size changed check, we keep mScreenRect.width() and mScreenRect.height()// in view local space.mLocation[0] = getWidth();mLocation[1] = getHeight();mScreenRect.set(mWindowSpaceLeft, mWindowSpaceTop,mWindowSpaceLeft + mLocation[0], mWindowSpaceTop + mLocation[1]);if (mTranslator != null) {mTranslator.translateRectInAppWindowToScreen(mScreenRect);}if (mSurfaceControl == null) {return;}if (!isHardwareAccelerated() || !mRtHandlingPositionUpdates) {try {setParentSpaceRectangle(mScreenRect, -1);} catch (Exception ex) {Log.e(TAG, "Exception configuring surface", ex);}}}}}

通过这个方法，surface已经创建完毕，并且位置和大小也确定了，所以会留下一个黑洞。如果想要绘制内容，需要实现 SurfaceHolder.Callback。下面是个demo，结合我上面的分析，能够对SV的绘制机制有更深入了解。

public class SurfaceViewDemo extends SurfaceView implements SurfaceHolder.Callback{private SurfaceHolder mSurfaceHolder;private Canvas mCanvas;private Paint paint;public SurfaceViewDemo(Context context) {this(context,null,0);}public SurfaceViewDemo(Context context, AttributeSet attrs) {this(context, attrs,0);}public SurfaceViewDemo(Context context, AttributeSet attrs, int defStyleAttr) {super(context, attrs, defStyleAttr);init();}private void init() {mSurfaceHolder = getHolder();mSurfaceHolder.addCallback(this);setFocusable(true);setFocusableInTouchMode(true);this.setKeepScreenOn(true);paint = new Paint(Paint.ANTI_ALIAS_FLAG);paint.setColor(Color.RED);paint.setStrokeWidth(5);paint.setStyle(Paint.Style.STROKE);}@Overridepublic void surfaceCreated(SurfaceHolder holder) {System.out.println("=========surfaceCreated========");new Thread(new Runnable() {@Overridepublic void run() {draw();}}).start();}private void draw() {try {System.out.println("============draw========");mCanvas = mSurfaceHolder.lockCanvas();mCanvas.drawCircle(500,500,300,paint);mCanvas.drawCircle(100,100,20,paint);} catch (Exception e) {e.printStackTrace();} finally {if (mCanvas != null)mSurfaceHolder.unlockCanvasAndPost(mCanvas);}}@Overridepublic void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {System.out.println("=========surfaceChanged========");}@Overridepublic void surfaceDestroyed(SurfaceHolder holder) {System.out.println("=========surfaceDestroyed========");}
}

通过对SurfaceView机制的分析，可以得知，实际上SV是自己独占一个Surface。好处就是摆脱了View自带的绘制机制，比如说主线程绘制，必须是VSYNC 才能绘制等缺点。

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