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View绘制的三部曲，测量，布局，绘画
现在我们分析绘画部分
测量和布局 在前两篇文章中已经分析过了。不了解的可以去我的博客里找一下

下面进入正题，开始分析调用以及函数原理

private void performDraw() { if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) { return; } final boolean fullRedrawNeeded = mFullRedrawNeeded; mFullRedrawNeeded = false; mIsDrawing = true; Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw"); try { //调用内部实现方法，来实现分发绘画的工作 draw(fullRedrawNeeded); } finally { mIsDrawing = false; Trace.traceEnd(Trace.TRACE_TAG_VIEW); } //... }

这个函数调用内部draw方法去处理绘画前的工作,来继续完成绘制工作

private void draw(boolean fullRedrawNeeded) { Surface surface = mSurface; if (!surface.isValid()) { return; } if (DEBUG_FPS) { trackFPS(); } 　　(!sFirstDrawComplete) { synchronized (sFirstDrawHandlers) { sFirstDrawComplete = true; final int count = sFirstDrawHandlers.size(); for (int i = 0; i< count; i++) { mHandler.post(sFirstDrawHandlers.get(i)); } } } 　　//当界面需要滚动的时候，这个方法会触发Scroller类下的startScroll函数 scrollToRectOrFocus(null, false); 　　 (mAttachInfo.mViewScrollChanged) { mAttachInfo.mViewScrollChanged = false; mAttachInfo.mTreeObserver.dispatchOnScrollChanged(); } boolean animating = mScroller != null && mScroller.computeScrollOffset(); final int curScrollY; if (animating) { curScrollY = mScroller.getCurrY(); } else { curScrollY = mScrollY; } if (mCurScrollY != curScrollY) { mCurScrollY = curScrollY; fullRedrawNeeded = true; if (mView instanceof RootViewSurfaceTaker) { ((RootViewSurfaceTaker) mView).onRootViewScrollYChanged(mCurScrollY); } } final float appScale = mAttachInfo.mApplicationScale; final boolean scalingRequired = mAttachInfo.mScalingRequired; int resizeAlpha = 0; if (mResizeBuffer != null) { long deltaTime = SystemClock.uptimeMillis() - mResizeBufferStartTime; if (deltaTime < mResizeBufferDuration) { float amt = deltaTime/(float) mResizeBufferDuration; amt = mResizeInterpolator.getInterpolation(amt); animating = true; resizeAlpha = 255 - (int)(amt*255); } else { disposeResizeBuffer(); } } final Rect dirty = mDirty; if (mSurfaceHolder != null) { // The app owns the surface, we won‘t draw. dirty.setEmpty(); if (animating) { if (mScroller != null) { mScroller.abortAnimation(); } disposeResizeBuffer(); } return; } if (fullRedrawNeeded) { mAttachInfo.mIgnoreDirtyState = true; dirty.set(0, 0, (int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f)); } if (DEBUG_ORIENTATION || DEBUG_DRAW) { Log.v(TAG, "Draw " + mView + "http://mamicode.com/" + mWindowAttributes.getTitle() + ": dirty={" + dirty.left + "," + dirty.top + "," + dirty.right + "," + dirty.bottom + "} surface=" + surface + " surface.isValid()=" + surface.isValid() + ", appScale:" + appScale + ",,color: #000000"> mHeight); } 　　//如果有注册TreeObserver下的监听，在调用onDraw之前会触发 mAttachInfo.mTreeObserver.dispatchOnDraw(); int xOffset = 0; int yOffset = curScrollY; final WindowManager.LayoutParams params = mWindowAttributes; final Rect surfaceInsets = params != null ? params.surfaceInsets : null; if (surfaceInsets != null) { xOffset -= surfaceInsets.left; yOffset -= surfaceInsets.top; // Offset dirty rect for surface insets. dirty.offset(surfaceInsets.left, surfaceInsets.right); } boolean accessibilityFocusDirty = false; final Drawable drawable = mAttachInfo.mAccessibilityFocusDrawable; if (drawable != null) { final Rect bounds = mAttachInfo.mTmpInvalRect; final boolean hasFocus = getAccessibilityFocusedRect(bounds); if (!hasFocus) { bounds.setEmpty(); } if (!bounds.equals(drawable.getBounds())) { accessibilityFocusDirty = true; } } mAttachInfo.mDrawingTime = mChoreographer.getFrameTimeNanos() / TimeUtils.NANOS_PER_MS; if (!dirty.isEmpty() || mIsAnimating || accessibilityFocusDirty) { if (mAttachInfo.mHardwareRenderer != null && mAttachInfo.mHardwareRenderer.isEnabled()) { invalidateRoot = accessibilityFocusDirty; // Draw with hardware renderer. mIsAnimating = false; if (mHardwareYOffset != yOffset || mHardwareXOffset != xOffset) { mHardwareYOffset = yOffset; mHardwareXOffset = xOffset; invalidateRoot = true; } mResizeAlpha = resizeAlpha; if (invalidateRoot) { mAttachInfo.mHardwareRenderer.invalidateRoot(); } dirty.setEmpty(); mBlockResizeBuffer = false; mAttachInfo.mHardwareRenderer.draw(mView, mAttachInfo, this); } else { // If we get here with a disabled & requested hardware renderer, something went // wrong (an invalidate posted right before we destroyed the hardware surface // for instance) so we should just bail out. Locking the surface with software // rendering at this point would lock it forever and prevent hardware renderer // from doing its job when it comes back. // Before we request a new frame we must however attempt to reinitiliaze the // hardware renderer if it‘s in requested state. This would happen after an (mAttachInfo.mHardwareRenderer != null && !mAttachInfo.mHardwareRenderer.isEnabled() && mAttachInfo.mHardwareRenderer.isRequested()) { try { mAttachInfo.mHardwareRenderer.initializeIfNeeded( mWidth, mHeight, mAttachInfo, mSurface, surfaceInsets); } catch (OutOfResourcesException e) { handleOutOfResourcesException(e); return; } mFullRedrawNeeded = true; scheduleTraversals(); return; } 　　　　　(!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) { return; } } } if (animating) { mFullRedrawNeeded = true; scheduleTraversals(); } }

 这个函数在调用view的draw之前做了很多处理，大概总结一下就是view的滚动设置和硬件加速功能绘画还有一些比较深入的属性比如插图之类，这些我们不怎么关心
我们最关心的是 drawSoftWare 方法