View 的绘制系列文章：
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Android View 绘制流程之 DecorView 与 ViewRootImpl
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Android View 的绘制流程之 Measure 过程详解 (一)
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Android View 的绘制流程之 Layout 和 Draw 过程详解 (二)
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Android View 的事件分发原理解析
在上一篇 Android View 的绘制流程之 Measure 过程详解 (一)，已经详细的分析了 DecorView 和其子 View 的测量过程，接下去就要开始讲 layout 和 draw 流程。下面开始进入分析：
DecorView Layout 阶段
在 ViewRootImpl 中，调用 performLayout 方法来确定 DecorView 在屏幕中的位置，下面看下具体的代码逻辑：
// ViewRootImpl
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth, int desiredWindowHeight) { mLayoutRequested = false; mScrollMayChange = true; mInLayout = true; final View host = mView; if (host == null) { return; } if (DEBUG_ORIENTATION || DEBUG_LAYOUT) { Log.v(mTag, "Laying out " + host + " to (" + host.getMeasuredWidth() + ", " + host.getMeasuredHeight() + ")"); } Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout"); try {
// 根据测量结果进行绘制 host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); mInLayout = false; int numViewsRequestingLayout = mLayoutRequesters.size(); if (numViewsRequestingLayout > 0) { // requestLayout() was called during layout. // If no layout-request flags are set on the requesting views, there is no problem. // If some requests are still pending, then we need to clear those flags and do // a full request/measure/layout pass to handle this situation. ArrayList<View> validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, false); if (validLayoutRequesters != null) { // Set this flag to indicate that any further requests are happening during // the second pass, which may result in posting those requests to the next // frame instead mHandlingLayoutInLayoutRequest = true; // Process fresh layout requests, then measure and layout int numValidRequests = validLayoutRequesters.size(); for (int i = 0; i < numValidRequests; ++i) { final View view = validLayoutRequesters.get(i); Log.w("View", "requestLayout() improperly called by " + view + " during layout: running second layout pass"); view.requestLayout(); } measureHierarchy(host, lp, mView.getContext().getResources(), desiredWindowWidth, desiredWindowHeight); mInLayout = true; host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); mHandlingLayoutInLayoutRequest = false; // Check the valid requests again, this time without checking/clearing the // layout flags, since requests happening during the second pass get noop'd validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, true); if (validLayoutRequesters != null) { final ArrayList<View> finalRequesters = validLayoutRequesters; // Post second-pass requests to the next frame getRunQueue().post(new Runnable() { @Override public void run() { int numValidRequests = finalRequesters.size(); for (int i = 0; i < numValidRequests; ++i) { final View view = finalRequesters.get(i); Log.w("View", "requestLayout() improperly called by " + view + " during second layout pass: posting in next frame"); view.requestLayout(); } } }); } } } } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); } mInLayout = false; }
当在 layout 绘制过程中，收到了关于重新 layout 的请求，会先判断这些请求里面哪些是有效的，如果是有效的，那么就必须先处理，清除 layout-request flags （View.PFLAG_FORCE_LAYOUT）这些标记，再做一次彻底的重绘工作：重新测量，layout。
那么对于 DecorView 来说，调用 layout 方法，就是对它自身进行布局，注意到传递的参数分别是 0，0， host.getMeasuredWidth, host.getMeasuredHeigh，它们分别代表了一个 View 的上下左右四个位置，显然，DecorView 的左上位置为 0，然后宽高为它的测量宽高，下面来看 layout 的具体代码：
// ViewGroup public final void layout(int l, int t, int r, int b) { if (!mSuppressLayout && (mTransition == null || !mTransition.isChangingLayout())) { if (mTransition != null) { mTransition.layoutChange(this); } super.layout(l, t, r, b); } else { // record the fact that we noop'd it; request layout when transition finishes mLayoutCalledWhileSuppressed = true; } }
由于 ViewGroup 的 layout 方法是 final 类型，子类不能重写，这里调用了父类的 View#layout 方法，下面看看该方法是如何操作的：
// View
public void layout(int l, int t, int r, int b) { if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) { onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec); mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT; } int oldL = mLeft; int oldT = mTop; int oldB = mBottom; int oldR = mRight; // 设置界面的显示大小 boolean changed = isLayoutModeOptical(mParent) ? setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b); // 发生了改变，或者是需要重新 layout，那么就会进入 onLayout 逻辑 if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
// 开始 onLayout(changed, l, t, r, b); if (shouldDrawRoundScrollbar()) { if(mRoundScrollbarRenderer == null) { mRoundScrollbarRenderer = new RoundScrollbarRenderer(this); } } else { mRoundScrollbarRenderer = null; } // 清除请求 layout 的标记 mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED; ListenerInfo li = mListenerInfo; if (li != null && li.mOnLayoutChangeListeners != null) { ArrayList<OnLayoutChangeListener> listenersCopy = (ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone(); int numListeners = listenersCopy.size(); for (int i = 0; i < numListeners; ++i) { listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB); } } } final boolean wasLayoutValid = isLayoutValid(); mPrivateFlags &= ~PFLAG_FORCE_LAYOUT; mPrivateFlags3 |= PFLAG3_IS_LAID_OUT; if (!wasLayoutValid && isFocused()) { mPrivateFlags &= ~PFLAG_WANTS_FOCUS; if (canTakeFocus()) { // We have a robust focus, so parents should no longer be wanting focus. clearParentsWantFocus(); } else if (getViewRootImpl() == null || !getViewRootImpl().isInLayout()) { // This is a weird case. Most-likely the user, rather than ViewRootImpl, called // layout. In this case, there's no guarantee that parent layouts will be evaluated // and thus the safest action is to clear focus here. clearFocusInternal(null, /* propagate */ true, /* refocus */ false); clearParentsWantFocus(); } else if (!hasParentWantsFocus()) { // original requestFocus was likely on this view directly, so just clear focus clearFocusInternal(null, /* propagate */ true, /* refocus */ false); } // otherwise, we let parents handle re-assigning focus during their layout passes. } else if ((mPrivateFlags & PFLAG_WANTS_FOCUS) != 0) { mPrivateFlags &= ~PFLAG_WANTS_FOCUS; View focused = findFocus(); if (focused != null) { // Try to restore focus as close as possible to our starting focus. if (!restoreDefaultFocus() && !hasParentWantsFocus()) { // Give up and clear focus once we've reached the top-most parent which wants // focus. focused.clearFocusInternal(null, /* propagate */ true, /* refocus */ false); } } } if ((mPrivateFlags3 & PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT) != 0) { mPrivateFlags3 &= ~PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT; notifyEnterOrExitForAutoFillIfNeeded(true); } }
调用了 setFrame 方法，并把四个位置信息传递进去，这个方法用于确定 View 的四个顶点的位置，看下具体的代码：
protected boolean setFrame(int left, int top, int right, int bottom) { boolean changed = false; if (DBG) { Log.d(VIEW_LOG_TAG, this + " View.setFrame(" + left + "," + top + "," + right + "," + bottom + ")"); } // 有一个不一样说明发生了改变 if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) { changed = true; // Remember our drawn bit int drawn = mPrivateFlags & PFLAG_DRAWN; int oldWidth = mRight - mLeft; int oldHeight = mBottom - mTop; int newWidth = right - left; int newHeight = bottom - top; boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight); // Invalidate our old position invalidate(sizeChanged); mLeft = left; mTop = top; mRight = right; mBottom = bottom; mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom); // 做好标记 mPrivateFlags |= PFLAG_HAS_BOUNDS; // size 改变的回调 if (sizeChanged) { sizeChange(newWidth, newHeight, oldWidth, oldHeight); } if ((mViewFlags & VISIBILITY_MASK) == VISIBLE || mGhostView != null) { // If we are visible, force the DRAWN bit to on so that // this invalidate will go through (at least to our parent). // This is because someone may have invalidated this view // before this call to setFrame came in, thereby clearing // the DRAWN bit. mPrivateFlags |= PFLAG_DRAWN; invalidate(sizeChanged); // parent display list may need to be recreated based on a change in the bounds // of any child invalidateParentCaches(); } // Reset drawn bit to original value (invalidate turns it off) mPrivateFlags |= drawn; mBackgroundSizeChanged = true; mDefaultFocusHighlightSizeChanged = true; if (mForegroundInfo != null) { mForegroundInfo.mBoundsChanged = true; } notifySubtreeAccessibilityStateChangedIfNeeded(); } return changed; }
这里我们看到它对 mLeft、mTop、mRight、mBottom 这四个值进行了重新赋值，对于每一个View，包括 ViewGroup 来说，以上四个值保存了 Viwe 的位置信息，所以这四个值是最终宽高，也即是说，如果要得到 View 的位置信息，那么就应该在 layout 方法完成后调用 getLeft()、getTop() 等方法来取得最终宽高，如果是在此之前调用相应的方法，只能得到 0 的结果。当初始化完毕后，ViewGroup 的布局流程也就完成了。
赋值后，前后对比大小，如果发生了改变，就会调用了 sizeChange（）方法，最终会回调 onSizeChanged，标明 view 的尺寸发生了变化，第一次 laout 的时候也会调用。在自定义view 的时候，可以在这里获取控件的宽和高度。
private void sizeChange(int newWidth, int newHeight, int oldWidth, int oldHeight) { onSizeChanged(newWidth, newHeight, oldWidth, oldHeight); ...... }
子 View Layout 流程
当 DecorView 确定好了自己的位置之后，开始调用 onLayout 来确定子 view 的位置。对于 onLayout 方法，View 和 ViewGroup 类是空实现，接下来看 FrameLayout 的实现：
// FrameLayout protected void onLayout(boolean changed, int left, int top, int right, int bottom) { layoutChildren(left, top, right, bottom, false /* no force left gravity */); }
可以看到，该方法调用了 layoutChildren 来确定子 view 的位置。
// FrameLaout
void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) { final int count = getChildCount(); // 获取 DecoverView 剩余的空间范围 final int parentLeft = getPaddingLeftWithForeground(); final int parentRight = right - left - getPaddingRightWithForeground(); final int parentTop = getPaddingTopWithForeground(); final int parentBottom = bottom - top - getPaddingBottomWithForeground(); for (int i = 0; i < count; i++) { final View child = getChildAt(i); if (child.getVisibility() != GONE) { final LayoutParams lp = (LayoutParams) child.getLayoutParams(); final int width = child.getMeasuredWidth(); final int height = child.getMeasuredHeight(); int childLeft; int childTop;
// DEFAULT_CHILD_GRAVITY = Gravity.TOP | Gravity.START 默认是在左上角
int gravity = lp.gravity; if (gravity == -1) { gravity = DEFAULT_CHILD_GRAVITY; } final int layoutDirection = getLayoutDirection(); final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection); final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK; // 确定左边起始点 switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) { case Gravity.CENTER_HORIZONTAL: childLeft = parentLeft + (parentRight - parentLeft - width) / 2 + lp.leftMargin - lp.rightMargin; break; case Gravity.RIGHT: if (!forceLeftGravity) { childLeft = parentRight - width - lp.rightMargin; break; } case Gravity.LEFT: default: childLeft = parentLeft + lp.leftMargin; } // 确定上边起始点 switch (verticalGravity) { case Gravity.TOP: childTop = parentTop + lp.topMargin; break; case Gravity.CENTER_VERTICAL: childTop = parentTop + (parentBottom - parentTop - height) / 2 + lp.topMargin - lp.bottomMargin; break; case Gravity.BOTTOM: childTop = parentBottom - height - lp.bottomMargin; break; default: childTop = parentTop + lp.topMargin; } child.layout(childLeft, childTop, childLeft + width, childTop + height); } } }
先梳理一下以上逻辑：
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首先先获取父容器的 padding 值，得到 DecorView 的可用于显示的空间范围。
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然后遍历其每一个子 View，根据子 View 的 layout_gravity 属性、子 View 的测量宽高、父容器的 padding 值、来确定子 View 的左上角的坐标位置
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然后调用 child.layout 方法，参数是左上角坐标和自身宽高结合起来的，这样就可以确定子 View 的位置。
最终调用 layout 是 View#layout，前面已经分析过，就不在分析了。
可以看到子 View 的布局流程也很简单，如果子 View 是一个 ViewGroup，那么就会重复以上步骤，如果是一个 View，那么会直接调用 View#layout 方法，根据以上分析，在该方法内部会设置 view 的四个布局参数，接着调用 onLayout 方法 ：
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}
此方法是一个空方法，也就是说需要子类去实现此方法，不同的 View 实现方式不同，这里就不分析了。
layout阶段的基本思想也是由根View开始，递归地完成整个控件树的布局（layout）工作。
对于宽高的获取这里在总结下：
这里需要注意一下，在非一般情况下，也就是通过人为设置，重写View的layout()强行设置，这种情况下，测量的值与最终的值是不一样的。
Layout 整体的流程图
上面分别从 View 和 ViewGroup 的角度讲解了布局流程，这里再以流程图的形式归纳一下整个 Layout 过程，便于加深记忆：
DecorView Draw 流程
Draw 的入口也是在 ViewRootImpl 中，执行 ViewRootImpl#performTraversals 中会执行 ViewRootIml#performDraw：
private void performDraw() { ... //fullRedrawNeeded，它的作用是判断是否需要重新绘制全部视图 draw(fullRedrawNeeded); ... }
然后会执行到 ViewRootImpl#draw：
private void draw(boolean fullRedrawNeeded) { ... //获取mDirty，该值表示需要重绘的区域 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; } //如果fullRedrawNeeded为真，则把dirty区域置为整个屏幕，表示整个视图都需要绘制 //第一次绘制流程，需要绘制所有视图 if (fullRedrawNeeded) { mAttachInfo.mIgnoreDirtyState = true; dirty.set(0, 0, (int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f)); } ... if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) { return; } }
接着会执行到 ViewRootIml#drawSoftware，然后在 ViewRootIml#drawSoftware 会执行到 mView.draw(canvas)。
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff, boolean scalingRequired, Rect dirty) { final Canvas canvas; //锁定canvas区域，由dirty区域决定 //这个canvas就是我们想在上面绘制东西的画布 canvas = mSurface.lockCanvas(dirty); ... //画布支持位图的密度，和手机分辨率相关 canvas.setDensity(mDensity); ... if (!canvas.isOpaque() || yoff != 0 || xoff != 0) { canvas.drawColor(0, PorterDuff.Mode.CLEAR); } ... canvas.translate(-xoff, -yoff); ... //正式开始绘制 mView.draw(canvas); ... //提交需要绘制的东西 surface.unlockCanvasAndPost(canvas); }
mView.draw(canvas) 开始真正的绘制。此处 mView 就是 DecorView，先看 DecorView 中 Draw 的方法：
public void draw(Canvas canvas) { super.draw(canvas); if (mMenuBackground != null) { mMenuBackground.draw(canvas); } }
里面会调用 View#draw：
public void draw(Canvas canvas) { final int privateFlags = mPrivateFlags; final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE && (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState); mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN; /* * Draw traversal performs several drawing steps which must be executed * in the appropriate order: * * 1. Draw the background * 2. If necessary, save the canvas' layers to prepare for fading * 3. Draw view's content * 4. Draw children * 5. If necessary, draw the fading edges and restore layers * 6. Draw decorations (scrollbars for instance) */ // Step 1, draw the background, if needed int saveCount; //绘制背景 if (!dirtyOpaque) { drawBackground(canvas); } // 如果可以跳过2和5步 final int viewFlags = mViewFlags; //判断是否有绘制衰退边缘的标示 boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0; boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0; // 如果没有绘制衰退边缘只需要3,4,6步 if (!verticalEdges && !horizontalEdges) { // Step 3, draw the content if (!dirtyOpaque) onDraw(canvas); // Step 4, draw the children dispatchDraw(canvas); // Overlay is part of the content and draws beneath Foreground if (mOverlay != null && !mOverlay.isEmpty()) { mOverlay.getOverlayView().dispatchDraw(canvas); } // Step 6, draw decorations (foreground, scrollbars) onDrawForeground(canvas); // we're done... return; } /* * Here we do the full fledged routine... * (this is an uncommon case where speed matters less, * this is why we repeat some of the tests that have been * done above) */ boolean drawTop = false; boolean drawBottom = false; boolean drawLeft = false; boolean drawRight = false; float topFadeStrength = 0.0f; float bottomFadeStrength = 0.0f; float leftFadeStrength = 0.0f; float rightFadeStrength = 0.0f; // Step 2, save the canvas' layers int paddingLeft = mPaddingLeft; final boolean offsetRequired = isPaddingOffsetRequired(); if (offsetRequired) { paddingLeft += getLeftPaddingOffset(); } int left = mScrollX + paddingLeft; int right = left + mRight - mLeft - mPaddingRight - paddingLeft; int top = mScrollY + getFadeTop(offsetRequired); int bottom = top + getFadeHeight(offsetRequired); if (offsetRequired) { right += getRightPaddingOffset(); bottom += getBottomPaddingOffset(); } final ScrollabilityCache scrollabilityCache = mScrollCache; final float fadeHeight = scrollabilityCache.fadingEdgeLength; int length = (int) fadeHeight; // clip the fade length if top and bottom fades overlap // overlapping fades produce odd-looking artifacts if (verticalEdges && (top + length > bottom - length)) { length = (bottom - top) / 2; } // also clip horizontal fades if necessary if (horizontalEdges && (left + length > right - length)) { length = (right - left) / 2; } if (verticalEdges) { topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength())); drawTop = topFadeStrength * fadeHeight > 1.0f; bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength())); drawBottom = bottomFadeStrength * fadeHeight > 1.0f; } if (horizontalEdges) { leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength())); drawLeft = leftFadeStrength * fadeHeight > 1.0f; rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength())); drawRight = rightFadeStrength * fadeHeight > 1.0f; } saveCount = canvas.getSaveCount(); int solidColor = getSolidColor(); if (solidColor == 0) { final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG; if (drawTop) { canvas.saveLayer(left, top, right, top + length, null, flags); } if (drawBottom) { canvas.saveLayer(left, bottom - length, right, bottom, null, flags); } if (drawLeft) { canvas.saveLayer(left, top, left + length, bottom, null, flags); } if (drawRight) { canvas.saveLayer(right - length, top, right, bottom, null, flags); } } else { scrollabilityCache.setFadeColor(solidColor); } // Step 3, draw the content if (!dirtyOpaque) onDraw(canvas); // Step 4, draw the children dispatchDraw(canvas); // Step 5, draw the fade effect and restore layers final Paint p = scrollabilityCache.paint; final Matrix matrix = scrollabilityCache.matrix; final Shader fade = scrollabilityCache.shader; if (drawTop) { matrix.setScale(1, fadeHeight * topFadeStrength); matrix.postTranslate(left, top); fade.setLocalMatrix(matrix); p.setShader(fade); canvas.drawRect(left, top, right, top + length, p); } if (drawBottom) { matrix.setScale(1, fadeHeight * bottomFadeStrength); matrix.postRotate(180); matrix.postTranslate(left, bottom); fade.setLocalMatrix(matrix); p.setShader(fade); canvas.drawRect(left, bottom - length, right, bottom, p); } if (drawLeft) { matrix.setScale(1, fadeHeight * leftFadeStrength); matrix.postRotate(-90); matrix.postTranslate(left, top); fade.setLocalMatrix(matrix); p.setShader(fade); canvas.drawRect(left, top, left + length, bottom, p); } if (drawRight) { matrix.setScale(1, fadeHeight * rightFadeStrength); matrix.postRotate(90); matrix.postTranslate(right, top); fade.setLocalMatrix(matrix); p.setShader(fade); canvas.drawRect(right - length, top, right, bottom, p); } canvas.restoreToCount(saveCount); // Overlay is part of the content and draws beneath Foreground if (mOverlay != null && !mOverlay.isEmpty()) { mOverlay.getOverlayView().dispatchDraw(canvas); } // Step 6, draw decorations (foreground, scrollbars) onDrawForeground(canvas); }
可以看到，draw过程比较复杂，但是逻辑十分清晰，而官方注释也清楚地说明了每一步的做法。我们首先来看一开始的标记位 dirtyOpaque，该标记位的作用是判断当前 View 是否是透明的，如果 View 是透明的，那么根据下面的逻辑可以看出，将不会执行一些步骤，比如绘制背景、绘制内容等。这样很容易理解，因为一个 View 既然是透明的，那就没必要绘制它了。接着是绘制流程的六个步骤，这里先小结这六个步骤分别是什么，然后再展开来讲。绘制流程的六个步骤：
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对 View 的背景进行绘制
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保存当前的图层信息(可跳过)
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绘制 View 的内容
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对 View 的子 View 进行绘制(如果有子 View )
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绘制 View 的褪色的边缘，类似于阴影效果(可跳过)
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绘制 View 的装饰（例如：滚动条）
其中第2步和第5步是可以跳过的，我们这里不做分析，我们重点来分析其它步骤。
ViewGroup子类默认情况下就是不执行 onDraw 方法的，在 ViewGroup 源码中的 initViewGroup() 方法中设置了一个标记，源码如下：
private void initViewGroup() { // ViewGroup doesn't draw by default if (!debugDraw()) { setFlags(WILL_NOT_DRAW, DRAW_MASK); } ...... }
看第二行注释也知道，ViewGroup 默认情况下是不会 draw 的。第四行调用 setFlags 方法设置标记 WILL_NOT_DRAW，在回到 View 中 draw 方法看第2行代码：
1 final int privateFlags = mPrivateFlags; 2 final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE && 3 (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
setFlags 方法就是对 View中mPrivateFlags 值进行相应改变，我们设置标记 WILL_NOT_DRAW 那么 dirtyOpaque 得到的值就为 true，从而 if (!dirtyOpaque) 不成立，也就不会执行onDraw 方法。
1. 绘制背景
View#drawBackground
private void drawBackground(Canvas canvas) { //获取背景的Drawable，没有就不需要绘制 final Drawable background = mBackground; if (background == null) { return; } //确定背景Drawable边界 setBackgroundBounds(); ... //如果有偏移量先偏移画布再将drawable绘制上去 final int scrollX = mScrollX; final int scrollY = mScrollY; if ((scrollX | scrollY) == 0) { background.draw(canvas); } else { canvas.translate(scrollX, scrollY); //此处会执行各种Drawable对应的draw方法 background.draw(canvas); //把画布的原点移回去，drawable在屏幕上的位置不动 canvas.translate(-scrollX, -scrollY); } }
3. 绘制 View 的内容
先跳过第 2 步，是因为不是所有的 View 都需绘制褪色边缘。DecorView#onDraw：
public void onDraw(Canvas c) { super.onDraw(c); mBackgroundFallback.draw(this, mContentRoot, c, mWindow.mContentParent, mStatusColorViewState.view, mNavigationColorViewState.view); }
View#onDraw
protected void onDraw(Canvas canvas) {
}
onDraw 是空实现，需要子 View 自己去绘制。对于DecorView 一般也没啥内容，除了需要背景颜色等，所以本身并需要绘制啥。
4. 绘制子View
DecorView 绘制完成后，开始绘制子 View，所以 ViewGroup 的绘制需要绘制子 View，直接看看 ViewGroup#dispatchDraw：
protected void dispatchDraw(Canvas canvas) { boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode); final int childrenCount = mChildrenCount; final View[] children = mChildren; int flags = mGroupFlags; //ViewGroup是否有设置子View入场动画，如果有绑定到View // 启动动画控制器 ... //指定修改区域 int clipSaveCount = 0; final boolean clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;
// 不让子view绘制在pandding里面，也就是去除padding if (clipToPadding) { clipSaveCount = canvas.save(); canvas.clipRect(mScrollX + mPaddingLeft, mScrollY + mPaddingTop, mScrollX + mRight - mLeft - mPaddingRight, mScrollY + mBottom - mTop - mPaddingBottom); } ... for (int i = 0; i < childrenCount; i++) { //先取mTransientViews中的View，mTransientViews中的View通过addTransientView添加，它们只是容器渲染的一个item while (transientIndex >= 0 && mTransientIndices.get(transientIndex) == i) { final View transientChild = mTransientViews.get(transientIndex); if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE || transientChild.getAnimation() != null) { more |= drawChild(canvas, transientChild, drawingTime); } transientIndex++; if (transientIndex >= transientCount) { transientIndex = -1; } } int childIndex = customOrder ? getChildDrawingOrder(childrenCount, i) : i; final View child = (preorderedList == null) ? children[childIndex] : preorderedList.get(childIndex); if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) { more |= drawChild(canvas, child, drawingTime); } } ... }
ViewGroup#dispatchDraw 的流程是先启动第一次加到布局中的动画，然后确定绘制区域，遍历绘制 View，遍历 View 的时候优先绘制渲染的 mTransientViews，绘制 View 调用到ViewGroup#drawChild：
protected boolean drawChild(Canvas canvas, View child, long drawingTime) { //View中有两个draw方法 //这个多参数的draw用于view绘制自身内容 return child.draw(canvas, this, drawingTime); }
View#draw(canvas, this, drawingTime）
boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) { boolean drawingWithRenderNode = mAttachInfo != null && mAttachInfo.mHardwareAccelerated && hardwareAcceleratedCanvas; ... //主要判断是否有绘制缓存，如果有，直接使用缓存，如果没有，调用 draw(canvas)方法 if (!drawingWithDrawingCache) { if (drawingWithRenderNode) { mPrivateFlags &= ~PFLAG_DIRTY_MASK; ((DisplayListCanvas) canvas).drawRenderNode(renderNode); } else { // Fast path for layouts with no backgrounds if ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) { mPrivateFlags &= ~PFLAG_DIRTY_MASK; dispatchDraw(canvas); } else { draw(canvas); } } } else if (cache != null) { mPrivateFlags &= ~PFLAG_DIRTY_MASK; if (layerType == LAYER_TYPE_NONE) { // no layer paint, use temporary paint to draw bitmap Paint cachePaint = parent.mCachePaint; if (cachePaint == null) { cachePaint = new Paint(); cachePaint.setDither(false); parent.mCachePaint = cachePaint; } cachePaint.setAlpha((int) (alpha * 255)); canvas.drawBitmap(cache, 0.0f, 0.0f, cachePaint); } else { // use layer paint to draw the bitmap, merging the two alphas, but also restore int layerPaintAlpha = mLayerPaint.getAlpha(); mLayerPaint.setAlpha((int) (alpha * layerPaintAlpha)); canvas.drawBitmap(cache, 0.0f, 0.0f, mLayerPaint); mLayerPaint.setAlpha(layerPaintAlpha); } } }
首先判断是否已经有缓存，即之前是否已经绘制过一次了，如果没有，则会调用 draw(canvas) 方法，开始正常的绘制，即上面所说的六个步骤，否则利用缓存来显示。
这一步也可以归纳为 ViewGroup 绘制过程，它对子 View 进行了绘制，而子 View 又会调用自身的 draw 方法来绘制自身，这样不断遍历子 View 及子 View 的不断对自身的绘制，从而使得 View 树完成绘制。
对于自定义 View ，如果需要绘制东西的话，直接重新 onDraw 就可以了。
6. 绘制装饰
public void onDrawForeground(Canvas canvas) { //绘制滑动指示 onDrawScrollIndicators(canvas); //绘制ScrollBar onDrawScrollBars(canvas); //获取前景色的Drawable，绘制到canvas上 final Drawable foreground = mForegroundInfo != null ? mForegroundInfo.mDrawable : null; if (foreground != null) { if (mForegroundInfo.mBoundsChanged) { mForegroundInfo.mBoundsChanged = false; final Rect selfBounds = mForegroundInfo.mSelfBounds; final Rect overlayBounds = mForegroundInfo.mOverlayBounds; if (mForegroundInfo.mInsidePadding) { selfBounds.set(0, 0, getWidth(), getHeight()); } else { selfBounds.set(getPaddingLeft(), getPaddingTop(), getWidth() - getPaddingRight(), getHeight() - getPaddingBottom()); } final int ld = getLayoutDirection(); Gravity.apply(mForegroundInfo.mGravity, foreground.getIntrinsicWidth(), foreground.getIntrinsicHeight(), selfBounds, overlayBounds, ld); foreground.setBounds(overlayBounds); } foreground.draw(canvas); } }
2和5.绘制View的褪色边缘
当 horizontalEdges 或者 verticalEdges 有一个 true 的时候，表示需要绘制 View 的褪色边缘：
boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0; boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
这时候先计算出是否需要绘制上下左右的褪色边缘和它的参数，然后保存视图层：
int paddingLeft = mPaddingLeft; final boolean offsetRequired = isPaddingOffsetRequired(); if (offsetRequired) { paddingLeft += getLeftPaddingOffset(); } int left = mScrollX + paddingLeft; int right = left + mRight - mLeft - mPaddingRight - paddingLeft; int top = mScrollY + getFadeTop(offsetRequired); int bottom = top + getFadeHeight(offsetRequired); if (offsetRequired) { right += getRightPaddingOffset(); bottom += getBottomPaddingOffset(); } final ScrollabilityCache scrollabilityCache = mScrollCache; final float fadeHeight = scrollabilityCache.fadingEdgeLength; int length = (int) fadeHeight; if (verticalEdges && (top + length > bottom - length)) { length = (bottom - top) / 2; } if (horizontalEdges && (left + length > right - length)) { length = (right - left) / 2; } if (verticalEdges) { topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength())); drawTop = topFadeStrength * fadeHeight > 1.0f; bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength())); drawBottom = bottomFadeStrength * fadeHeight > 1.0f; } if (horizontalEdges) { leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength())); drawLeft = leftFadeStrength * fadeHeight > 1.0f; rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength())); drawRight = rightFadeStrength * fadeHeight > 1.0f; } saveCount = canvas.getSaveCount(); int solidColor = getSolidColor(); if (solidColor == 0) { final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG; if (drawTop) { canvas.saveLayer(left, top, right, top + length, null, flags); } if (drawBottom) { canvas.saveLayer(left, bottom - length, right, bottom, null, flags); } if (drawLeft) { canvas.saveLayer(left, top, left + length, bottom, null, flags); } if (drawRight) { canvas.saveLayer(right - length, top, right, bottom, null, flags); } } else { scrollabilityCache.setFadeColor(solidColor); }
绘制褪色边缘，恢复视图层 ：
final Paint p = scrollabilityCache.paint; final Matrix matrix = scrollabilityCache.matrix; final Shader fade = scrollabilityCache.shader; if (drawTop) { matrix.setScale(1, fadeHeight * topFadeStrength); matrix.postTranslate(left, top); fade.setLocalMatrix(matrix); p.setShader(fade); canvas.drawRect(left, top, right, top + length, p); } if (drawBottom) { matrix.setScale(1, fadeHeight * bottomFadeStrength); matrix.postRotate(180); matrix.postTranslate(left, bottom); fade.setLocalMatrix(matrix); p.setShader(fade); canvas.drawRect(left, bottom - length, right, bottom, p); } if (drawLeft) { matrix.setScale(1, fadeHeight * leftFadeStrength); matrix.postRotate(-90); matrix.postTranslate(left, top); fade.setLocalMatrix(matrix); p.setShader(fade); canvas.drawRect(left, top, left + length, bottom, p); } if (drawRight) { matrix.setScale(1, fadeHeight * rightFadeStrength); matrix.postRotate(90); matrix.postTranslate(right, top); fade.setLocalMatrix(matrix); p.setShader(fade); canvas.drawRect(right - length, top, right, bottom, p); } canvas.restoreToCount(saveCount);
所谓的绘制装饰，就是指 View 除了背景、内容、子 View 的其余部分，例如滚动条等。
最后附上 View 的 draw 流程：
到此，View 的绘制流程就讲完了，下一篇会讲自定义 View。