深入浅析Android中View的绘制流程
深入浅析Android中View的绘制流程?很多新手对此不是很清楚,为了帮助大家解决这个难题,下面小编将为大家详细讲解,有这方面需求的人可以来学习下,希望你能有所收获。
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从performDraw说起
三大工作流程始于ViewRootImpl#performTraversals,在这个方法内部会分别调用performMeasure,performLayout,performDraw三个方法来分别完成测量,布局,绘制流程。那么我们现在先从performDraw方法看起,ViewRootImpl#performDraw:
private void performDraw() { //... final boolean fullRedrawNeeded = mFullRedrawNeeded; try { draw(fullRedrawNeeded); } finally { mIsDrawing = false; Trace.traceEnd(Trace.TRACE_TAG_VIEW); } //省略... }
里面又调用了ViewRootImpl#draw方法,并传递了fullRedrawNeeded参数,而该参数由mFullRedrawNeeded成员变量获取,它的作用是判断是否需要重新绘制全部视图,如果是第一次绘制视图,那么显然应该绘制所以的视图,如果由于某些原因,导致了视图重绘,那么就没有必要绘制所有视图。我们来看看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)); } //省略... if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) { return; } }
这里省略了一部分代码,我们只看关键代码,首先是先获取了mDirty值,该值保存了需要重绘的区域的信息,关于视图重绘,后面会有文章专门叙述,这里先熟悉一下。接着根据fullRedrawNeeded来判断是否需要重置dirty区域,最后调用了ViewRootImpl#drawSoftware方法,并把相关参数传递进去,包括dirty区域,我们接着看该方法的源码:
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff, boolean scalingRequired, Rect dirty) { // Draw with software renderer. final Canvas canvas; try { final int left = dirty.left; final int top = dirty.top; final int right = dirty.right; final int bottom = dirty.bottom; //锁定canvas区域,由dirty区域决定 canvas = mSurface.lockCanvas(dirty); // The dirty rectangle can be modified by Surface.lockCanvas() //noinspection ConstantConditions if (left != dirty.left || top != dirty.top || right != dirty.right || bottom != dirty.bottom) { attachInfo.mIgnoreDirtyState = true; } canvas.setDensity(mDensity); } try { if (!canvas.isOpaque() || yoff != 0 || xoff != 0) { canvas.drawColor(0, PorterDuff.Mode.CLEAR); } dirty.setEmpty(); mIsAnimating = false; attachInfo.mDrawingTime = SystemClock.uptimeMillis(); mView.mPrivateFlags |= View.PFLAG_DRAWN; try { canvas.translate(-xoff, -yoff); if (mTranslator != null) { mTranslator.translateCanvas(canvas); } canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0); attachInfo.mSetIgnoreDirtyState = false; //正式开始绘制 mView.draw(canvas); } } return true; }
可以看书,首先是实例化了Canvas对象,然后锁定该canvas的区域,由dirty区域决定,接着对canvas进行一系列的属性赋值,最后调用了mView.draw(canvas)方法,前面分析过,mView就是DecorView,也就是说从DecorView开始绘制,前面所做的一切工作都是准备工作,而现在则是正式开始绘制流程。
View的绘制
由于ViewGroup没有重写draw方法,因此所有的View都是调用View#draw方法,因此,我们直接看它的源码:
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; if (!dirtyOpaque) { drawBackground(canvas); } // skip step 2 & 5 if possible (common case) final int viewFlags = mViewFlags; boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0; boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0; 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; } ... }
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:
1、对View的背景进行绘制
2、保存当前的图层信息(可跳过)
3、绘制View的内容
4、对View的子View进行绘制(如果有子View)
5、绘制View的褪色的边缘,类似于阴影效果(可跳过)
6、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。
Skip 1:绘制背景
这里调用了View#drawBackground方法,我们看它的源码:
private void drawBackground(Canvas canvas) { //mBackground是该View的背景参数,比如背景颜色 final Drawable background = mBackground; if (background == null) { return; } //根据View四个布局参数来确定背景的边界 setBackgroundBounds(); ... //获取当前View的mScrollX和mScrollY值 final int scrollX = mScrollX; final int scrollY = mScrollY; if ((scrollX | scrollY) == 0) { background.draw(canvas); } else { //如果scrollX和scrollY有值,则对canvas的坐标进行偏移,再绘制背景 canvas.translate(scrollX, scrollY); background.draw(canvas); canvas.translate(-scrollX, -scrollY); } }
可以看出,这里考虑到了view的偏移参数,scrollX和scrollY,绘制背景在偏移后的view中绘制。
Skip 3:绘制内容
这里调用了View#onDraw方法,View中该方法是一个空实现,因为不同的View有着不同的内容,这需要我们自己去实现,即在自定义View中重写该方法来实现。
Skip 4:绘制子View
如果当前的View是一个ViewGroup类型,那么就需要绘制它的子View,这里调用了dispatchDraw,而View中该方法是空实现,实际是ViewGroup重写了这个方法,那么我们来看看,ViewGroup#dispatchDraw:
protected void dispatchDraw(Canvas canvas) { boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode); final int childrenCount = mChildrenCount; final View[] children = mChildren; int flags = mGroupFlags; for (int i = 0; i < childrenCount; i++) { 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); } } //省略... }
源码很长,这里简单说明一下,里面主要遍历了所以子View,每个子View都调用了drawChild这个方法,我们找到这个方法,ViewGroup#drawChild:
protected boolean drawChild(Canvas canvas, View child, long drawingTime) { return child.draw(canvas, this, drawingTime); }
可以看出,这里调用了View的draw方法,但这个方法并不是上面所说的,因为参数不同,我们来看看这个方法,View#draw:
boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) { //省略... 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); } } }
我们主要来看核心部分,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。
Skip 6绘制装饰
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,我们看View#onDrawForeground:
public void onDrawForeground(Canvas canvas) { onDrawScrollIndicators(canvas); onDrawScrollBars(canvas); 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); } }
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文章标题:深入浅析Android中View的绘制流程
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