本文主要是介绍draw与dispatchDraw，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
View类中的draw方法：
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：绘制背景* 2. If necessary, save the canvas' layers to prepare for fading* ：如有必要，颜色渐变淡之前保存画布层(即锁定原有的画布内容)* 3. Draw view's content：绘制view的内容* 4. Draw children：绘制子view* 5. If necessary, draw the fading edges and restore layers* ：如有必要，绘制颜色渐变淡的边框，并恢复画布(即画布改变的内容附加到原有内容上)* 6. Draw decorations (scrollbars for instance)：绘制装饰，比如滚动条*/// Step 1, draw the background, if needed：//背景绘制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 contentif (!dirtyOpaque) onDraw(canvas);// Step 4, draw the childrendispatchDraw(canvas);// Overlay is part of the content and draws beneath Foregroundif (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' layersint 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 artifactsif (verticalEdges && (top + length > bottom - length)) {length = (bottom - top) / 2;}// also clip horizontal fades if necessaryif (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 ：//调用onDrawif (!dirtyOpaque) onDraw(canvas);// Step 4, draw the children ://绘制子viewdispatchDraw(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 Foregroundif (mOverlay != null && !mOverlay.isEmpty()) {mOverlay.getOverlayView().dispatchDraw(canvas);}// Step 6, draw decorations (foreground, scrollbars)onDrawForeground(canvas);
}
说明：
自定义一个view时，重写onDraw。调用view.invalidate(),会触发onDraw和computeScroll()。前提是该view被附加在当前窗口上.
Android中实现view的更新有两组方法，一组是invalidate，另一组是postInvalidate，其中前者是在UI线程自身中使用，而后者在非UI线程中使用。
自定义一个ViewGroup，重写onDraw。onDraw可能不会被调用，原因是需要先设置一个背景(颜色或图)。表示这个group有东西需要绘制了，才会触发draw，之后是onDraw。因此，一般直接重写dispatchDraw来绘制viewGroup
自定义一个ViewGroup,dispatchDraw会调用drawChild。
ViewGroup类dispatchDraw：
@Override
protected void dispatchDraw(Canvas canvas) {//......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;}}final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {more |= drawChild(canvas, child, drawingTime);}}while (transientIndex >= 0) {// there may be additional transient views after the normal viewsfinal View transientChild = mTransientViews.get(transientIndex);if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||transientChild.getAnimation() != null) {more |= drawChild(canvas, transientChild, drawingTime);}transientIndex++;if (transientIndex >= transientCount) {break;}}// Draw any disappearing views that have animationsif (mDisappearingChildren != null) {final ArrayList<View> disappearingChildren = mDisappearingChildren;final int disappearingCount = disappearingChildren.size() - 1;// Go backwards -- we may delete as animations finishfor (int i = disappearingCount; i >= 0; i--) {final View child = disappearingChildren.get(i);more |= drawChild(canvas, child, drawingTime);}}
}protected boolean drawChild(Canvas canvas, View child, long drawingTime) {return child.draw(canvas, this, drawingTime);
}
绘制VIew本身的内容，通过调用View.onDraw(canvas)函数实现,绘制自己的孩子通过dispatchDraw（canvas）实现
View组件的绘制会调用draw(Canvas canvas)方法，draw过程中主要是先画Drawable背景，对 drawable调用setBounds(),
然后是draw(Canvas c)方法.有点注意的是背景drawable的实际大小会影响view组件的大小，drawable的实际大小通过getIntrinsicWidth()和getIntrinsicHeight()获取，当背景比较大时view组件大小等于背景drawable的大小画完背景后，draw过程会调用onDraw(Canvas canvas)方法，
然后就是dispatchDraw(Canvas canvas)方法, dispatchDraw()主要是分发给子组件进行绘制，我们通常定制组件的时候重写的是onDraw()方法。
值得注意的是ViewGroup容器组件的绘制，当它没有背景时直接调用的是dispatchDraw()方法, 而绕过了draw()方法，当它有背景的时候就调用draw()方法，而draw()方法里包含了dispatchDraw()方法的调用。因此要在ViewGroup上绘制东西的时候往往重写的是dispatchDraw()方法而不是onDraw()方法，或者自定制一个Drawable，重写它的draw(Canvas c)和 getIntrinsicWidth(), getIntrinsicHeight()方法，然后设为背景。
这篇关于draw与dispatchDraw的文章就介绍到这儿，希望我们推荐的文章对编程师们有所帮助！
