android view draw源码过程分析

FrameLayout.java

  @Override    public void draw(Canvas canvas) {        super.draw(canvas);        if (mForeground != null) {            final Drawable foreground = mForeground;            if (mForegroundBoundsChanged) {                mForegroundBoundsChanged = false;                final Rect selfBounds = mSelfBounds;                final Rect overlayBounds = mOverlayBounds;                final int w = mRight-mLeft;                final int h = mBottom-mTop;                if (mForegroundInPadding) {                    selfBounds.set(0, 0, w, h);                } else {                    selfBounds.set(mPaddingLeft, mPaddingTop, w - mPaddingRight, h - mPaddingBottom);                }                final int layoutDirection = getLayoutDirection();                Gravity.apply(mForegroundGravity, foreground.getIntrinsicWidth(),                        foreground.getIntrinsicHeight(), selfBounds, overlayBounds,                        layoutDirection);                foreground.setBounds(overlayBounds);            }                        foreground.draw(canvas);        }    }

super.draw()直接调用view.draw(). viewgroup没实现重写draw方法

有mForeground时绘制mForeground，

view.java

    /**     * Manually render this view (and all of its children) to the given Canvas.     * The view must have already done a full layout before this function is     * called.  When implementing a view, implement     * {@link #onDraw(android.graphics.Canvas)} instead of overriding this method.     * If you do need to override this method, call the superclass version.     *     * @param canvas The Canvas to which the View is rendered.     */    public void draw(Canvas canvas) {        if (mClipBounds != null) {            canvas.clipRect(mClipBounds);        }        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           *      3. Draw view's content         *      4. Draw children  绘制子元素         *      5. If necessary, draw the fading edges and restore layers  绘制渐变         *      6. Draw decorations (scrollbars for instance) 绘制滑块         */        // Step 1, draw the background, if needed        int saveCount;        if (!dirtyOpaque) {            final Drawable background = mBackground;            if (background != null) {                final int scrollX = mScrollX;                final int scrollY = mScrollY;                if (mBackgroundSizeChanged) {                    background.setBounds(0, 0,  mRight - mLeft, mBottom - mTop);                    mBackgroundSizeChanged = false;                }                if ((scrollX | scrollY) == 0) {                    background.draw(canvas);                } else {                    canvas.translate(scrollX, scrollY);                    background.draw(canvas);                    canvas.translate(-scrollX, -scrollY);                }            }        }        // 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);            // Step 6, draw decorations (scrollbars)            onDrawScrollBars(canvas);            if (mOverlay != null && !mOverlay.isEmpty()) {                mOverlay.getOverlayView().dispatchDraw(canvas);            }            // we're done...            return;        }

onDraw(canvas); dispatchDraw(canvas); 都是空方法，对具体view重写

ViewGroup.java

 /**     * {@inheritDoc}     */    @Override    protected void dispatchDraw(Canvas canvas) {        final int count = mChildrenCount;        final View[] children = mChildren;        int flags = mGroupFlags;        if ((flags & FLAG_RUN_ANIMATION) != 0 && canAnimate()) {            final boolean cache = (mGroupFlags & FLAG_ANIMATION_CACHE) == FLAG_ANIMATION_CACHE;            final boolean buildCache = !isHardwareAccelerated();            for (int i = 0; i < count; i++) {                final View child = children[i];                if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {                    final LayoutParams params = child.getLayoutParams();                    attachLayoutAnimationParameters(child, params, i, count);                    bindLayoutAnimation(child);                    if (cache) {                        child.setDrawingCacheEnabled(true);                        if (buildCache) {                                                    child.buildDrawingCache(true);                        }                    }                }            }            final LayoutAnimationController controller = mLayoutAnimationController;            if (controller.willOverlap()) {                mGroupFlags |= FLAG_OPTIMIZE_INVALIDATE;            }            controller.start();            mGroupFlags &= ~FLAG_RUN_ANIMATION;            mGroupFlags &= ~FLAG_ANIMATION_DONE;            if (cache) {                mGroupFlags |= FLAG_CHILDREN_DRAWN_WITH_CACHE;            }            if (mAnimationListener != null) {                mAnimationListener.onAnimationStart(controller.getAnimation());            }        }        int saveCount = 0;        final boolean clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;        if (clipToPadding) {            saveCount = canvas.save();            canvas.clipRect(mScrollX + mPaddingLeft, mScrollY + mPaddingTop,                    mScrollX + mRight - mLeft - mPaddingRight,                    mScrollY + mBottom - mTop - mPaddingBottom);        }        // We will draw our child's animation, let's reset the flag        mPrivateFlags &= ~PFLAG_DRAW_ANIMATION;        mGroupFlags &= ~FLAG_INVALIDATE_REQUIRED;        boolean more = false;        final long drawingTime = getDrawingTime();        if ((flags & FLAG_USE_CHILD_DRAWING_ORDER) == 0) {            for (int i = 0; i < count; i++) {                final View child = children[i];                if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {                    more |= drawChild(canvas, child, drawingTime);                }            }        } else {            for (int i = 0; i < count; i++) {                final View child = children[getChildDrawingOrder(count, i)];                if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {                    more |= drawChild(canvas, child, drawingTime);                }            }        }        // Draw any disappearing views that have animations        if (mDisappearingChildren != null) {            final ArrayList<View> disappearingChildren = mDisappearingChildren;            final int disappearingCount = disappearingChildren.size() - 1;            // Go backwards -- we may delete as animations finish            for (int i = disappearingCount; i >= 0; i--) {                final View child = disappearingChildren.get(i);                more |= drawChild(canvas, child, drawingTime);            }        }        if (debugDraw()) {            onDebugDraw(canvas);        }        if (clipToPadding) {            canvas.restoreToCount(saveCount);        }        // mGroupFlags might have been updated by drawChild()        flags = mGroupFlags;        if ((flags & FLAG_INVALIDATE_REQUIRED) == FLAG_INVALIDATE_REQUIRED) {            invalidate(true);        }        if ((flags & FLAG_ANIMATION_DONE) == 0 && (flags & FLAG_NOTIFY_ANIMATION_LISTENER) == 0 &&                mLayoutAnimationController.isDone() && !more) {            // We want to erase the drawing cache and notify the listener after the            // next frame is drawn because one extra invalidate() is caused by            // drawChild() after the animation is over            mGroupFlags |= FLAG_NOTIFY_ANIMATION_LISTENER;            final Runnable end = new Runnable() {               public void run() {                   notifyAnimationListener();               }            };            post(end);        }    } /**     * Draw one child of this View Group. This method is responsible for getting     * the canvas in the right state. This includes clipping, translating so     * that the child's scrolled origin is at 0, 0, and applying any animation     * transformations.     *     * @param canvas The canvas on which to draw the child     * @param child Who to draw     * @param drawingTime The time at which draw is occurring     * @return True if an invalidate() was issued     */    protected boolean drawChild(Canvas canvas, View child, long drawingTime) {        return child.draw(canvas, this, drawingTime);    }

 dispatchDraw()的核心代码就是通过for循环调用drawChild()对ViewGroup的每个子视图进行绘制，上述代码中如果FLAG_USE_CHILD_DRAWING_ORDER为true，则子视图的绘制顺序通过getChildDrawingOrder来决定，默认的绘制顺序即是子视图加入ViewGroup的顺序，而我们可以重载getChildDrawingOrder函数来更改默认的绘制顺序，这会影响到子视图之间的重叠关系。
      drawChild()的核心过程就是为子视图分配合适的cavas剪切区，剪切区的大小正是由layout过程决定的，而剪切区的位置取决于滚动值以及子视图当前的动画。设置完剪切区后就会调用子视图的draw()函数进行具体的绘制，如果子视图的包含SKIP_DRAW标识，那么仅调用dispatchDraw()，即跳过子视图本身的绘制，但要绘制视图可能包含的字视图