android view draw原始碼過程分析

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()直接呼叫view.draw(). viewgroup沒實現重寫draw方法

有mForeground時繪製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   繪製背景
         *      2. If necessary, save the canvas' layers to prepare for fading  
         *      3. Draw view's content
         *      4. Draw children  繪製子元素
         *      5. If necessary, draw the fading edges and restore layers  繪製漸變
         *      6. Draw decorations (scrollbars for instance) 繪製滑塊
         */

        // 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，對具體view重寫

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()的核心程式碼就是通過for迴圈呼叫drawChild()對ViewGroup的每個子檢視進行繪製，上述程式碼中如果FLAG_USE_CHILD_DRAWING_ORDER為true，則子檢視的繪製順序通過getChildDrawingOrder來決定，預設的繪製順序即是子檢視加入ViewGroup的順序，而我們可以過載getChildDrawingOrder函式來更改預設的繪製順序，這會影響到子檢視之間的重疊關係。
      drawChild()的核心過程就是為子檢視分配合適的cavas剪下區，剪下區的大小正是由layout過程決定的，而剪下區的位置取決於滾動值以及子檢視當前的動畫。設定完剪下區後就會呼叫子檢視的draw()函式進行具體的繪製，如果子檢視的包含SKIP_DRAW標識，那麼僅呼叫dispatchDraw()，即跳過子檢視本身的繪製，但要繪製檢視可能包含的字檢視