Android(安卓)View深入学习（三），View的绘制（Draw）过程

View绘制最开始是从最顶层DecorView开始的，在ViewRootImpl中的performTraversals方法中，调用了 performDraw，开始对DecorView进行绘制：

 private void performDraw() {        if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) {            return;        }        final boolean fullRedrawNeeded = mFullRedrawNeeded;        mFullRedrawNeeded = false;        mIsDrawing = true;        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");        try {            draw(fullRedrawNeeded);        } finally {            mIsDrawing = false;            Trace.traceEnd(Trace.TRACE_TAG_VIEW);        }        // For whatever reason we didn't create a HardwareRenderer, end any        // hardware animations that are now dangling        if (mAttachInfo.mPendingAnimatingRenderNodes != null) {            final int count = mAttachInfo.mPendingAnimatingRenderNodes.size();            for (int i = 0; i < count; i++) {                mAttachInfo.mPendingAnimatingRenderNodes.get(i).endAllAnimators();            }            mAttachInfo.mPendingAnimatingRenderNodes.clear();        }        if (mReportNextDraw) {            mReportNextDraw = false;            if (mAttachInfo.mHardwareRenderer != null) {                mAttachInfo.mHardwareRenderer.fence();            }            if (LOCAL_LOGV) {                Log.v(TAG, "FINISHED DRAWING: " + mWindowAttributes.getTitle());            }            if (mSurfaceHolder != null && mSurface.isValid()) {                mSurfaceHolderCallback.surfaceRedrawNeeded(mSurfaceHolder);                SurfaceHolder.Callback callbacks[] = mSurfaceHolder.getCallbacks();                if (callbacks != null) {                    for (SurfaceHolder.Callback c : callbacks) {                        if (c instanceof SurfaceHolder.Callback2) {                            ((SurfaceHolder.Callback2)c).surfaceRedrawNeeded(                                    mSurfaceHolder);                        }                    }                }            }            try {                mWindowSession.finishDrawing(mWindow);            } catch (RemoteException e) {            }        }    }

12行调用draw(fullRedrawNeeded)，在draw方法里面，又调用了
drawSoftware：

 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;            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;            }            // TODO: Do this in native            canvas.setDensity(mDensity);        } catch (Surface.OutOfResourcesException e) {            handleOutOfResourcesException(e);            return false;        } catch (IllegalArgumentException e) {            Log.e(TAG, "Could not lock surface", e);            // Don't assume this is due to out of memory, it could be            // something else, and if it is something else then we could            // kill stuff (or ourself) for no reason.            mLayoutRequested = true;    // ask wm for a new surface next time.            return false;        }        try {            if (DEBUG_ORIENTATION || DEBUG_DRAW) {                Log.v(TAG, "Surface " + surface + " drawing to bitmap w="                        + canvas.getWidth() + ", h=" + canvas.getHeight());                //canvas.drawARGB(255, 255, 0, 0);            }            // If this bitmap's format includes an alpha channel, we            // need to clear it before drawing so that the child will            // properly re-composite its drawing on a transparent            // background. This automatically respects the clip/dirty region            // or            // If we are applying an offset, we need to clear the area            // where the offset doesn't appear to avoid having garbage            // left in the blank areas.            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;            if (DEBUG_DRAW) {                Context cxt = mView.getContext();                Log.i(TAG, "Drawing: package:" + cxt.getPackageName() +                        ", metrics=" + cxt.getResources().getDisplayMetrics() +                        ", compatibilityInfo=" + cxt.getResources().getCompatibilityInfo());            }            try {                canvas.translate(-xoff, -yoff);                if (mTranslator != null) {                    mTranslator.translateCanvas(canvas);                }                canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0);                attachInfo.mSetIgnoreDirtyState = false;                mView.draw(canvas);                drawAccessibilityFocusedDrawableIfNeeded(canvas);            } finally {                if (!attachInfo.mSetIgnoreDirtyState) {                    // Only clear the flag if it was not set during the mView.draw() call                    attachInfo.mIgnoreDirtyState = false;                }            }        } finally {            try {                surface.unlockCanvasAndPost(canvas);            } catch (IllegalArgumentException e) {                Log.e(TAG, "Could not unlock surface", e);                mLayoutRequested = true;    // ask wm for a new surface next time.                //noinspection ReturnInsideFinallyBlock                return false;            }            if (LOCAL_LOGV) {                Log.v(TAG, "Surface " + surface + " unlockCanvasAndPost");            }        }        return true;    }

73行，执行mView.draw(canvas)，mView就是DecorView的实例，会先调用DecorView的draw方法：

public void draw(Canvas canvas) {            super.draw(canvas);            if (mMenuBackground != null) {                mMenuBackground.draw(canvas);            }        }

在DecorView的draw方法里面，执行super.draw(canvas)，会调用FrameLayout的draw方法：

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);        }    }

在FrameLayout的draw方法里面，最先执行 super.draw(canvas)，也就是调用View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 * 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);            // Step 6, draw decorations (scrollbars)            onDrawScrollBars(canvas);            if (mOverlay != null && !mOverlay.isEmpty()) {                mOverlay.getOverlayView().dispatchDraw(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);        // Step 6, draw decorations (scrollbars)        onDrawScrollBars(canvas);        if (mOverlay != null && !mOverlay.isEmpty()) {            mOverlay.getOverlayView().dispatchDraw(canvas);        }    }

View的绘制一般分为6个步骤，在draw方法里面已经列出来了：
第一步：绘制View背景。
第二步：保存canvas的状态，准备绘制FadingEdge（边框阴影），这个不是必要的步骤。
第三步：调用onDraw方法，绘制View的内容。
第四步：调用dispatchDraw，如果是ViewGroup，则绘制子View。
第五步：绘制FadingEdge（边框阴影），恢复canvas的状态，这个不是必须的步骤。
第六步：绘制View的滑动条。
执行这六个步骤之后，View的绘制就完成了。重点关注第四步，执行dispatchDraw方法：

    protected void dispatchDraw(Canvas canvas) {    }

View的dispatchDraw是空的，只有ViewGroup才需要绘制子View，DecorView是FrameLayout，但是FrameLayout没有重写dispatchDraw方法，那么会执行ViewGroup的dispatchDraw方法：

 protected void dispatchDraw(Canvas canvas) {        boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode);        final int childrenCount = 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 < childrenCount; i++) {                final View child = children[i];                if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {                    final LayoutParams params = child.getLayoutParams();                    attachLayoutAnimationParameters(child, params, i,                            childrenCount);                    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 clipSaveCount = 0;        final boolean clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;        if (clipToPadding) {            clipSaveCount = 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 (usingRenderNodeProperties)            canvas.insertReorderBarrier();        // Only use the preordered list if not HW accelerated, since the HW        // pipeline will do the        // draw reordering internally        final ArrayList<View> preorderedList = usingRenderNodeProperties ? null                : buildOrderedChildList();        final boolean customOrder = preorderedList == null                && isChildrenDrawingOrderEnabled();        for (int i = 0; i < childrenCount; i++) {            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);            }        }        if (preorderedList != null)            preorderedList.clear();        // 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 (usingRenderNodeProperties)            canvas.insertInorderBarrier();        if (debugDraw()) {            onDebugDraw(canvas);        }        if (clipToPadding) {            canvas.restoreToCount(clipSaveCount);        }        // 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);        }    }

71行，开始遍历所有的子View，78行调用drawChild，开始绘制子View：

 protected boolean drawChild(Canvas canvas, View child, long drawingTime) {        return child.draw(canvas, this, drawingTime);    }

接着，又从从上往下，开始绘制，直到所有的View都绘制完成，整个绘制的流程如下：

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