Android學習分享：執行某ViewGroup的動畫時，子控件太多導致動畫執行卡頓的問題

最近在項目中遇到一個問題，我有一個LinearLayout，里面裝載了許多ImageView控件，ImageView控件顯示着自己的圖片，這個LinearLayout支持雙指縮放，縮放采用ScaleAnimation來實現，但是但是在縮放過程中，屏幕十分卡頓，縮放效果根本沒有跟上手指的縮放動作。后來在Google上查了一番，查到一個API，叫setAnimationDrawCacheEnabled(boolean enabled)：

    /**
     * Enables or disables the children's drawing cache during a layout animation.
     * By default, the drawing cache is enabled but this will prevent nested
     * layout animations from working. To nest animations, you must disable the
     * cache.
     *
     * @param enabled true to enable the animation cache, false otherwise
     *
     * @see #isAnimationCacheEnabled()
     * @see View#setDrawingCacheEnabled(boolean)
     */
    public void setAnimationCacheEnabled(boolean enabled) {
        setBooleanFlag(FLAG_ANIMATION_CACHE, enabled);
    }

方法的注解我這里簡單翻譯一下：在執行一個Layout動畫時開啟或關閉子控件的繪制緩存。默認情況下，繪制緩存是開啟的，但是這將阻止嵌套Layout動畫的正常執行。對於嵌套動畫，你必須禁用這個緩存。

先說drawing cache，繪制緩存的概念，Android為了提高View視圖的繪制效率，提出了一個緩存的概念，其實就是一個Bitmap，用來存儲View當前的繪制內容，在View的內容或者尺寸未發生改變時，這個緩存應該始終不被銷毀，銷毀了如果下次還用(開啟了繪圖緩存的前提下，API為setDrawingCacheEnabled(enabled)，另外還可以設置繪圖緩存Bitmap的質量，API為setDrawingCacheQuality(quality))就必須重建。

關於繪圖緩存的相關介紹，可搜索這些相關API的介紹：

1）setDrawingCacheQuality(int quality)

2）setDrawingCacheEnabled(enabled)

3）setDrawingCacheBackgroundColor(color)

先看一段代碼：

    /**
     * <p>Forces the drawing cache to be built if the drawing cache is invalid.</p>
     *
     * <p>If you call {@link #buildDrawingCache()} manually without calling
     * {@link #setDrawingCacheEnabled(boolean) setDrawingCacheEnabled(true)}, you
     * should cleanup the cache by calling {@link #destroyDrawingCache()} afterwards.</p>
     *
     * <p>Note about auto scaling in compatibility mode: When auto scaling is not enabled,
     * this method will create a bitmap of the same size as this view. Because this bitmap
     * will be drawn scaled by the parent ViewGroup, the result on screen might show
     * scaling artifacts. To avoid such artifacts, you should call this method by setting
     * the auto scaling to true. Doing so, however, will generate a bitmap of a different
     * size than the view. This implies that your application must be able to handle this
     * size.</p>
     *
     * <p>You should avoid calling this method when hardware acceleration is enabled. If
     * you do not need the drawing cache bitmap, calling this method will increase memory
     * usage and cause the view to be rendered in software once, thus negatively impacting
     * performance.</p>
     *
     * @see #getDrawingCache()
     * @see #destroyDrawingCache()
     */
    public void buildDrawingCache(boolean autoScale) {
        if ((mPrivateFlags & PFLAG_DRAWING_CACHE_VALID) == 0 || (autoScale ?
                mDrawingCache == null : mUnscaledDrawingCache == null)) {
            mCachingFailed = false;

            int width = mRight - mLeft;
            int height = mBottom - mTop;

            final AttachInfo attachInfo = mAttachInfo;
            final boolean scalingRequired = attachInfo != null && attachInfo.mScalingRequired;

            if (autoScale && scalingRequired) {
                width = (int) ((width * attachInfo.mApplicationScale) + 0.5f);
                height = (int) ((height * attachInfo.mApplicationScale) + 0.5f);
            }

            final int drawingCacheBackgroundColor = mDrawingCacheBackgroundColor;
　　　　　　　// 1.這里 final boolean opaque = drawingCacheBackgroundColor != 0 || isOpaque();
            final boolean use32BitCache = attachInfo != null && attachInfo.mUse32BitDrawingCache;

            final long projectedBitmapSize = width * height * (opaque && !use32BitCache ? 2 : 4);
            final long drawingCacheSize =
                    ViewConfiguration.get(mContext).getScaledMaximumDrawingCacheSize();
            if (width <= 0 || height <= 0 || projectedBitmapSize > drawingCacheSize) {
                if (width > 0 && height > 0) {
                    Log.w(VIEW_LOG_TAG, "View too large to fit into drawing cache, needs "
                            + projectedBitmapSize + " bytes, only "
                            + drawingCacheSize + " available");
                }
                destroyDrawingCache();
                mCachingFailed = true;
                return;
            }

            boolean clear = true;
            Bitmap bitmap = autoScale ? mDrawingCache : mUnscaledDrawingCache;

            if (bitmap == null || bitmap.getWidth() != width || bitmap.getHeight() != height) {
                Bitmap.Config quality;
　　　　　　　　　 // 2.這里 if (!opaque) {
                    // Never pick ARGB_4444 because it looks awful
                    // Keep the DRAWING_CACHE_QUALITY_LOW flag just in case
                    switch (mViewFlags & DRAWING_CACHE_QUALITY_MASK) {
                        case DRAWING_CACHE_QUALITY_AUTO:
                        case DRAWING_CACHE_QUALITY_LOW:
                        case DRAWING_CACHE_QUALITY_HIGH:
                        default:
                            quality = Bitmap.Config.ARGB_8888;
                            break;
                    }
                } else {
                    // Optimization for translucent windows
                    // If the window is translucent, use a 32 bits bitmap to benefit from memcpy()
                    quality = use32BitCache ? Bitmap.Config.ARGB_8888 : Bitmap.Config.RGB_565;
                }

                // Try to cleanup memory
                if (bitmap != null) bitmap.recycle();

                try {
                    bitmap = Bitmap.createBitmap(mResources.getDisplayMetrics(),
                            width, height, quality);
                    bitmap.setDensity(getResources().getDisplayMetrics().densityDpi);
                    if (autoScale) {
                        mDrawingCache = bitmap;
                    } else {
                        mUnscaledDrawingCache = bitmap;
                    }
                    if (opaque && use32BitCache) bitmap.setHasAlpha(false);
                } catch (OutOfMemoryError e) {
                    // If there is not enough memory to create the bitmap cache, just
                    // ignore the issue as bitmap caches are not required to draw the
                    // view hierarchy
                    if (autoScale) {
                        mDrawingCache = null;
                    } else {
                        mUnscaledDrawingCache = null;
                    }
                    mCachingFailed = true;
                    return;
                }

                clear = drawingCacheBackgroundColor != 0;
            }

            Canvas canvas;
            if (attachInfo != null) {
                canvas = attachInfo.mCanvas;
                if (canvas == null) {
                    canvas = new Canvas();
                }
                canvas.setBitmap(bitmap);
                // Temporarily clobber the cached Canvas in case one of our children
                // is also using a drawing cache. Without this, the children would
                // steal the canvas by attaching their own bitmap to it and bad, bad
                // thing would happen (invisible views, corrupted drawings, etc.)
                attachInfo.mCanvas = null;
            } else {
                // This case should hopefully never or seldom happen
                canvas = new Canvas(bitmap);
            }

            if (clear) {
                bitmap.eraseColor(drawingCacheBackgroundColor);
            }

            computeScroll();
            final int restoreCount = canvas.save();

            if (autoScale && scalingRequired) {
                final float scale = attachInfo.mApplicationScale;
                canvas.scale(scale, scale);
            }

            canvas.translate(-mScrollX, -mScrollY);

            mPrivateFlags |= PFLAG_DRAWN;
            if (mAttachInfo == null || !mAttachInfo.mHardwareAccelerated ||
                    mLayerType != LAYER_TYPE_NONE) {
                mPrivateFlags |= PFLAG_DRAWING_CACHE_VALID;
            }

            // Fast path for layouts with no backgrounds
            if ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) {
                mPrivateFlags &= ~PFLAG_DIRTY_MASK;
                dispatchDraw(canvas);
                if (mOverlay != null && !mOverlay.isEmpty()) {
                    mOverlay.getOverlayView().draw(canvas);
                }
            } else {
                draw(canvas);
            }

            canvas.restoreToCount(restoreCount);
            canvas.setBitmap(null);

            if (attachInfo != null) {
                // Restore the cached Canvas for our siblings
                attachInfo.mCanvas = canvas;
            }
        }
    }

兩個標注了紅色的地方，說明了這個mDrawingCacheBackgroundColor變量的作用，因此如果使用默認值，那么緩存Bitmap使用的是Bitmap.Config.ARGB_8888，比Bitmap.Config.RGB_565多占用了一半的內存，因此如果不想使用太大的內存，擔心內存泄露，可以設置給mDrawingCacheBackgroundColor一個值，例如：

setDrawingCacheBackgroundColor(0xFF0C0C0C);

那么，那么，這個繪圖緩存如何優化繪圖速率，又怎么阻礙了Animation的執行？

先看兩個方法：

1）ViewGroup -> dispatchDraw(Canvas canvas) 方法：

    /**
     * {@inheritDoc}
     */
    @Override
    protected void dispatchDraw(Canvas canvas) {
        final int count = mChildrenCount;
        final View[] children = mChildren;
        int flags = mGroupFlags;
　　　　　// 關鍵字：FLAG_RUN_ANIMATION，FLAG_ANIMATION_CACHE，cache，buildCache 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);
        }
    }

從中可以看出這個FLAG_ANIMATION_CACHE的作用了，當然還和硬件加速扯上了關系，這里先不補充相關知識，想了解的可以度娘or谷歌。

附：對硬件加速帶源碼分析的比較好的一篇文章：

Android硬件加速繪制過程源碼分析(一)

Android硬件加速繪制過程源碼分析(二)——DisplayList錄制繪制操作

Android硬件加速繪制過程源碼分析(三)——DisplayList的繪制過程

Android硬件加速繪制過程源碼分析(四)——離屏硬件緩存HardwareLayer

2）View -> draw(Canvas canvas, ViewGroup parent, long drawingTime) 方法;

boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {
    boolean caching;
    ……………
    final int flags = parent.mGroupFlags;
    …………..
    if ((flags & ViewGroup.FLAG_CHILDREN_DRAWN_WITH_CACHE) != 0 ||
                (flags & ViewGroup.FLAG_ALWAYS_DRAWN_WITH_CACHE) != 0) {
            caching = true;
            // Auto-scaled apps are not hw-accelerated, no need to set scaling flag on DisplayList
            if (mAttachInfo != null) scalingRequired =         mAttachInfo.mScalingRequired;
        } else {
            caching = (layerType != LAYER_TYPE_NONE) || hardwareAccelerated;
        }
    …………..
    if (caching) {
            if (!hardwareAccelerated) {
                if (layerType != LAYER_TYPE_NONE) {
                    layerType = LAYER_TYPE_SOFTWARE;
                    buildDrawingCache(true);
                }
                cache = getDrawingCache(true);
            } else {
                switch (layerType) {
                    case LAYER_TYPE_SOFTWARE:
                        if (useDisplayListProperties) {
                            hasDisplayList = canHaveDisplayList();
                        } else {
                            buildDrawingCache(true);
                            cache = getDrawingCache(true);
                        }
                        break;
                    case LAYER_TYPE_HARDWARE:
                        if (useDisplayListProperties) {
                            hasDisplayList = canHaveDisplayList();
                        }
                        break;
                    case LAYER_TYPE_NONE:
                        // Delay getting the display list until animation-driven alpha values are
                        // set up and possibly passed on to the view
                        hasDisplayList = canHaveDisplayList();
                        break;
                }
            }
        }
     …………………..

}    

從上面的代碼可以分析出來，如果不禁止繪圖緩存，那么每次繪制子View時都要更新緩存並且將緩存畫到畫布中。這無疑是多了一步，畫一個bitmap，animation需要不停的畫所以也就多了很多操作，但是這個緩存不是說是對繪制視圖的優化嘛，這個秘密就在View的invalidate中，當子View需要 invalidate時，事實上也是交給父布局去分發的。

    /**
     * This is where the invalidate() work actually happens. A full invalidate()
     * causes the drawing cache to be invalidated, but this function can be called with
     * invalidateCache set to false to skip that invalidation step for cases that do not
     * need it (for example, a component that remains at the same dimensions with the same
     * content).
     *
     * @param invalidateCache Whether the drawing cache for this view should be invalidated as
     * well. This is usually true for a full invalidate, but may be set to false if the
     * View's contents or dimensions have not changed.
　　　* 指示在視圖刷新時，是否也要刷新繪圖緩存，對於一個完全的刷新操作，比如視圖內容發生了變化，
　　　* 或者控件尺寸發生變化了，那么應該設置true，但是如果不是二者任何一個，則應該設置為false。
     */
    void invalidate(boolean invalidateCache) {
        if (skipInvalidate()) {
            return;
        }
        if ((mPrivateFlags & (PFLAG_DRAWN | PFLAG_HAS_BOUNDS)) == (PFLAG_DRAWN | PFLAG_HAS_BOUNDS) ||
                (invalidateCache && (mPrivateFlags & PFLAG_DRAWING_CACHE_VALID) == PFLAG_DRAWING_CACHE_VALID) ||
                (mPrivateFlags & PFLAG_INVALIDATED) != PFLAG_INVALIDATED || isOpaque() != mLastIsOpaque) {
            mLastIsOpaque = isOpaque();
            mPrivateFlags &= ~PFLAG_DRAWN;
            mPrivateFlags |= PFLAG_DIRTY;
            if (invalidateCache) {
                mPrivateFlags |= PFLAG_INVALIDATED;
                mPrivateFlags &= ~PFLAG_DRAWING_CACHE_VALID;
            }
            final AttachInfo ai = mAttachInfo;
            final ViewParent p = mParent;
            //noinspection PointlessBooleanExpression,ConstantConditions
            if (!HardwareRenderer.RENDER_DIRTY_REGIONS) {
                if (p != null && ai != null && ai.mHardwareAccelerated) {
                    // fast-track for GL-enabled applications; just invalidate the whole hierarchy
                    // with a null dirty rect, which tells the ViewAncestor to redraw everything
                    p.invalidateChild(this, null);
                    return;
                }
            }

            if (p != null && ai != null) {
                final Rect r = ai.mTmpInvalRect;
                r.set(0, 0, mRight - mLeft, mBottom - mTop);
                // Don't call invalidate -- we don't want to internally scroll
                // our own bounds
                p.invalidateChild(this, r);
            }
        }
    }

接着，咱們再看一個ViewGroup的方法，setPersistentDrawingCache(int drawingCacheToKeep)：

    /**
     * Indicates what types of drawing caches should be kept in memory after
     * they have been created.
     *
     * @see #getPersistentDrawingCache()
     * @see #setAnimationCacheEnabled(boolean)
     *
     * @param drawingCacheToKeep one or a combination of {@link #PERSISTENT_NO_CACHE},
     *        {@link #PERSISTENT_ANIMATION_CACHE}, {@link #PERSISTENT_SCROLLING_CACHE}
     *        and {@link #PERSISTENT_ALL_CACHES}
     */
    public void setPersistentDrawingCache(int drawingCacheToKeep) {
        mPersistentDrawingCache = drawingCacheToKeep & PERSISTENT_ALL_CACHES;
    }

這個方法的作用，便是控制繪圖緩存在被創建之后，什么時候使用。

方法的可用參數，系統提供了四個值：

1）PERSISTENT_ANIMATION_CACHE：動畫前不可用，動畫結束時可用，並保存此時的Cache。

2）PERSISTENT_SCROLLING_CACHE：滾動式不可用，滾動結束時可用，並保存此時的Cache。

3）PERSISTENT_ALL_CACHES：不管在什么時候，都是用緩存。

4）PERSISTENT_NO_CACHE：不適用緩存。

因此，你可以手動的控制AnimationDrawCache(在執行動畫前禁用，執行完畢后啟用)或者調用這個方法傳入適用於相應場景的參數值就可以自動實現控制了。

大概的明白了，有木有！其實我理解的也不深入，都是在別人分析的基礎上總結出來，希望對大家有用，也對自己有用。

Over！

 參考：

1）Android應用優化（2）View cache的優化

2）關於android ui的優化 view 的繪制速度

3）對View DrawingCache的理解

4）Android View animation - poor performance on big screens