android UI 优化系列之 创设RGB565的缓存

public void buildDrawingCache(boolean autoScale) {        if ((mPrivateFlags & DRAWING_CACHE_VALID) == 0 || (autoScale ?                (mDrawingCache == null || mDrawingCache.get() == null) :                (mUnscaledDrawingCache == null || mUnscaledDrawingCache.get() == null))) {            if (ViewDebug.TRACE_HIERARCHY) {                ViewDebug.trace(this, ViewDebug.HierarchyTraceType.BUILD_CACHE);            }            if (Config.DEBUG && ViewDebug.profileDrawing) {                EventLog.writeEvent(60002, hashCode());            }            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;            final boolean opaque = drawingCacheBackgroundColor != 0 ||                (mBGDrawable != null && mBGDrawable.getOpacity() == PixelFormat.OPAQUE);            if (width <= 0 || height <= 0 ||                    (width * height * (opaque ? 2 : 4) > // Projected bitmap size in bytes                            ViewConfiguration.get(mContext).getScaledMaximumDrawingCacheSize())) {                destroyDrawingCache();                return;            }            boolean clear = true;            Bitmap bitmap = autoScale ? (mDrawingCache == null ? null : mDrawingCache.get()) :                    (mUnscaledDrawingCache == null ? null : mUnscaledDrawingCache.get());            if (bitmap == null || bitmap.getWidth() != width || bitmap.getHeight() != height) {                Bitmap.Config quality;                if (!opaque) {                    switch (mViewFlags & DRAWING_CACHE_QUALITY_MASK) {                        case DRAWING_CACHE_QUALITY_AUTO:                            quality = Bitmap.Config.ARGB_8888;                            break;                        case DRAWING_CACHE_QUALITY_LOW:                            quality = Bitmap.Config.ARGB_4444;                            break;                        case DRAWING_CACHE_QUALITY_HIGH:                            quality = Bitmap.Config.ARGB_8888;                            break;                        default:                            quality = Bitmap.Config.ARGB_8888;                            break;                    }                } else {                    quality = Bitmap.Config.RGB_565;                }                // Try to cleanup memory                if (bitmap != null) bitmap.recycle();                try {                    bitmap = Bitmap.createBitmap(width, height, quality);                    bitmap.setDensity(getResources().getDisplayMetrics().densityDpi);                    if (autoScale) {                        mDrawingCache = new SoftReference<Bitmap>(bitmap);                    } else {                        mUnscaledDrawingCache = new SoftReference<Bitmap>(bitmap);                    }                } 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;                    }                    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 |= DRAWN;            // Fast path for layouts with no backgrounds            if ((mPrivateFlags & SKIP_DRAW) == SKIP_DRAW) {                if (ViewDebug.TRACE_HIERARCHY) {                    ViewDebug.trace(this, ViewDebug.HierarchyTraceType.DRAW);                }                mPrivateFlags &= ~DIRTY_MASK;                dispatchDraw(canvas);            } else {                draw(canvas);            }            canvas.restoreToCount(restoreCount);            if (attachInfo != null) {                // Restore the cached Canvas for our siblings                attachInfo.mCanvas = canvas;            }            mPrivateFlags |= DRAWING_CACHE_VALID;        }    }

final Bitmap cache = mContent.getDrawingCache();            if (cache != null) {             Config cfg = cache.getConfig();             Log.d(TAG, "----------------------- cache.getConfig() = " + cfg);?        }