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android UI 优化系列之 创建RGB565的缓存

2011-11-23  windli笔记

关于如何优化activity的启动速度, view 的绘制速度, 可参考这个sdk里的文档。 android-sdk-windows-1.5_r1/docs/resources/articles/window-bg-speed.html。
看完后你就知道 android:windowBackground 太重要了,影响到绘制效率。
这里要说的是另外一点, 不是这个windowBackground 。
android 为了提高滚动等各方面的绘制速度,可以为每一个view建立一个缓存,使用 View.buildDrawingCache为自己的view 建立相应的缓存,
这个所谓的缓存,实际上就是一个Bitmap对象。只是 这个 bitmap 对象可以有多种格式而已,如
     Bitmap.Config.ARGB_8888;
     Bitmap.Config.ARGB_4444;
     Bitmap.Config.ARGB_8888;
     Bitmap.Config.ARGB_8888;
     Bitmap.Config.RGB_565;
   默认的格式是Bitmap.Config.ARGB_8888.,但大多数嵌入式设备使用的显示格式都是Bitmap.Config.RGB_565. 对于后者, 并没有
alpha 值,所以绘制的时候不需要计算alpha合成,速递当让快些。其次,RGB_565可以直接使用优化了的memcopy函数,效率相对高出许多。


  所以, 在用buildDrawingCache建立缓存时, 可以使用RGB_565格式。但是如何制定这个格式呢 ?buildDrawingCache有两个版本,     buildDrawingCache(boolean) buildDrawingCache() 。并没有任何参数可以设置rgb格式,看看源码先:

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;
        }
    }

看完后明白了,至少跟两个因素有关 drawingCacheBackgroundColor 和 mBGDrawable.
用setDrawingCacheBackgroundColor(0xffff0000)设置为 非默认颜色后,建立的缓存就是rgb565了,可以用下列方法验证一下:

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

        }

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