getDrawingCache的问题。

[xingshi310]

每次想把View装换为Bitmap的时候都会调用函数getDrawingCache，但是一般返回的Bitmap都是ARGB8888的，解析了一下getDrawingCache的代码，是可以返回RGB565的Bitmap的，但是如何能够返回这样的图片不是很清楚，有达人能够给解释一下的么？

[fontlose]

参考这个
http://www.bangchui.org/read.php?tid=8458
按上面的方法设置了setDrawingCacheBackgroundColor(0xffff0000)
我试了下去layout的Cache返回的格式是565的了但背景成红色了。

[fontlose]

  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 || isOpaque();
            final boolean use32BitCache = attachInfo != null && attachInfo.mUse32BitDrawingCache;

            if (width <= 0 || height <= 0 ||
                     // Projected bitmap size in bytes
                    (width * height * (opaque && !use32BitCache ? 2 : 4) >
                            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 {
                    // 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(width, height, quality);
                    bitmap.setDensity(getResources().getDisplayMetrics().densityDpi);
                    if (autoScale) {
                        mDrawingCache = new SoftReference<Bitmap>(bitmap);
                    } else {
                        mUnscaledDrawingCache = new SoftReference<Bitmap>(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;
                    }
                    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;
            mPrivateFlags |= DRAWING_CACHE_VALID;

            // 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;
            }
        }
    }

[fontlose]

  final boolean opaque = drawingCacheBackgroundColor != 0 || isOpaque();

实现565格式，
1）只要drawingCacheBackgroundColor非零
setDrawingCacheBackgroundColor(1);
setDrawingCacheEnabled(true) ;也可以实现565格式。
2）isOpaque();
setBackgroundColor(0xFF000000) ;
setDrawingCacheEnabled(true) ;也可以实现565格式

[ch_984326013]

[Quote=引用 1 楼 yiyaaixuexi 的回复:]
View.buildDrawingCache为自己的view 建立相应的缓存，实际上就是一个Bitmap对象。
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.Co……
[/Quote]
up+
学习了

[xingshi310]

这个方法是不行的，真正的代码里面有两个判断，就算是把背景颜色改了也不能把图片改成565

[念茜]

View.buildDrawingCache为自己的view 建立相应的缓存，实际上就是一个Bitmap对象。
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.
Bitmap.Config.RGB_565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。


在用buildDrawingCache建立缓存时， 可以使用RGB_565格式
用 setDrawingCacheBackgroundColor(0xffff0000)设置为 非默认颜色后，建立的缓存就是rgb565了。