解析View的getDrawingCache方法
1． View 的getDrawingCache方法
有时候需要将某个view的内容以图片的方式保存下来，感觉就和截图差不多，可以使用View 的getDrawingCache方法，返回一个Bitmap对象。
2． View的getDrawingCache的具体实现
查看View的getDrawingCache（）方法
/** * <p>Calling this method is equivalent to calling <code>getDrawingCache(false)</code>.</p> * * @return A non-scaled bitmap representing this view or null if cache is disabled. * * @see #getDrawingCache(boolean) */ public Bitmap getDrawingCache() { return getDrawingCache(false); }
看代码继续调用了getDrawingCache(false)方法，继续查看getDrawingCache(false)方法。
/** * <p>Returns the bitmap in which this view drawing is cached. The returned bitmap * is null when caching is disabled. If caching is enabled and the cache is not ready, * this method will create it. Calling {@link #draw(android.graphics.Canvas)} will not * draw from the cache when the cache is enabled. To benefit from the cache, you must * request the drawing cache by calling this method and draw it on screen if the * returned bitmap is not null.</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> * * @param autoScale Indicates whether the generated bitmap should be scaled based on * the current density of the screen when the application is in compatibility * mode. * * @return A bitmap representing this view or null if cache is disabled. * * @see #setDrawingCacheEnabled(boolean) * @see #isDrawingCacheEnabled() * @see #buildDrawingCache(boolean) * @see #destroyDrawingCache() */ public Bitmap getDrawingCache(boolean autoScale) { if ((mViewFlags & WILL_NOT_CACHE_DRAWING) == WILL_NOT_CACHE_DRAWING) { return null; } if ((mViewFlags & DRAWING_CACHE_ENABLED) == DRAWING_CACHE_ENABLED) { buildDrawingCache(autoScale); } return autoScale ? mDrawingCache : mUnscaledDrawingCache; }
查看getDrawingCache(false)方法，如果该视图的标志是WILL_NOT_CACHE_DEAWING（表示该view没有任何绘图缓存）则直接返回null，如果视图的标志是DRWING_CACHE_ENABLED（表示该view将自己的绘图缓存成一个bitmap），则调用buildDrawingCache(autoScale)方法。
因为传递过来的autoScale为false，则返回的Bitmap是mUnscaledDrawingCache。
查看buildDrawingCache(autoScale)方法：
/** * <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)) { if (Trace.isTagEnabled(Trace.TRACE_TAG_VIEW)) { Trace.traceBegin(Trace.TRACE_TAG_VIEW, "buildDrawingCache/SW Layer for " + getClass().getSimpleName()); } try { buildDrawingCacheImpl(autoScale); } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); } } }
如果mPrivateFlags与PFLAG_DRAWING_CACHE_VALID与运算为0，或者mUnscaledDrawingCache为null，则调用buildDrawingCacheImpl(autoScale)方法。
查看buildDrawingCacheImpl(autoScale)方法,
/** * private, internal implementation of buildDrawingCache, used to enable tracing */ private void buildDrawingCacheImpl(boolean autoScale) { 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; 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, getClass().getSimpleName() + " not displayed because it is" + " too large to fit into a software layer (or 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; 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; } }
代码中AttachInfo类是连接到他的父window时给view的一组信息。
参数autoScale的值为false，可以忽略一些if判断语句。
方法分为三步：
第一步：得到view的宽width与高height。
int width = mRight - mLeft; int height = mBottom - mTop;
第二步，生成bitmap。
bitmap = Bitmap.createBitmap(mResources.getDisplayMetrics(),width, height, quality);
bitmap.setDensity(getResources().getDisplayMetrics().densityDpi);
use32BitCache数据代表的是view是否需要使用32-bit绘图缓存，当window为半透明的时候，使用32位绘图缓存。
第三步，绘制canvas。
canvas.setBitmap(bitmap); canvas = new Canvas(bitmap); draw(canvas);
3． 自定义view的getDrawingCache方法
有时候自定义的view虽然继承View，但是调用View的getDrawingCache()方法的时候会出现一些问题，返回的bitmap为null，这个时候就需要自己写一个getDrawingCache方法，可以参考buildDrawingCacheImpl方法去实现，实现如下：
public Bitmap getBitmap() { Bitmap bitmap = null; int width = getRight() - getLeft(); int height = getBottom() - getTop(); final boolean opaque = getDrawingCacheBackgroundColor() != 0 || isOpaque(); Bitmap.Config quality; if (!opaque) { switch (getDrawingCacheQuality()) { case DRAWING_CACHE_QUALITY_AUTO: case DRAWING_CACHE_QUALITY_LOW: case DRAWING_CACHE_QUALITY_HIGH: default: quality = Bitmap.Config.ARGB_8888; break; } } else { quality = Bitmap.Config.RGB_565; } if (opaque) bitmap.setHasAlpha(false); bitmap = Bitmap.createBitmap(getResources().getDisplayMetrics(), width, height, quality); bitmap.setDensity(getResources().getDisplayMetrics().densityDpi); boolean clear = getDrawingCacheBackgroundColor() != 0; Canvas canvas = new Canvas(bitmap); if (clear) { bitmap.eraseColor(getDrawingCacheBackgroundColor()); } computeScroll(); final int restoreCount = canvas.save(); canvas.translate(-getScrollX(), -getScrollY()); draw(canvas); canvas.restoreToCount(restoreCount); canvas.setBitmap(null); return bitmap; }
