当我们想截取某个视图成图片的时候，可以使用View提供的DrawingCache方式来快速简单的获取视图的缓存。具体如下：
View view = activity.getWindow().getDecorView();
view.setDrawingCacheEnabled(true);
view.buildDrawingCache();
Bitmap bitmap = Bitmap.createBitmap(view.getDrawingCache());
从方法名可以看出buildDrawingCache(boolean autoScale)方法是构建视图缓存的地方，实际上也就是调用的buildDrawingCacheImpl(boolean autoScale)方法。如下：
private void buildDrawingCacheImpl(boolean autoScale) {
mCachingFailed = false;
//获取视图大小信息
int width = mRight - mLeft;
int height = mBottom - mTop;
final AttachInfo attachInfo = mAttachInfo;
final boolean scalingRequired = attachInfo != null && attachInfo.mScalingRequired;
//根据缩放比调整大小
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;
//生成的Bitmap最终大小
final long projectedBitmapSize = width * height * (opaque && !use32BitCache ? 2 : 4);
//系统允许的Bitmap Cache的最大值
final long drawingCacheSize =
ViewConfiguration.get(mContext).getScaledMaximumDrawingCacheSize();
//判断大小不符合条件 清除Cache 缓存失败
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;
//根据设置的是否缩放换回不同的Bitmap 默认是false
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();
}
//将Bitmap设置给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);
drawAutofilledHighlight(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;
}
}
主要的逻辑在代码中有注释。总结的说就是
- 获取View视图的信息.宽，高，背景颜色，透明度，缓存所需大小
- 如果宽高值小于0，或者所需要的Cache大小超过系统限制的大小则直接清空Cache,缓存失败。
- 根据当前是否设置Scale获取不同的Bitmap,然后设置一堆Bitmap配置，最后将其设置给Canvas
- Canvas通过dispatchDraw或者draw方法绘制，因为之前通过Canvas.setBitmap方法将bitmap设置给了Canvas，所以Canvas所有的绘制元素最终都将绘制在bitmap上
根据第二点可知，如果某种情况下getDrawingCache = null，那么可能的原因是因为无法获取到正确的view 信息(由于没有进行measure、layout操作，无法得到正确的width\height)，所以直接返回null
你可能有多个地方需要进行截图，或许你会想写一个通用的方法，比如下面所示：
public static Bitmap getFullScreenImage(Activity activity) {
if (activity == null) {
return null;
}
View view = activity.getWindow().getDecorView();
view.setDrawingCacheEnabled(true);
view.buildDrawingCache();
Bitmap bitmap = Bitmap.createBitmap(view.getDrawingCache());
return bitmap;
}
那么你会“惊喜”的发现每次获取到的Bitmap都是一样的。其实源码的注释是已经有说明的。“Calling will not draw from the cache when * the cache is enabled”，那么为什么会这样呢，值得我们继续分析
所以说每次使用完记得设置setDrawingCacheEnabled(false);