ScrollView嵌套子View的getDrawingCache为空的解决方法
问题
将组件的显示布局改为可以滚动的，然后用ScrollView作为了View的父类，发现View的getDrawingCache返回为null了，组件的滚动是必须要实现的，所以探究一下getDrawingCache为空的解决方法。
问题原因
查看日志可以发现
`02-01 14:21:33.512 3461-3461/com.example.sample.job W/View: View too large to fit into drawing cache, needs 9338145 bytes, only 3686400 available
这条信息，意思是我的view太大了不能放入图像缓存中。
考虑到之前没有ScrollView的时候，并没有出现这个错误，所以问题是出在ScrollView嵌套了View上，这样的布局结构导致在使用getDrawingCache()时，系统因缓存超量直接取消了写入操作，drawingCache中成为了空值。
通常，使用getDrawngCache()方法获取控件截图的代码是：
public Bitmap getBitmap(View view) {
view.setDrawingCacheEnabled(true);
view.buildDrawingCache();
Bitmap bitmap = Bitmap.createBitmap(view.getDrawingCache(), 0, 0, view.getMeasuredWidth(), view.getMeasuredHeight() - view.getPaddingBottom());
view.setDrawingCacheEnabled(false);
view.destroyDrawingCache();
return bitmap;
}
查看一下getDrawingCache的实现：
/**
* <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);
}
发现getDrawingCache()方法调用了getDrawingCache(false)方法，接着查看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)没有看到有用的信息，所以继续查看调用的方法buildDrawingCache(autoScale)：
/**
* <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);
}
}
}
查看buildDrawingCache(boolean autoScale)方法也没有看到有用的信息，所以查看调用的方法buildDrawingCacheImpl(boolean autoScale)：
/**
* 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;
}
}
在这个方法中，我们可以看到我们的日志报错的内容
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;
}
可以看到报错的原因是projectedBitmapSize > drawingCacheSize，可以看到这两个值得计算：
final long projectedBitmapSize = width * height * (opaque && !use32BitCache ? 2 : 4);
final long drawingCacheSize =
ViewConfiguration.get(mContext).getScaledMaximumDrawingCacheSize();
从这两个变量的计算上看，drawingCacheSize是当前设备系统所允许的最大绘制缓存值，一个固定的值，问题可能就是projectedBitmapSize的值有问题，而(opaque && !use32BitCache ? 2 : 4)不可能为0，而width是屏幕的宽，没有问题，那么就是height的值出现了异常，所以，接着查看视图高度的计算代码：
/**
* Return the height of your view.
*
* @return The height of your view, in pixels.
*/
@ViewDebug.ExportedProperty(category = "layout")
public final int getHeight() {
return mBottom - mTop;
}
从代码可以看到height的值是mBottom - mTop，其中mBottom指的是视图自身的底边到父视图顶边的距离，mTop指的是视图自身的顶边到父视图顶边的距离。如果对子视图设置layout_height="match_parent"，子视图充满父视图，那么getHeight()实际就可以看做是父视图的高。但是在ScrollView中不同，设置它的子View为match_parent是没用的（并且一般设置的都是wrap_content），ScrollView的高度会随着子View的高度变化而变化，而且意味着getHeight()返回的不再是屏幕可见范围的高度，而是整个加载出来的实际界面内容高度。ScrollView的子控件执行的截屏操作会将用户未看到的图片一并截取。
所以，使用ScrollView嵌套的子View，采用buildDrawingCache方法进行截图时，getHeight()获得的数值远大于当前屏幕高度。所以projectedBitmapSize > drawingCacheSize的判断正确，报出了错误。
解决问题
既然ScrollView嵌套的子View，使用getDrawingCache()截图失败，那就换成使用canvas直接复制视图内容的方式实现：
public Bitmap getBitmapByCanvas(View view) {
Bitmap bitmap = Bitmap.createBitmap(view.getWidth(), view.getHeight(), Bitmap.Config.ARGB_8888);
Canvas canvas = new Canvas(bitmap);
if (Build.VERSION.SDK_INT >= 11) {
view.measure(
View.MeasureSpec.makeMeasureSpec(
view.getWidth(), View.MeasureSpec.EXACTLY),
View.MeasureSpec.makeMeasureSpec(
view.getHeight(), View.MeasureSpec.EXACTLY)
);
} else {
view.measure(
View.MeasureSpec.makeMeasureSpec(
0, View.MeasureSpec.UNSPECIFIED),
View.MeasureSpec.makeMeasureSpec(
0, View.MeasureSpec.UNSPECIFIED)
);
view.layout(0, 0, view.getMeasuredWidth(), view.getMeasuredHeight());
}
view.draw(canvas);
return bitmap;
}
问题就这样解决了。