view.getDrawingCache() 为 null

App开发功能之一 – 截图：

代码如下:

view.destroyDrawingCache();
view.setDrawingCacheEnabled(true);
view.buildDrawingCache();
Bitmap map = view.getDrawingCache();

但是在部分机型上总是会有 map 为 null 的情况发生。

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日志分析：

View: LinearLayout not displayed because it is too large to fit into a software layer (or drawing cache), needs 15223680 bytes, only 8294400 available

貌似是触发了系统的某些限制，说是控件太大了。当然也确实是比较长的一个控件。

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源码分析：

view.buildDrawingCache() -> 
buildDrawingCache(false) -> 
buildDrawingCacheImpl(autoScale)

在 buildDrawingCacheImpl 中：

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;
}

width / height 在View正常显示的情况下都不为0，那么就是 projectedBitmapSize > drawingCacheSize 这个条件件被触发了：

final long projectedBitmapSize = 
                width * height * (opaque && !use32BitCache ? 2 : 4);
final long drawingCacheSize =
                ViewConfiguration.get(mContext).getScaledMaximumDrawingCacheSize();

ViewConfiguration.get(mContext).getScaledMaximumDrawingCacheSize() 得到的是系统所提供的最大的DrawingCache的值。

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从上面的分析中可以看到View过大，超出了系统DrawingCache的限制导致了view.buildDrawingCache() 方法没有生效，最终 Bitmap map = view.getDrawingCache() 中map为null。

解决方案：

public static BitmapWithHeight getSimpleViewToBitmap(final View view, int width) throws OutOfMemoryError {
        if (view.getWidth() <= 0 || view.getHeight() <= 0) {
            view.measure(View.MeasureSpec.makeMeasureSpec(width, View.MeasureSpec.EXACTLY),
                    View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED));
            view.layout(0, 0, view.getMeasuredWidth(), view.getMeasuredHeight());
        }
        view.destroyDrawingCache();
        view.setDrawingCacheEnabled(true);
        view.buildDrawingCache();
        Bitmap map = view.getDrawingCache();
        if (view.getHeight() > 0 && map == null)
            map = getViewBitmap(view);
        return new BitmapWithHeight(map, view.getMeasuredWidth(), view.getMeasuredHeight());
    }


    public static Bitmap getViewBitmap(View view) {
        Bitmap bitmap;
        if (view.getWidth() > 0 && view.getHeight() > 0)
            bitmap = Bitmap.createBitmap(view.getWidth(), view.getHeight(), Bitmap.Config.ARGB_8888);
        else if (view.getMeasuredWidth() > 0 && view.getMeasuredHeight() > 0)
            bitmap = Bitmap.createBitmap(view.getMeasuredWidth(), view.getMeasuredHeight(), Bitmap.Config.ARGB_8888);
        else
            bitmap = Bitmap.createBitmap(com.blankj.utilcode.utils.ScreenUtils.getScreenWidth(), com.blankj.utilcode.utils.ScreenUtils.getScreenHeight() * 2, Bitmap.Config.ARGB_8888);
        Canvas canvas = new Canvas(bitmap);
        view.draw(canvas);
        return bitmap;
    }