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：

    /**
     * 
Calling this method is equivalent to calling getDrawingCache(false).
     *
     * @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：

    /**
     * 
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.
     *
     * 
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.
     *
     * @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)：

    /**
     * 
Forces the drawing cache to be built if the drawing cache is invalid.
     *
     * 
If you call {@link #buildDrawingCache()} manually without calling
     * {@link #setDrawingCacheEnabled(boolean) setDrawingCacheEnabled(true)}, you
     * should cleanup the cache by calling {@link #destroyDrawingCache()} afterwards.
     *
     * 
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.
     *
     * 
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.
     *
     * @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  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  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。

参考文章

https://blog.csdn.net/lz8362/article/details/79284492