Android View之DrawingCache的使用

Android View类中，有这样一组神奇的截图方法：

setDrawingCacheEnabled(boolean enabled)

buildDrawingCache()

getDrawingCache()

destroyDrawingCache()

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四个方法配合使用，让你可以获取任意View的Bitmap缓存，达到截图等作用。先来看一下这四个方法都代表什么：

1.setDrawingCacheEnabled(boolean enabled)：

/** *

Enables or disables the drawing cache. When the drawing cache is enabled, the next call * to {@link #getDrawingCache()} or {@link #buildDrawingCache()} will draw the view in a * bitmap. 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 {@link #getDrawingCache()} and draw it on screen if the returned bitmap is not * null.

*/

public void setDrawingCacheEnabled(boolean enabled) {

    mCachingFailed = false; setFlags(enabled ? DRAWING_CACHE_ENABLED : 0, DRAWING_CACHE_ENABLED);

}

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直译理解，就是一个控制方法，enable设为true则可以使用buildDrawingCache()与getDrawingCache()，反之则无法使用。默认为true。

2.buildDrawingCache()&&buildDrawingCache(boolean 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.

*/

public void buildDrawingCache(boolean autoScale) {

...//此处省略杂七杂八的一堆代码

}

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巴拉巴拉一堆注释，总之就是只要调用了这个方法，View就会生成一张自身的Bitmap图片。那么这个方法如何生成的，往下看真正的实现方法：

/**

*这才是真正的实现方法

*/

private void buildDrawingCacheImpl(boolean autoScale) {

    ...

    int width = mRight - mLeft;

    int height = mBottom - mTop;

    ...

    try {

        //重点：新建了一个跟View宽高一样的Bitmap

        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;

        }

    ...

    Canvas canvas;

    ...

    //重点：设置bitmap到 canvas中，准备绘制

    canvas = new Canvas(bitmap);

    ...

    //绘制

    dispatchDraw(canvas);

}

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上面代码省略了很多，这里只展示部分关键代码。总结一下，在buildDrawingCache()中，主要做了两件事情：

    1）创建了一个空的bitmap（宽高跟autoScale有关，如果autoScale为false，宽高则为View的宽高）

    2）使用Canvas将视图绘制到bitmap上

3.getDrawingCache()

4.destroyDrawingCache()

这两个方法没啥可说的了，顾名思义就不贴源码了。getDrawingCache()在没有检测到生成过bitmap的时候，会先调用一次buildDrawingCache()生成一张bitmap，如果检测到有的话，则返回之前生成的。

destroyDrawingCache()则是用来销毁之前生成的bitmap，释放资源。如果需要反复获取View的Bitmap视图，则需要配合destroyDrawingCache()方法，不然容易反复获取同一张bitmap。