学步园

这篇文章其实也可以起另外一个标题：android 如何高效的展示图片
原文地址：http://developer.android.com/training/displaying-bitmaps/index.html

学习和使用常见的技术去处理和加载图片，能让你的用户界面快速响应，并且能避免动不动就超出了受限内存。可能你一不小心，就会导致内存消耗完毕从而crash
,抛出java.lang.OutofMemoryError: bitmap size exceeds VM budget，在应用程序中加载图片是个棘手的问题，主要包括以下几个原因：

1.移动设备通常有约束的系统资源，android 设备中可能在一个应用里只有16M的可用内存。Android兼容定义文档(CDD),3.7节，指出了在
不同屏幕大小和密度所需的最小应用程序内存。应用程序应该优化过并且在这个最小的内存限制之下执行。当然，请记住许多设备配置更高的限制。

2.图片需要消耗大量的内存，尤其是丰富的图像比如照片。例如,摄像头拍照的Galaxy Nexus 2592 x1936像素(像素)。
如果位图配置使用ARGB 8888(默认从Android 2.3起)然后加载这个图像到内存需要大约19 mb的内存(2592 * 1936 * 4字节),在一些设备会立即消耗完每个应用拥有的内存。



3.android App 的UI 可能需要频繁的将多个图片一次性加载 ,比如 ListView,GridView,ViewPager 一般是在屏幕上包含多张图片，同时在屏幕之外也可能即将需要展示（也许就在你滑动的一瞬间就显示了）。


内容分为五个部分，分别是：
1，高效的加载大图片：在不会超过单个应用限制内存这一条例下进行decode bitmap.
2, 异步加载图片：通过AsyncTask 异步处理图片，同时也提一下如何处理并发的情况。
3. 缓存图片：通过内存和磁盘缓存，来提高UI的响应速度和流程性。
4. 内存管理：如何管理位图内存来最大化你的应用程序的性能。
4. 在你的UI上展示图片：用例子说明如何在ViewPager和GridView中 使用后台线程来和图片缓存来加载图片。



一、高效的加载大图片
在decode一张Bitmap之前，先检查这张图片的尺寸（除非你明确的知道这张图片的尺寸，并且确定一定不会产生内存溢出），可以设置BitmapFactory.Options对象options的属性inJustDecodeBounds为true,因为他不会去开辟内存生成一张图片，却能够知道图片的宽度和高度 


BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeResource(getResources(), R.id.myimage, options);
int imageHeight = options.outHeight;
int imageWidth = options.outWidth;
String imageType = options.outMimeType;
在知道宽度和高度后，可根据需要生成采样率(说明一点：如果生成的采样率是2的幂,如2,4,8,16...那么解码一张图片将会更快更高效)
public static int calculateInSampleSize(
            BitmapFactory.Options options, int reqWidth, int reqHeight) {
    // Raw height and width of image
    final int height = options.outHeight;
    final int width = options.outWidth;
    int inSampleSize = 1;

    if (height > reqHeight || width > reqWidth) {

        // Calculate ratios of height and width to requested height and width
        final int heightRatio = Math.round((float) height / (float) reqHeight);
        final int widthRatio = Math.round((float) width / (float) reqWidth);

        // Choose the smallest ratio as inSampleSize value, this will guarantee
        // a final image with both dimensions larger than or equal to the
        // requested height and width.
        inSampleSize = heightRatio < widthRatio ? heightRatio : widthRatio;
    }

    return inSampleSize;
}
在知道采样率后，就可以生成图片了（这时要设置inSampleSize=获取的采样率，inJustDecodeBounds=false）
public static Bitmap decodeSampledBitmapFromResource(Resources res, int resId,
        int reqWidth, int reqHeight) {

    // First decode with inJustDecodeBounds=true to check dimensions
    final BitmapFactory.Options options = new BitmapFactory.Options();
    options.inJustDecodeBounds = true;
    BitmapFactory.decodeResource(res, resId, options);

    // Calculate inSampleSize
    options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);

    // Decode bitmap with inSampleSize set
    options.inJustDecodeBounds = false;
    return BitmapFactory.decodeResource(res, resId, options);
}
在获取图片后，就可以设置UI组件的图片相关属性（这里是举例，通常在UI中需要异步回调进行设置，而不是直接在UI线程中设置，如果需要在SD卡中读取，或者网络读取的话，会因为耗时导致阻塞UI线程，从而产出ANR）：
mImageView.setImageBitmap(decodeSampledBitmapFromResource(getResources(), R.id.myimage, 100, 100));

二、异步加载图片
class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> {
    private final WeakReference<ImageView> imageViewReference;
    private int data = 0;

    public BitmapWorkerTask(ImageView imageView) {
        // Use a WeakReference to ensure the ImageView can be garbage collected
        imageViewReference = new WeakReference<ImageView>(imageView);
    }

    // Decode image in background.
    @Override
    protected Bitmap doInBackground(Integer... params) {
        data = params[0];
        return decodeSampledBitmapFromResource(getResources(), data, 100, 100));
    }

    // Once complete, see if ImageView is still around and set bitmap.
    @Override
    protected void onPostExecute(Bitmap bitmap) {
        if (imageViewReference != null && bitmap != null) {
            final ImageView imageView = imageViewReference.get();
            if (imageView != null) {
                imageView.setImageBitmap(bitmap);
            }
        }
    }
}
注意：这里的ImageView 使用弱引用的目的 是为了确保AsyncTask不会阻止ImageView或者它引用的资源被系统回收
所以要异步加载一张图片很简单了，调用如下即可：
public void loadBitmap(int resId, ImageView imageView) {
    BitmapWorkerTask task = new BitmapWorkerTask(imageView);
    task.execute(resId);
}
上面主要是针对普通的View ,但是ListView,GridView,ViewPage等这些滚动时有重用自己的child view又该怎么办呢？因为如果每一个child View 都触发一个AsyncTask的话，就无法
保证一种情况：AsyncTask已经完成，但与之相关连的child View却没有被回收，转而被重用了，你这样设置的话，显示的图片不是会错了么。。。就是顺序无法保证一致。
这种情况下的解决方案是：ImageView存储一个最近的AsyncTask的引用，并且在完成的时候再次判断一下，不就可以了吗！


仿照AsyncTask存储一个ImageView软应用的方法，我们可以自定义一个Drawable,也存储一个AsyncTask的软应用，使用了相同的思想


static class AsyncDrawable extends BitmapDrawable {
    private final WeakReference<BitmapWorkerTask> bitmapWorkerTaskReference;

    public AsyncDrawable(Resources res, Bitmap bitmap,
            BitmapWorkerTask bitmapWorkerTask) {
        super(res, bitmap);
        bitmapWorkerTaskReference =
            new WeakReference<BitmapWorkerTask>(bitmapWorkerTask);
    }

    public BitmapWorkerTask getBitmapWorkerTask() {
        return bitmapWorkerTaskReference.get();
    }
}

在执行AsyncTask之前,我们new AsyncDrawable 绑定到ImageView

public void loadBitmap(int resId, ImageView imageView) {
    if (cancelPotentialWork(resId, imageView)) {
        final BitmapWorkerTask task = new BitmapWorkerTask(imageView);
        final AsyncDrawable asyncDrawable =
                new AsyncDrawable(getResources(), mPlaceHolderBitmap, task);
        imageView.setImageDrawable(asyncDrawable);
        task.execute(resId);
    }
}

绑定之前，先清掉以前的Task即可：

public static boolean cancelPotentialWork(int data, ImageView imageView) {
    final BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(imageView);

    if (bitmapWorkerTask != null) {
        final int bitmapData = bitmapWorkerTask.data;
        if (bitmapData != data) {
            // Cancel previous task
            bitmapWorkerTask.cancel(true);
        } else {
            // The same work is already in progress
            return false;
        }
    }
    // No task associated with the ImageView, or an existing task was cancelled
    return true;
}
private static BitmapWorkerTask getBitmapWorkerTask(ImageView imageView) {
   if (imageView != null) {
       final Drawable drawable = imageView.getDrawable();
       if (drawable instanceof AsyncDrawable) {
           final AsyncDrawable asyncDrawable = (AsyncDrawable) drawable;
           return asyncDrawable.getBitmapWorkerTask();
       }
    }
    return null;
}
所以在你的BitmapWorkerTask 中onPostExecute() 需要再次检查当前的task是不是ImageView相匹配的Task


class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> {
    ...

    @Override
    protected void onPostExecute(Bitmap bitmap) {
        if (isCancelled()) {
            bitmap = null;
        }

        if (imageViewReference != null && bitmap != null) {
            final ImageView imageView = imageViewReference.get();
            final BitmapWorkerTask bitmapWorkerTask =
                    getBitmapWorkerTask(imageView);
            if (this == bitmapWorkerTask && imageView != null) {
                imageView.setImageBitmap(bitmap);
            }
        }
    }
}
这个时候你就可以在ListView,GridView的getView()方法中执行loadBitmap方法即可了。


三、缓存图片


a.内存缓存。就是使用我们常说的LRU进行缓存了。
这里注意一点：在过去，很流行使用软应用和弱应用来缓存图片，但是现在已经不推荐使用了，因为从Android 2.3(API级别9)以后，垃圾收集器是更积极收集软/弱引用，这使得它们相当无效，

Android 3.0(API级别11) 之前，支持数据的位图存储在本地内存（而不是虚拟机内存），并且不是显示方式释放,可能导致应用程序超过其内存限制和崩溃。

如何为LruCahce选定一个合适的大小，需要考虑一系列的因素，如：
1.内存是如何加强你的activity/application.
2.有多少图片需要马上显示，有多少图片准备显示。
3.设备的尺寸和密度是什么，高密度设备在缓存相同数量的图片需要的更大的缓存
4.图片的尺寸和配置是什么，需要消耗多少的内存。
5.你是否访问频繁？如果频繁访问，你可能需要将某些图片内存常驻，或者使用多个LruCache(不同场合不同大小不同定义)
6.需要平衡数量与质量。
所以没有一个特定的大小或者适合所有的解决方案，得自己去分析APP,综合想出一个解决方案，缓存太小，导致额外的开销，缓存太大，容易再次使内存溢出或者留下很少的内存供给其它的用途。

这里举个例子：

private LruCache<String, Bitmap> mMemoryCache;

@Override
protected void onCreate(Bundle savedInstanceState) {
    ...
    // Get max available VM memory, exceeding this amount will throw an
    // OutOfMemory exception. Stored in kilobytes as LruCache takes an
    // int in its constructor.
    final int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024);

    // Use 1/8th of the available memory for this memory cache.
    final int cacheSize = maxMemory / 8;

    mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {
        @Override
        protected int sizeOf(String key, Bitmap bitmap) {
            // The cache size will be measured in kilobytes rather than
            // number of items.
            return bitmap.getByteCount() / 1024;
        }
    };
    ...
}

public void addBitmapToMemoryCache(String key, Bitmap bitmap) {
    if (getBitmapFromMemCache(key) == null) {
        mMemoryCache.put(key, bitmap);
    }
}

public Bitmap getBitmapFromMemCache(String key) {
    return mMemoryCache.get(key);
}

所以之前的loadBitmap方法，使用LRUCache,就可以先check一下：

public void loadBitmap(int resId, ImageView imageView) {
    final String imageKey = String.valueOf(resId);

    final Bitmap bitmap = getBitmapFromMemCache(imageKey);
    if (bitmap != null) {
        mImageView.setImageBitmap(bitmap);
    } else {
        mImageView.setImageResource(R.drawable.image_placeholder);
        BitmapWorkerTask task = new BitmapWorkerTask(mImageView);
        task.execute(resId);
    }
}
同时BitmapWorkerTask也需要更新LRUCache:

class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> {
    ...
    // Decode image in background.
    @Override
    protected Bitmap doInBackground(Integer... params) {
        final Bitmap bitmap = decodeSampledBitmapFromResource(
                getResources(), params[0], 100, 100));
        addBitmapToMemoryCache(String.valueOf(params[0]), bitmap);
        return bitmap;
    }
    ...
}



b.DISK卡缓存
注意：如果访问特别的频繁，ContentProvider可能是一个合适的地方存储缓存的图片。
DiskLruCache的例子：

private DiskLruCache mDiskLruCache;
private final Object mDiskCacheLock = new Object();
private boolean mDiskCacheStarting = true;
private static final int DISK_CACHE_SIZE = 1024 * 1024 * 10; // 10MB
private static final String DISK_CACHE_SUBDIR = "thumbnails";

@Override
protected void onCreate(Bundle savedInstanceState) {
    ...
    // Initialize memory cache
    ...
    // Initialize disk cache on background thread
    File cacheDir = getDiskCacheDir(this, DISK_CACHE_SUBDIR);
    new InitDiskCacheTask().execute(cacheDir);
    ...
}

class InitDiskCacheTask extends AsyncTask<File, Void, Void> {
    @Override
    protected Void doInBackground(File... params) {
        synchronized (mDiskCacheLock) {
            File cacheDir = params[0];
            mDiskLruCache = DiskLruCache.open(cacheDir, DISK_CACHE_SIZE);
            mDiskCacheStarting = false; // Finished initialization
            mDiskCacheLock.notifyAll(); // Wake any waiting threads
        }
        return null;
    }
}

class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> {
    ...
    // Decode image in background.
    @Override
    protected Bitmap doInBackground(Integer... params) {
        final String imageKey = String.valueOf(params[0]);

        // Check disk cache in background thread
        Bitmap bitmap = getBitmapFromDiskCache(imageKey);

        if (bitmap == null) { // Not found in disk cache
            // Process as normal
            final Bitmap bitmap = decodeSampledBitmapFromResource(
                    getResources(), params[0], 100, 100));
        }

        // Add final bitmap to caches
        addBitmapToCache(imageKey, bitmap);

        return bitmap;
    }
    ...
}

public void addBitmapToCache(String key, Bitmap bitmap) {
    // Add to memory cache as before
    if (getBitmapFromMemCache(key) == null) {
        mMemoryCache.put(key, bitmap);
    }

    // Also add to disk cache
    synchronized (mDiskCacheLock) {
        if (mDiskLruCache != null && mDiskLruCache.get(key) == null) {
            mDiskLruCache.put(key, bitmap);
        }
    }
}

public Bitmap getBitmapFromDiskCache(String key) {
    synchronized (mDiskCacheLock) {
        // Wait while disk cache is started from background thread
        while (mDiskCacheStarting) {
            try {
                mDiskCacheLock.wait();
            } catch (InterruptedException e) {}
        }
        if (mDiskLruCache != null) {
            return mDiskLruCache.get(key);
        }
    }
    return null;
}

// Creates a unique subdirectory of the designated app cache directory. Tries to use external
// but if not mounted, falls back on internal storage.
public static File getDiskCacheDir(Context context, String uniqueName) {
    // Check if media is mounted or storage is built-in, if so, try and use external cache dir
    // otherwise use internal cache dir
    final String cachePath =
            Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState()) ||
                    !isExternalStorageRemovable() ? getExternalCacheDir(context).getPath() :
                            context.getCacheDir().getPath();

    return new File(cachePath + File.separator + uniqueName);
}
内存缓存检查在UI线程中，Disk卡缓存检查在非UI线程中，但是图片完成后，内存缓存和Disk缓存都需要添加。


举个例子,当Configuration 改变时候，如横竖屏切换。

private LruCache<String, Bitmap> mMemoryCache;

@Override
protected void onCreate(Bundle savedInstanceState) {
    ...
    RetainFragment retainFragment =
            RetainFragment.findOrCreateRetainFragment(getFragmentManager());
    mMemoryCache = retainFragment.mRetainedCache;
    if (mMemoryCache == null) {
        mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {
            ... // Initialize cache here as usual
        }
        retainFragment.mRetainedCache = mMemoryCache;
    }
    ...
}

class RetainFragment extends Fragment {
    private static final String TAG = "RetainFragment";
    public LruCache<String, Bitmap> mRetainedCache;

    public RetainFragment() {}

    public static RetainFragment findOrCreateRetainFragment(FragmentManager fm) {
        RetainFragment fragment = (RetainFragment) fm.findFragmentByTag(TAG);
        if (fragment == null) {
            fragment = new RetainFragment();
        }
        return fragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setRetainInstance(true);
    }
}
四、内存管理
主要讲的是促进垃圾回收期回收，和使图片重用。
先讲下关于版本的一些变化知识：



1.在Android上Android 2.2(API级别8)和低版本时,当垃圾收集发生时,您的应用程序的线程被暂停，这导致滞后,降低性能。Android 2.3增加了并发垃圾收集,这意味着内存位图不再被引用不久后就会被回收。
2.在安卓2.3.3(API级别10)和低版本,位图像素数据存储在Native内存。它是独立于位图本身（存储在Dalvik堆）。像素数据在Native内存不是显式的方式释放,可能导致应用程序超过其内存限制和崩溃。在Android 3.0(API级别11),像素数据连同相关的位图存储在Dalvik堆。




2.3.3以及以前的优化：
通过引用计数的方式来判断图片是否可以回收了。

private int mCacheRefCount = 0;
private int mDisplayRefCount = 0;
...
// Notify the drawable that the displayed state has changed.
// Keep a count to determine when the drawable is no longer displayed.
public void setIsDisplayed(boolean isDisplayed) {
    synchronized (this) {
        if (isDisplayed) {
            mDisplayRefCount++;
            mHasBeenDisplayed = true;
        } else {
            mDisplayRefCount--;
        }
    }
    // Check to see if recycle() can be called.
    checkState();
}

// Notify the drawable that the cache state has changed.
// Keep a count to determine when the drawable is no longer being cached.
public void setIsCached(boolean isCached) {
    synchronized (this) {
        if (isCached) {
            mCacheRefCount++;
        } else {
            mCacheRefCount--;
        }
    }
    // Check to see if recycle() can be called.
    checkState();
}

private synchronized void checkState() {
    // If the drawable cache and display ref counts = 0, and this drawable
    // has been displayed, then recycle.
    if (mCacheRefCount <= 0 && mDisplayRefCount <= 0 && mHasBeenDisplayed
            && hasValidBitmap()) {
        getBitmap().recycle();
    }
}

private synchronized boolean hasValidBitmap() {
    Bitmap bitmap = getBitmap();
    return bitmap != null && !bitmap.isRecycled();
}
3.0以及以后版本的优化：

3.0以后引进了BitmapFactory.Options.inBitmap 这个属性，如果option设置了这个属性的话，当load一张图片的时候，它将尝试去复用一张已经存在的图片：
就是复用之前那种图片的内存，而不用频繁的去开辟/回收内存，从而提高了效率。
当然是有条件的：复用图片的大小必须和新生成的图片大小一致（确保所占用的内存一致）

HashSet<SoftReference<Bitmap>> mReusableBitmaps;
private LruCache<String, BitmapDrawable> mMemoryCache;

// If you're running on Honeycomb or newer, create
// a HashSet of references to reusable bitmaps.
if (Utils.hasHoneycomb()) {
    mReusableBitmaps = new HashSet<SoftReference<Bitmap>>();
}

mMemoryCache = new LruCache<String, BitmapDrawable>(mCacheParams.memCacheSize) {

    // Notify the removed entry that is no longer being cached.
    @Override
    protected void entryRemoved(boolean evicted, String key,
            BitmapDrawable oldValue, BitmapDrawable newValue) {
        if (RecyclingBitmapDrawable.class.isInstance(oldValue)) {
            // The removed entry is a recycling drawable, so notify it
            // that it has been removed from the memory cache.
            ((RecyclingBitmapDrawable) oldValue).setIsCached(false);
        } else {
            // The removed entry is a standard BitmapDrawable.
            if (Utils.hasHoneycomb()) {
                // We're running on Honeycomb or later, so add the bitmap
                // to a SoftReference set for possible use with inBitmap later.
                mReusableBitmaps.add
                        (new SoftReference<Bitmap>(oldValue.getBitmap()));
            }
        }
    }
....
}

public static Bitmap decodeSampledBitmapFromFile(String filename,
        int reqWidth, int reqHeight, ImageCache cache) {

    final BitmapFactory.Options options = new BitmapFactory.Options();
    ...
    BitmapFactory.decodeFile(filename, options);
    ...

    // If we're running on Honeycomb or newer, try to use inBitmap.
    if (Utils.hasHoneycomb()) {
        addInBitmapOptions(options, cache);
    }
    ...
    return BitmapFactory.decodeFile(filename, options);
}

private static void addInBitmapOptions(BitmapFactory.Options options,
        ImageCache cache) {
    // inBitmap only works with mutable bitmaps, so force the decoder to
    // return mutable bitmaps.
    options.inMutable = true;

    if (cache != null) {
        // Try to find a bitmap to use for inBitmap.
        Bitmap inBitmap = cache.getBitmapFromReusableSet(options);

        if (inBitmap != null) {
            // If a suitable bitmap has been found, set it as the value of
            // inBitmap.
            options.inBitmap = inBitmap;
        }
    }
}

// This method iterates through the reusable bitmaps, looking for one 
// to use for inBitmap:
protected Bitmap getBitmapFromReusableSet(BitmapFactory.Options options) {
        Bitmap bitmap = null;

    if (mReusableBitmaps != null && !mReusableBitmaps.isEmpty()) {
        final Iterator<SoftReference<Bitmap>> iterator
                = mReusableBitmaps.iterator();
        Bitmap item;

        while (iterator.hasNext()) {
            item = iterator.next().get();

            if (null != item && item.isMutable()) {
                // Check to see it the item can be used for inBitmap.
                if (canUseForInBitmap(item, options)) {
                    bitmap = item;

                    // Remove from reusable set so it can't be used again.
                    iterator.remove();
                    break;
                }
            } else {
                // Remove from the set if the reference has been cleared.
                iterator.remove();
            }
        }
    }
    return bitmap;
}

private static boolean canUseForInBitmap(
        Bitmap candidate, BitmapFactory.Options targetOptions) {
    int width = targetOptions.outWidth / targetOptions.inSampleSize;
    int height = targetOptions.outHeight / targetOptions.inSampleSize;

    // Returns true if "candidate" can be used for inBitmap re-use with
    // "targetOptions".
    return candidate.getWidth() == width && candidate.getHeight() == height;
}





邮箱：zz7zz7zz@163.com
微博：http://weibo.com/u/3209971935