android之图片缓存的探究

下面是一个简单的DiskLruCache实现。然而推荐的实现DiskLruCache方案请参考Android4.0中（libcore/luni/src/main/java/libcore/io/DiskLruCache.java）源码。本文使用的是之前版本中的简单实现（Quick Search中是另外的实现）.

显示是简单实现DiskLruCache更新后的例子：

private DiskLruCache mDiskCache; 
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 
    ... 
    File cacheDir = getCacheDir(this, DISK_CACHE_SUBDIR); 
    mDiskCache = DiskLruCache.openCache(this, cacheDir, DISK_CACHE_SIZE); 
    ... 
} 
                                   
class BitmapWorkerTask extends AsyncTask { 
    ... 
    // 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(String.valueOf(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 
    if (!mDiskCache.containsKey(key)) { 
        mDiskCache.put(key, bitmap); 
    } 
} 
                                   
public Bitmap getBitmapFromDiskCache(String key) { 
    return mDiskCache.get(key); 
} 
                                   
// 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 getCacheDir(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.getExternalStorageState() == Environment.MEDIA_MOUNTED 
            || !Environment.isExternalStorageRemovable() ? 
                    context.getExternalCacheDir().getPath() : context.getCacheDir().getPath(); 
                                   
    return new File(cachePath + File.separator + uniqueName); 
}

内存缓存检查在UI线程中做，磁盘缓存的检查在后台线程中。硬盘操作不应在UI线程中。图片处理完成后应将其加入正在使用的内存、磁盘缓存中。

如何处理配置的改变？

应用运行中配置改变时，如屏幕方向改变时为了应用新的配置Android会销毁重新运行当前的Activity，此时，为了给用户快速、平缓的用户体验你可能不想重新加载图片。

多亏你运行了缓存技术，缓存可以通过 setRetainInstance(true))传递给新的Activity，在Activity重启后，你可以通过附着的Fragment重新使用已存在的缓存，这样就可以快速加载到ImageView中了。

下面是一个当配置改变时用Fragment重用已有的缓存的例子：

private LruCache mMemoryCache; 
                  
@Override
protected void onCreate(Bundle savedInstanceState) { 
    ... 
    RetainFragment mRetainFragment = 
            RetainFragment.findOrCreateRetainFragment(getFragmentManager()); 
    mMemoryCache = RetainFragment.mRetainedCache; 
    if (mMemoryCache == null) { 
        mMemoryCache = new LruCache(cacheSize) { 
            ... // Initialize cache here as usual 
        } 
        mRetainFragment.mRetainedCache = mMemoryCache; 
    } 
    ... 
} 
                  
class RetainFragment extends Fragment { 
    private static final String TAG = "RetainFragment"; 
    public LruCache 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); 
    } 
}

为了测试，在使用和未使用Fragment的情况下，强制旋转屏幕，你会发现从保留内存缓存加载图片时几乎没有滞后。