缓存图片资源

最新推荐文章于 2025-01-10 00:35:01 发布

原创最新推荐文章于 2025-01-10 00:35:01 发布 · 281 阅读

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#Android #缓存 #图片

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本文介绍如何使用内存缓存和磁盘缓存来提高图片加载效率，包括LruCache的使用和DiskLruCache的实现，并讨论了如何处理配置变化。

一、使用内存缓存，通过使用LruCache类，需要确定缓存的大小，如使用app运行内存的1/8：

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;
        }
    };
    ...
}

Bitmap存入缓存（调用put()方法）：

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

从缓存中读出Bitmap（调用get()方法）：

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

注：在常见的hpdi设备上，程序的运行内存的1/8通常为4MB，一幅全屏800*480分辨率的图片大约要1.5MB空间，

所以该缓存可以存储大约2.5张图片。

在加载图片时，先检测LruCache中是否有该图片，如果没有则开启子线程去加载：

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);
    }
}

二、使用磁盘缓存

使用内存缓存有不小的局限性，如当你的进程因为系统运行内存不够被杀死时，缓存的文件就被破坏了，

这样下次又需要重新加载，可以通过磁盘进行缓存。以下实例使用了开源项目DiskLruCache：

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;
    }
    ...
}

private void addBitmapToDiskCache(String key, Bitmap bitmap) {
    synchronized (mDiskCacheLock) {
        if (mDiskCache != null) {
            DiskLruCache.Editor editor = null;
            try {
                editor = mDiskCache.edit(key);
                if (editor != null) {
                    OutputStream stream = editor.newOutputStream(0);
                    ByteArrayOutputStream baos = new ByteArrayOutputStream();
                    bitmap.compress(Bitmap.CompressFormat.PNG, 100, stream);
                    byte[] byteArray = baos.toByteArray();
                    stream.write(byteArray);
                    editor.commit();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

 private Bitmap getBitmapFromDiskCache(String key) {
        synchronized (mDiskCacheLock) {
            //如果磁盘缓存初始化未结束，则等待结束
            while (mDiskCacheStarting) {
                try {
                    mDiskCacheLock.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            if (mDiskCache != null) {
                try {
                    DiskLruCache.Snapshot snapshot = mDiskCache.get(key);
                    if (snapshot != null) {
                        InputStream inputStream = snapshot.getInputStream(0);
                        return BitmapFactory.decodeStream(inputStream);
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
        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);}

注：由于初始化缓存是在子线程中进行，这样有可能用户读取缓存文件时初始化还没完成，因此给初始化过程加锁；

磁盘操作不应该在主线程中进行。

三、处理程序运行时配置改变：

当用户转动屏幕时，界面会被重新创建，因此缓存对象被销毁，此时需要重新处理缓存文件。因此为了给用户一个流畅的使用体验，

可以给该界面保留一个Fragment，通过该Fragment把缓存对象传递到新的界面实例，这样你的界面就可以重新访问内存中的缓存：

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();
            fm.beginTransaction().add(fragment, TAG).commit();
        }
        return fragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setRetainInstance(true);
    }
}

注：记得在Fragment调用setRetainInstance()方法，如上述的onCreate（）方法。