Question

单击ListView片段中的项目，控制将传递到详细活动，该活动将下载与此项目相关的数据，并为此活动中的视图页面设置适当的适配器。在此活动中，如果用户单击图像，全屏活动启动，以全屏显示此特定图像。

现在我的问题是：当用户通过点击关闭图标或后退按钮遍历离开此FullScreen活动时，如何重用详细活动数据（此项目先前已下载）而不是再次下载。解决方案当用户单击“详细活动”上的新项目时，也应该适用于场景，数据下载应该从此项目开始。

不想使用SQLLite，FileSystem进行存储。我可以在ListView中单击项目时下载数据，但是当用户从全屏活动中返回时不能下载数据

Answer 1

您需要缓存数据，然后不要下载相同的数据。除了sqlite之外还有其他存储选项。

由您来决定实施缓存的对象

http://developer.android.com/guide/topics/data/data-storage.html

这是实施

public class ImageCache {
    private static final String TAG = "ImageCache";

    // Default memory cache size
    private static final int DEFAULT_MEM_CACHE_SIZE = 1024 * 1024 * 5; // 5MB

    // Default disk cache size
    private static final int DEFAULT_DISK_CACHE_SIZE = 1024 * 1024 * 10; // 10MB

    // Compression settings when writing images to disk cache
    private static final CompressFormat DEFAULT_COMPRESS_FORMAT = CompressFormat.JPEG;
    private static final int DEFAULT_COMPRESS_QUALITY = 70;
    private static final int DISK_CACHE_INDEX = 0;

    // Constants to easily toggle various caches
    private static final boolean DEFAULT_MEM_CACHE_ENABLED = true;
    private static final boolean DEFAULT_DISK_CACHE_ENABLED = true;
    private static final boolean DEFAULT_CLEAR_DISK_CACHE_ON_START = false;
    private static final boolean DEFAULT_INIT_DISK_CACHE_ON_CREATE = false;

    private DiskLruCache mDiskLruCache;
    private LruCache<String, Bitmap> mMemoryCache;
    private ImageCacheParams mCacheParams;
    private final Object mDiskCacheLock = new Object();
    private boolean mDiskCacheStarting = true;

    /**
     * Creating a new ImageCache object using the specified parameters.
     *
     * @param cacheParams The cache parameters to use to initialize the cache
     */
    public ImageCache(ImageCacheParams cacheParams) {
        init(cacheParams);
    }

    /**
     * Creating a new ImageCache object using the default parameters.
     *
     * @param context The context to use
     * @param uniqueName A unique name that will be appended to the cache directory
     */
    public ImageCache(Context context, String uniqueName) {
        init(new ImageCacheParams(context, uniqueName));
    }

    /**
     * Find and return an existing ImageCache stored in a {@link RetainFragment}, if not found a new
     * one is created using the supplied params and saved to a {@link RetainFragment}.
     *
     * @param fragmentManager The fragment manager to use when dealing with the retained fragment.
     * @param cacheParams The cache parameters to use if creating the ImageCache
     * @return An existing retained ImageCache object or a new one if one did not exist
     */
    public static ImageCache findOrCreateCache(
            FragmentManager fragmentManager, ImageCacheParams cacheParams) {

        // Search for, or create an instance of the non-UI RetainFragment
        final RetainFragment mRetainFragment = findOrCreateRetainFragment(fragmentManager);

        // See if we already have an ImageCache stored in RetainFragment
        ImageCache imageCache = (ImageCache) mRetainFragment.getObject();

        // No existing ImageCache, create one and store it in RetainFragment
        if (imageCache == null) {
            imageCache = new ImageCache(cacheParams);
            mRetainFragment.setObject(imageCache);
        }

        return imageCache;
    }

    /**
     * Initialize the cache, providing all parameters.
     *
     * @param cacheParams The cache parameters to initialize the cache
     */
    private void init(ImageCacheParams cacheParams) {
        mCacheParams = cacheParams;

        // Set up memory cache
        if (mCacheParams.memoryCacheEnabled) {
            if (BuildConfig.DEBUG) {
                Log.d(TAG, "Memory cache created (size = " + mCacheParams.memCacheSize + ")");
            }
            mMemoryCache = new LruCache<String, Bitmap>(mCacheParams.memCacheSize) {
                /**
                 * Measure item size in bytes rather than units which is more practical
                 * for a bitmap cache
                 */
                @Override
                protected int sizeOf(String key, Bitmap bitmap) {
                    return getBitmapSize(bitmap);
                }
            };
        }

        // By default the disk cache is not initialized here as it should be initialized
        // on a separate thread due to disk access.
        if (cacheParams.initDiskCacheOnCreate) {
            // Set up disk cache
            initDiskCache();
        }
    }

    /**
     * Initializes the disk cache.  Note that this includes disk access so this should not be
     * executed on the main/UI thread. By default an ImageCache does not initialize the disk
     * cache when it is created, instead you should call initDiskCache() to initialize it on a
     * background thread.
     */
    public void initDiskCache() {
        // Set up disk cache
        synchronized (mDiskCacheLock) {
            if (mDiskLruCache == null || mDiskLruCache.isClosed()) {
                File diskCacheDir = mCacheParams.diskCacheDir;
                if (mCacheParams.diskCacheEnabled && diskCacheDir != null) {
                    if (!diskCacheDir.exists()) {
                        diskCacheDir.mkdirs();
                    }
                    if (getUsableSpace(diskCacheDir) > mCacheParams.diskCacheSize) {
                        try {
                            mDiskLruCache = DiskLruCache.open(
                                    diskCacheDir, 1, 1, mCacheParams.diskCacheSize);
                            if (BuildConfig.DEBUG) {
                                Log.d(TAG, "Disk cache initialized");
                            }
                        } catch (final IOException e) {
                            mCacheParams.diskCacheDir = null;
                            Log.e(TAG, "initDiskCache - " + e);
                        }
                    }
                }
            }
            mDiskCacheStarting = false;
            mDiskCacheLock.notifyAll();
        }
    }

    /**
     * Adds a bitmap to both memory and disk cache.
     * @param data Unique identifier for the bitmap to store
     * @param bitmap The bitmap to store
     */
    public void addBitmapToCache(String data, Bitmap bitmap) {
        if (data == null || bitmap == null) {
            return;
        }

        // Add to memory cache
        if (mMemoryCache != null && mMemoryCache.get(data) == null) {
            mMemoryCache.put(data, bitmap);
        }

        synchronized (mDiskCacheLock) {
            // Add to disk cache
            if (mDiskLruCache != null) {
                final String key = hashKeyForDisk(data);
                OutputStream out = null;
                try {
                    DiskLruCache.Snapshot snapshot = mDiskLruCache.get(key);
                    if (snapshot == null) {
                        final DiskLruCache.Editor editor = mDiskLruCache.edit(key);
                        if (editor != null) {
                            out = editor.newOutputStream(DISK_CACHE_INDEX);
                            bitmap.compress(
                                    mCacheParams.compressFormat, mCacheParams.compressQuality, out);
                            editor.commit();
                            out.close();
                        }
                    } else {
                        snapshot.getInputStream(DISK_CACHE_INDEX).close();
                    }
                } catch (final IOException e) {
                    Log.e(TAG, "addBitmapToCache - " + e);
                } catch (Exception e) {
                    Log.e(TAG, "addBitmapToCache - " + e);
                } finally {
                    try {
                        if (out != null) {
                            out.close();
                        }
                    } catch (IOException e) {}
                }
            }
        }
    }

    /**
     * Get from memory cache.
     *
     * @param data Unique identifier for which item to get
     * @return The bitmap if found in cache, null otherwise
     */
    public Bitmap getBitmapFromMemCache(String data) {
        if (mMemoryCache != null) {
            final Bitmap memBitmap = mMemoryCache.get(data);
            if (memBitmap != null) {
                if (BuildConfig.DEBUG) {
                    Log.d(TAG, "Memory cache hit");
                }
                return memBitmap;
            }
        }
        return null;
    }

    /**
     * Get from disk cache.
     *
     * @param data Unique identifier for which item to get
     * @return The bitmap if found in cache, null otherwise
     */
    public Bitmap getBitmapFromDiskCache(String data) {
        final String key = hashKeyForDisk(data);
        synchronized (mDiskCacheLock) {
            while (mDiskCacheStarting) {
                try {
                    mDiskCacheLock.wait();
                } catch (InterruptedException e) {}
            }
            if (mDiskLruCache != null) {
                InputStream inputStream = null;
                try {
                    final DiskLruCache.Snapshot snapshot = mDiskLruCache.get(key);
                    if (snapshot != null) {
                        if (BuildConfig.DEBUG) {
                            Log.d(TAG, "Disk cache hit");
                        }
                        inputStream = snapshot.getInputStream(DISK_CACHE_INDEX);
                        if (inputStream != null) {
                            final Bitmap bitmap = BitmapFactory.decodeStream(inputStream);
                            return bitmap;
                        }
                    }
                } catch (final IOException e) {
                    Log.e(TAG, "getBitmapFromDiskCache - " + e);
                } finally {
                    try {
                        if (inputStream != null) {
                            inputStream.close();
                        }
                    } catch (IOException e) {}
                }
            }
            return null;
        }
    }

    /**
     * Clears both the memory and disk cache associated with this ImageCache object. Note that
     * this includes disk access so this should not be executed on the main/UI thread.
     */
    public void clearCache() {
        if (mMemoryCache != null) {
            mMemoryCache.evictAll();
            if (BuildConfig.DEBUG) {
                Log.d(TAG, "Memory cache cleared");
            }
        }

        synchronized (mDiskCacheLock) {
            mDiskCacheStarting = true;
            if (mDiskLruCache != null && !mDiskLruCache.isClosed()) {
                try {
                    mDiskLruCache.delete();
                    if (BuildConfig.DEBUG) {
                        Log.d(TAG, "Disk cache cleared");
                    }
                } catch (IOException e) {
                    Log.e(TAG, "clearCache - " + e);
                }
                mDiskLruCache = null;
                initDiskCache();
            }
        }
    }

    /**
     * Flushes the disk cache associated with this ImageCache object. Note that this includes
     * disk access so this should not be executed on the main/UI thread.
     */
    public void flush() {
        synchronized (mDiskCacheLock) {
            if (mDiskLruCache != null) {
                try {
                    mDiskLruCache.flush();
                    if (BuildConfig.DEBUG) {
                        Log.d(TAG, "Disk cache flushed");
                    }
                } catch (IOException e) {
                    Log.e(TAG, "flush - " + e);
                }
            }
        }
    }

    /**
     * Closes the disk cache associated with this ImageCache object. Note that this includes
     * disk access so this should not be executed on the main/UI thread.
     */
    public void close() {
        synchronized (mDiskCacheLock) {
            if (mDiskLruCache != null) {
                try {
                    if (!mDiskLruCache.isClosed()) {
                        mDiskLruCache.close();
                        mDiskLruCache = null;
                        if (BuildConfig.DEBUG) {
                            Log.d(TAG, "Disk cache closed");
                        }
                    }
                } catch (IOException e) {
                    Log.e(TAG, "close - " + e);
                }
            }
        }
    }

    /**
     * A holder class that contains cache parameters.
     */
    public static class ImageCacheParams {
        public int memCacheSize = DEFAULT_MEM_CACHE_SIZE;
        public int diskCacheSize = DEFAULT_DISK_CACHE_SIZE;
        public File diskCacheDir;
        public CompressFormat compressFormat = DEFAULT_COMPRESS_FORMAT;
        public int compressQuality = DEFAULT_COMPRESS_QUALITY;
        public boolean memoryCacheEnabled = DEFAULT_MEM_CACHE_ENABLED;
        public boolean diskCacheEnabled = DEFAULT_DISK_CACHE_ENABLED;
        public boolean clearDiskCacheOnStart = DEFAULT_CLEAR_DISK_CACHE_ON_START;
        public boolean initDiskCacheOnCreate = DEFAULT_INIT_DISK_CACHE_ON_CREATE;

        public ImageCacheParams(Context context, String uniqueName) {
            diskCacheDir = getDiskCacheDir(context, uniqueName);
        }

        public ImageCacheParams(File diskCacheDir) {
            this.diskCacheDir = diskCacheDir;
        }

        /**
         * Sets the memory cache size based on a percentage of the device memory class.
         * Eg. setting percent to 0.2 would set the memory cache to one fifth of the device memory
         * class. Throws {@link IllegalArgumentException} if percent is < 0.05 or > .8.
         *
         * This value should be chosen carefully based on a number of factors
         * Refer to the corresponding Android Training class for more discussion:
         * http://developer.android.com/training/displaying-bitmaps/
         *
         * @param context Context to use to fetch memory class
         * @param percent Percent of memory class to use to size memory cache
         */
        public void setMemCacheSizePercent(Context context, float percent) {
            if (percent < 0.05f || percent > 0.8f) {
                throw new IllegalArgumentException("setMemCacheSizePercent - percent must be "
                        + "between 0.05 and 0.8 (inclusive)");
            }
            memCacheSize = Math.round(percent * getMemoryClass(context) * 1024 * 1024);
        }

        private static int getMemoryClass(Context context) {
            return ((ActivityManager) context.getSystemService(
                    Context.ACTIVITY_SERVICE)).getMemoryClass();
        }
    }

    /**
     * Get a usable cache directory (external if available, internal otherwise).
     *
     * @param context The context to use
     * @param uniqueName A unique directory name to append to the cache dir
     * @return The cache dir
     */
    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);
    }

    /**
     * A hashing method that changes a string (like a URL) into a hash suitable for using as a
     * disk filename.
     */
    public static String hashKeyForDisk(String key) {
        String cacheKey;
        try {
            final MessageDigest mDigest = MessageDigest.getInstance("MD5");
            mDigest.update(key.getBytes());
            cacheKey = bytesToHexString(mDigest.digest());
        } catch (NoSuchAlgorithmException e) {
            cacheKey = String.valueOf(key.hashCode());
        }
        return cacheKey;
    }

    private static String bytesToHexString(byte[] bytes) {
        // https://stackoverflow.com/questions/332079
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < bytes.length; i++) {
            String hex = Integer.toHexString(0xFF & bytes[i]);
            if (hex.length() == 1) {
                sb.append('0');
            }
            sb.append(hex);
        }
        return sb.toString();
    }

    /**
     * Get the size in bytes of a bitmap.
     * @param bitmap
     * @return size in bytes
     */
    @TargetApi(12)
    public static int getBitmapSize(Bitmap bitmap) {
        if (Utils.hasHoneycombMR1()) {
            return bitmap.getByteCount();
        }
        // Pre HC-MR1
        return bitmap.getRowBytes() * bitmap.getHeight();
    }

    /**
     * Check if external storage is built-in or removable.
     *
     * @return True if external storage is removable (like an SD card), false
     *         otherwise.
     */
    @TargetApi(9)
    public static boolean isExternalStorageRemovable() {
        if (Utils.hasGingerbread()) {
            return Environment.isExternalStorageRemovable();
        }
        return true;
    }

    /**
     * Get the external app cache directory.
     *
     * @param context The context to use
     * @return The external cache dir
     */
    @TargetApi(8)
    public static File getExternalCacheDir(Context context) {
        if (Utils.hasFroyo()) {
            return context.getExternalCacheDir();
        }

        // Before Froyo we need to construct the external cache dir ourselves
        final String cacheDir = "/Android/data/" + context.getPackageName() + "/cache/";
        return new File(Environment.getExternalStorageDirectory().getPath() + cacheDir);
    }

    /**
     * Check how much usable space is available at a given path.
     *
     * @param path The path to check
     * @return The space available in bytes
     */
    @TargetApi(9)
    public static long getUsableSpace(File path) {
        if (Utils.hasGingerbread()) {
            return path.getUsableSpace();
        }
        final StatFs stats = new StatFs(path.getPath());
        return (long) stats.getBlockSize() * (long) stats.getAvailableBlocks();
    }

    /**
     * Locate an existing instance of this Fragment or if not found, create and
     * add it using FragmentManager.
     *
     * @param fm The FragmentManager manager to use.
     * @return The existing instance of the Fragment or the new instance if just
     *         created.
     */
    public static RetainFragment findOrCreateRetainFragment(FragmentManager fm) {
        // Check to see if we have retained the worker fragment.
        RetainFragment mRetainFragment = (RetainFragment) fm.findFragmentByTag(TAG);

        // If not retained (or first time running), we need to create and add it.
        if (mRetainFragment == null) {
            mRetainFragment = new RetainFragment();
            fm.beginTransaction().add(mRetainFragment, TAG).commitAllowingStateLoss();
        }

        return mRetainFragment;
    }

    /**
     * A simple non-UI Fragment that stores a single Object and is retained over configuration
     * changes. It will be used to retain the ImageCache object.
     */
    public static class RetainFragment extends Fragment {
        private Object mObject;

        /**
         * Empty constructor as per the Fragment documentation
         */
        public RetainFragment() {}

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

            // Make sure this Fragment is retained over a configuration change
            setRetainInstance(true);
        }

        /**
         * Store a single object in this Fragment.
         *
         * @param object The object to store
         */
        public void setObject(Object object) {
            mObject = object;
        }

        /**
         * Get the stored object.
         *
         * @return The stored object
         */
        public Object getObject() {
            return mObject;
        }
    }

}

文档中有一个例子

http://developer.android.com/training/displaying-bitmaps/index.html

下载zip文件DisplayingBitmaps.zip。该示例具有图像缓存的实现。

http://developer.android.com/training/displaying-bitmaps/cache-bitmap.html

使用通用图像加载器

Caching images and displaying

编辑：

BitmapFun示例快照。

下载并缓存图像

enter image description here