控件进入下一个活动后，保持活动状态，用于后退按钮遍历

Question

OnClicking an Item in A ListView Fragment, Control passes to Detail Activity, which downloads the data related to this item and sets the appropriate adapters for the viewpagers in this activity.Within this activity, if a user clicks on a image, a FullScreen Activity starts which shows this particular image in fullscreen.

Now My problem is: when user leaves this FullScreen Activity by either tapping Close Icon or Back Button Traversing, How do i reuse the Detail Activity Data(Downloaded earlier for this item) instead of downloading it again. Solution Should also work for scenario when user clicks a new item on Detail Activity,Data Downloading should begin for this item.

Don't want to use SQLLite, FileSystem for storage. I am ok with downloading data when a item is clicked in a ListView but not when user traverses back from the fullscreen activity

Answer 1

You need to cache the Data so then you don't download the same. There are storage options other then sqlite.

Its upto you to decide who you implement the cache

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

Here's the implementation

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;
        }
    }

}

There is an example in the docs

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

Download the zip file DisplayingBitmaps.zip . The example has a implementation of cache for images.

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

Using Universal Image Loader

Caching images and displaying

Edit:

BitmapFun sample snap shots.

Images are downloaded and cached

Detailed View

Now loading other images when scrolled. Images are not yet cached

With all the three its easy to understand.