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Commit 9977ddde authored by Adam Koch's avatar Adam Koch
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Updates to "Displaying Bitmaps Efficiently" class. 

Changes:
 -Updated code sample (see http://ag/214812)
 -Updated code snippets to match updated sample
 -Fixed <> in code snippets
 -Updated disk cache section
 -Some other minor updates

Change-Id: Id7ca4d161f165814d71f238f940b2c5bfbc220aa
parent 116748d0

docs/downloads/training/BitmapFun.zip

+80.4 KiB (389 KiB)

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docs/html/training/displaying-bitmaps/cache-bitmap.jd

+58 −28
Original line number Diff line number Diff line
@@ -96,7 +96,7 @@ and leave the rest of your app little memory to work with.</p> 
<p>Here’s an example of setting up a {@link android.util.LruCache} for bitmaps:</p>



<pre>

private LruCache<String, Bitmap> mMemoryCache;

private LruCache&lt;String, Bitmap&gt; mMemoryCache;



&#64;Override

protected void onCreate(Bundle savedInstanceState) {
@@ -109,7 +109,7 @@ protected void onCreate(Bundle savedInstanceState) { 
    // Use 1/8th of the available memory for this memory cache.

    final int cacheSize = 1024 * 1024 * memClass / 8;



    mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {

    mMemoryCache = new LruCache&lt;String, Bitmap&gt;(cacheSize) {

        &#64;Override

        protected int sizeOf(String key, Bitmap bitmap) {

            // The cache size will be measured in bytes rather than number of items.
@@ -159,7 +159,7 @@ public void loadBitmap(int resId, ImageView imageView) { 
updated to add entries to the memory cache:</p>



<pre>

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

class BitmapWorkerTask extends AsyncTask&lt;Integer, Void, Bitmap&gt; {

    ...

    // Decode image in background.

    &#64;Override
@@ -179,7 +179,7 @@ class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> { 
rely on images being available in this cache. Components like {@link android.widget.GridView} with

larger datasets can easily fill up a memory cache. Your application could be interrupted by another

task like a phone call, and while in the background it might be killed and the memory cache

destroyed. Once the user resumes, your application it has to process each image again.</p>

destroyed. Once the user resumes, your application has to process each image again.</p>



<p>A disk cache can be used in these cases to persist processed bitmaps and help decrease loading

times where images are no longer available in a memory cache. Of course, fetching images from disk
@@ -190,18 +190,14 @@ be unpredictable.</p> 
appropriate place to store cached images if they are accessed more frequently, for example in an

image gallery application.</p>



<p>Included in the sample code of this class is a basic {@code DiskLruCache} implementation.

However, a more robust and recommended {@code DiskLruCache} solution is included in the Android 4.0

source code ({@code libcore/luni/src/main/java/libcore/io/DiskLruCache.java}). Back-porting this

class for use on previous Android releases should be fairly straightforward (a <a

href="http://www.google.com/search?q=disklrucache">quick search</a> shows others who have already

implemented this solution).</p>



<p>Here’s updated example code that uses the simple {@code DiskLruCache} included in the sample

application of this class:</p>

<p>The sample code of this class uses a {@code DiskLruCache} implementation that is pulled from the 

<a href="https://android.googlesource.com/platform/libcore/+/master/luni/src/main/java/libcore/io/DiskLruCache.java">Android source</a>. Here’s updated example code that adds a disk cache in addition

to the existing memory cache:</p>



<pre>

private DiskLruCache mDiskCache;

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";
@@ -210,12 +206,26 @@ protected void onCreate(Bundle savedInstanceState) { 
    ...

    // Initialize memory cache

    ...

    File cacheDir = getCacheDir(this, DISK_CACHE_SUBDIR);

    mDiskCache = DiskLruCache.openCache(this, cacheDir, DISK_CACHE_SIZE);

    // Initialize disk cache on background thread

    File cacheDir = getDiskCacheDir(this, DISK_CACHE_SUBDIR);

    new InitDiskCacheTask().execute(cacheDir);

    ...

}



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

class InitDiskCacheTask extends AsyncTask&lt;File, Void, Void&gt; {

    &#64;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&lt;Integer, Void, Bitmap&gt; {

    ...

    // Decode image in background.

    &#64;Override
@@ -232,7 +242,7 @@ class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> { 
        }



        // Add final bitmap to caches

        addBitmapToCache(String.valueOf(imageKey, bitmap);

        addBitmapToCache(imageKey, bitmap);



        return bitmap;

    }
@@ -246,28 +256,48 @@ public void addBitmapToCache(String key, Bitmap bitmap) { 
    }



    // Also add to disk cache

    if (!mDiskCache.containsKey(key)) {

        mDiskCache.put(key, bitmap);

    synchronized (mDiskCacheLock) {

        if (mDiskLruCache != null && mDiskLruCache.get(key) == null) {

            mDiskLruCache.put(key, bitmap);

        }

    }

}



public Bitmap getBitmapFromDiskCache(String key) {

    return mDiskCache.get(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 getCacheDir(Context context, String uniqueName) {

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.getExternalStorageState() == Environment.MEDIA_MOUNTED

            || !Environment.isExternalStorageRemovable() ?

                    context.getExternalCacheDir().getPath() : context.getCacheDir().getPath();

    final String cachePath =

            Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState()) ||

                    !isExternalStorageRemovable() ? getExternalCacheDir(context).getPath() :

                            context.getCacheDir().getPath();



    return new File(cachePath + File.separator + uniqueName);

}

</pre>



<p class="note"><strong>Note:</strong> Even initializing the disk cache requires disk operations

and therefore should not take place on the main thread. However, this does mean there's a chance

the cache is accessed before initialization. To address this, in the above implementation, a lock

object ensures that the app does not read from the disk cache until the cache has been

initialized.</p>



<p>While the memory cache is checked in the UI thread, the disk cache is checked in the background

thread. Disk operations should never take place on the UI thread. When image processing is

complete, the final bitmap is added to both the memory and disk cache for future use.</p>
@@ -292,7 +322,7 @@ android.widget.ImageView} objects.</p> 
changes using a {@link android.app.Fragment}:</p>



<pre>

private LruCache<String, Bitmap> mMemoryCache;

private LruCache&lt;String, Bitmap&gt; mMemoryCache;



&#64;Override

protected void onCreate(Bundle savedInstanceState) {
@@ -301,7 +331,7 @@ protected void onCreate(Bundle savedInstanceState) { 
            RetainFragment.findOrCreateRetainFragment(getFragmentManager());

    mMemoryCache = RetainFragment.mRetainedCache;

    if (mMemoryCache == null) {

        mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {

        mMemoryCache = new LruCache&lt;String, Bitmap&gt;(cacheSize) {

            ... // Initialize cache here as usual

        }

        mRetainFragment.mRetainedCache = mMemoryCache;
@@ -311,7 +341,7 @@ protected void onCreate(Bundle savedInstanceState) { 


class RetainFragment extends Fragment {

    private static final String TAG = "RetainFragment";

    public LruCache<String, Bitmap> mRetainedCache;

    public LruCache&lt;String, Bitmap&gt; mRetainedCache;



    public RetainFragment() {}

docs/html/training/displaying-bitmaps/display-bitmap.jd

+13 −11
Original line number Diff line number Diff line
@@ -103,7 +103,8 @@ public class ImageDetailActivity extends FragmentActivity { 
}

</pre>



<p>The details {@link android.app.Fragment} holds the {@link android.widget.ImageView} children:</p>

<p>Here is an implementation of the details {@link android.app.Fragment} which holds the {@link android.widget.ImageView} children. This might seem like a perfectly reasonable approach, but can

you see the drawbacks of this implementation? How could it be improved?</p>



<pre>

public class ImageDetailFragment extends Fragment {
@@ -146,11 +147,11 @@ public class ImageDetailFragment extends Fragment { 
}

</pre>



<p>Hopefully you noticed the issue with this implementation; The images are being read from

resources on the UI thread which can lead to an application hanging and being force closed. Using an

{@link android.os.AsyncTask} as described in the <a href="process-bitmap.html">Processing Bitmaps Off

the UI Thread</a> lesson, it’s straightforward to move image loading and processing to a background

thread:</p>

<p>Hopefully you noticed the issue: the images are being read from resources on the UI thread,

which can lead to an application hanging and being force closed. Using an

{@link android.os.AsyncTask} as described in the <a href="process-bitmap.html">Processing Bitmaps

Off the UI Thread</a> lesson, it’s straightforward to move image loading and processing to a

background thread:</p>



<pre>

public class ImageDetailActivity extends FragmentActivity {
@@ -190,7 +191,7 @@ modifications for a memory cache:</p> 
<pre>

public class ImageDetailActivity extends FragmentActivity {

    ...

    private LruCache<String, Bitmap> mMemoryCache;

    private LruCache&lt;String, Bitmap&gt; mMemoryCache;



    &#64;Override

    public void onCreate(Bundle savedInstanceState) {
@@ -229,7 +230,8 @@ UI remains fluid, memory usage remains under control and concurrency is handled 
the way {@link android.widget.GridView} recycles its children views).</p>



<p>To start with, here is a standard {@link android.widget.GridView} implementation with {@link

android.widget.ImageView} children placed inside a {@link android.app.Fragment}:</p>

android.widget.ImageView} children placed inside a {@link android.app.Fragment}. Again, this might

seem like a perfectly reasonable approach, but what would make it better?</p>



<pre>

public class ImageGridFragment extends Fragment implements AdapterView.OnItemClickListener {
@@ -261,7 +263,7 @@ public class ImageGridFragment extends Fragment implements AdapterView.OnItemCli 
    }



    &#64;Override

    public void onItemClick(AdapterView<?> parent, View v, int position, long id) {

    public void onItemClick(AdapterView&lt;?&gt; parent, View v, int position, long id) {

        final Intent i = new Intent(getActivity(), ImageDetailActivity.class);

        i.putExtra(ImageDetailActivity.EXTRA_IMAGE, position);

        startActivity(i);
@@ -345,13 +347,13 @@ public class ImageGridFragment extends Fragment implements AdapterView.OnItemCli 
    }



    static class AsyncDrawable extends BitmapDrawable {

        private final WeakReference<BitmapWorkerTask> bitmapWorkerTaskReference;

        private final WeakReference&lt;BitmapWorkerTask&gt; bitmapWorkerTaskReference;



        public AsyncDrawable(Resources res, Bitmap bitmap,

                BitmapWorkerTask bitmapWorkerTask) {

            super(res, bitmap);

            bitmapWorkerTaskReference =

                new WeakReference<BitmapWorkerTask>(bitmapWorkerTask);

                new WeakReference&lt;BitmapWorkerTask&gt;(bitmapWorkerTask);

        }



        public BitmapWorkerTask getBitmapWorkerTask() {

docs/html/training/displaying-bitmaps/index.jd

+3 −3
Original line number Diff line number Diff line
@@ -43,8 +43,8 @@ exception:<br />{@code java.lang.OutofMemoryError: bitmap size exceeds VM budget 
  perform under this minimum memory limit. However, keep in mind many devices are configured with

  higher limits.</li>

  <li>Bitmaps take up a lot of memory, especially for rich images like photographs. For example, the

  camera on the <a href="http://www.google.com/nexus/">Galaxy Nexus</a> takes photos up to 2592x1936

  pixels (5 megapixels). If the bitmap configuration used is {@link

  camera on the <a href="http://www.android.com/devices/detail/galaxy-nexus">Galaxy Nexus</a> takes 

  photos up to 2592x1936 pixels (5 megapixels). If the bitmap configuration used is {@link

  android.graphics.Bitmap.Config ARGB_8888} (the default from the Android 2.3 onward) then loading

  this image into memory takes about 19MB of memory (2592*1936*4 bytes), immediately exhausting the

  per-app limit on some devices.</li>

docs/html/training/displaying-bitmaps/process-bitmap.jd

+7 −7
Original line number Diff line number Diff line
@@ -62,13 +62,13 @@ decodeSampledBitmapFromResource()}</a>: </p> 


<a name="BitmapWorkerTask"></a>

<pre>

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

    private final WeakReference<ImageView> imageViewReference;

class BitmapWorkerTask extends AsyncTask&lt;Integer, Void, Bitmap&gt; {

    private final WeakReference&lt;ImageView&gt; 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);

        imageViewReference = new WeakReference&lt;ImageView&gt;(imageView);

    }



    // Decode image in background.
@@ -133,13 +133,13 @@ completes:</p> 
<a name="AsyncDrawable"></a>

<pre>

static class AsyncDrawable extends BitmapDrawable {

    private final WeakReference<BitmapWorkerTask> bitmapWorkerTaskReference;

    private final WeakReference&lt;BitmapWorkerTask&gt; bitmapWorkerTaskReference;



    public AsyncDrawable(Resources res, Bitmap bitmap,

            BitmapWorkerTask bitmapWorkerTask) {

        super(res, bitmap);

        bitmapWorkerTaskReference =

            new WeakReference<BitmapWorkerTask>(bitmapWorkerTask);

            new WeakReference&lt;BitmapWorkerTask&gt;(bitmapWorkerTask);

    }



    public BitmapWorkerTask getBitmapWorkerTask() {
@@ -211,7 +211,7 @@ one associated with the {@link android.widget.ImageView}:</p> 


<a name="BitmapWorkerTaskUpdated"></a>

<pre>

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

class BitmapWorkerTask extends AsyncTask&lt;Integer, Void, Bitmap&gt; {

    ...



    &#64;Override