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The
BitmapFactory.decode*
methods, discussed in the
Load Large Bitmaps
Efficiently
lesson, should not be executed on the main UI thread if the source data is read from
disk or a network location (or really any source other than memory). The time this data takes to
load is unpredictable and depends on a variety of factors (speed of reading from disk or network,
size of image, power of CPU, etc.). If one of these tasks blocks the UI thread, the system flags
your application as non-responsive and the user has the option of closing it (see
Designing for Responsiveness
for
more information).
This lesson walks you through processing bitmaps in a background thread using
AsyncTask
and shows you how to handle concurrency issues.
Use an AsyncTask
The
AsyncTask
class provides an easy way to execute some work in a background
thread and publish the results back on the UI thread. To use it, create a subclass and override the
provided methods. Here’s an example of loading a large image into an
ImageView
using
AsyncTask
and
decodeSampledBitmapFromResource()
:
class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> { private final WeakReference<ImageView> 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); } // Decode image in background. @Override protected Bitmap doInBackground(Integer... params) { data = params[0]; return decodeSampledBitmapFromResource(getResources(), data, 100, 100)); } // Once complete, see if ImageView is still around and set bitmap. @Override protected void onPostExecute(Bitmap bitmap) { if (imageViewReference != null && bitmap != null) { final ImageView imageView = imageViewReference.get(); if (imageView != null) { imageView.setImageBitmap(bitmap); } } } }
The
WeakReference
to the
ImageView
ensures that the
AsyncTask
does not prevent the
ImageView
and anything it
references from being garbage collected. There’s no guarantee the
ImageView
is still around when the task finishes, so you must also check the reference in
onPostExecute()
. The
ImageView
may no longer exist, if for example, the user navigates away from the activity or if a
configuration change happens before the task finishes.
To start loading the bitmap asynchronously, simply create a new task and execute it:
public void loadBitmap(int resId, ImageView imageView) { BitmapWorkerTask task = new BitmapWorkerTask(imageView); task.execute(resId); }
Handle Concurrency
Common view components such as
ListView
and
GridView
introduce another issue when used in conjunction with the
AsyncTask
as demonstrated in the previous section. In order to be efficient with memory,
these components recycle child views as the user scrolls. If each child view triggers an
AsyncTask
, there is no guarantee that when it completes, the associated view has not
already been recycled for use in another child view. Furthermore, there is no guarantee that the
order in which asynchronous tasks are started is the order that they complete.
The blog post
Multithreading
for Performance
further discusses dealing with concurrency, and offers a solution where the
ImageView
stores a reference to the most recent
AsyncTask
which can later be checked when the task completes. Using a similar method, the
AsyncTask
from the previous section can be extended to follow a similar pattern.
Create a dedicated
Drawable
subclass to store a reference
back to the worker task. In this case, a
BitmapDrawable
is used so
that a placeholder image can be displayed in the
ImageView
while the task
completes:
static class AsyncDrawable extends BitmapDrawable { private final WeakReference<BitmapWorkerTask> bitmapWorkerTaskReference; public AsyncDrawable(Resources res, Bitmap bitmap, BitmapWorkerTask bitmapWorkerTask) { super(res, bitmap); bitmapWorkerTaskReference = new WeakReference<BitmapWorkerTask>(bitmapWorkerTask); } public BitmapWorkerTask getBitmapWorkerTask() { return bitmapWorkerTaskReference.get(); } }
Before executing the
BitmapWorkerTask
, you create an
AsyncDrawable
and bind it to the target
ImageView
:
public void loadBitmap(int resId, ImageView imageView) { if (cancelPotentialWork(resId, imageView)) { final BitmapWorkerTask task = new BitmapWorkerTask(imageView); final AsyncDrawable asyncDrawable = new AsyncDrawable(getResources(), mPlaceHolderBitmap, task); imageView.setImageDrawable(asyncDrawable); task.execute(resId); } }
The
cancelPotentialWork
method referenced in the code sample above checks if another
running task is already associated with the
ImageView
. If so, it attempts to
cancel the previous task by calling
cancel()
. In a small number
of cases, the new task data matches the existing task and nothing further needs to happen. Here is
the implementation of
cancelPotentialWork
:
public static boolean cancelPotentialWork(int data, ImageView imageView) { final BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(imageView); if (bitmapWorkerTask != null) { final int bitmapData = bitmapWorkerTask.data; // If bitmapData is not yet set or it differs from the new data if (bitmapData == 0 || bitmapData != data) { // Cancel previous task bitmapWorkerTask.cancel(true); } else { // The same work is already in progress return false; } } // No task associated with the ImageView, or an existing task was cancelled return true; }
A helper method,
getBitmapWorkerTask()
, is used above to retrieve the task associated
with a particular
ImageView
:
private static BitmapWorkerTask getBitmapWorkerTask(ImageView imageView) { if (imageView != null) { final Drawable drawable = imageView.getDrawable(); if (drawable instanceof AsyncDrawable) { final AsyncDrawable asyncDrawable = (AsyncDrawable) drawable; return asyncDrawable.getBitmapWorkerTask(); } } return null; }
The last step is updating
onPostExecute()
in
BitmapWorkerTask
so that it checks if the task is cancelled and if the current task matches the
one associated with the
ImageView
:
class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> { ... @Override protected void onPostExecute(Bitmap bitmap) { if (isCancelled()) { bitmap = null; } if (imageViewReference != null && bitmap != null) { final ImageView imageView = imageViewReference.get(); final BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(imageView); if ( this == bitmapWorkerTask && imageView != null) { imageView.setImageBitmap(bitmap); } } } }
This implementation is now suitable for use in
ListView
and
GridView
components as well as any other components that recycle their child
views. Simply call
loadBitmap
where you normally set an image to your
ImageView
. For example, in a
GridView
implementation this
would be in the
getView()
method of the backing adapter.