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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.