提到的AsyncTask退出范例：
svn checkout http://shelves.googlecode.com/svn/trunk/ shelves-read-only
Threads
When an application is launched, the system creates a thread of execution for the application, called "main." This thread is very important because it is in charge of dispatching events to the appropriate user interface widgets, including drawing events. It is also the thread in which your application interacts with components from the Android UI toolkit (components from the android.widget and android.view packages). As such, the main thread is also sometimes called the UI thread.
The system does not create a separate thread for each instance of a component. All components that run in the same process are instantiated in the UI thread, and system calls to each component are dispatched from that thread. Consequently, methods that respond to system callbacks (such as onKeyDown() to report user actions or a lifecycle callback method) always run in the UI thread of the process.
For instance, when the user touches a button on the screen, your app's UI thread dispatches the touch event to the widget, which in turn sets its pressed state and posts an invalidate request to the event queue. The UI thread dequeues the request and notifies the widget that it should redraw itself.
When your app performs intensive work in response to user interaction, this single thread model can yield poor performance unless you implement your application properly. Specifically, if everything is happening in the UI thread, performing long operations such as network access or database queries will block the whole UI. When the thread is blocked, no events can be dispatched, including drawing events. From the user's perspective, the application appears to hang. Even worse, if the UI thread is blocked for more than a few seconds (about 5 seconds currently) the user is presented with the infamous "application not responding" (ANR) dialog. The user might then decide to quit your application and uninstall it if they are unhappy.
Additionally, the Andoid UI toolkit is not thread-safe. So, you must not manipulate your UI from a worker thread—you must do all manipulation to your user interface from the UI thread. Thus, there are simply two rules to Android's single thread model:
- Do not block the UI thread
- Do not access the Android UI toolkit from outside the UI thread
Worker threads
Because of the single thread model described above, it's vital to the responsiveness of your application's UI that you do not block the UI thread. If you have operations to perform that are not instantaneous, you should make sure to do them in separate threads ("background" or "worker" threads).
For example, below is some code for a click listener that downloads an image from a separate thread and displays it in an ImageView:
public void onClick(View v) { new Thread(new Runnable() { public void run() { Bitmap b = loadImageFromNetwork("http://example.com/image.png");
mImageView.setImageBitmap(b); } }).start(); }
At first, this seems to work fine, because it creates a new thread to handle the network operation. However, it violates the second rule of the single-threaded model: do not access the Android UI toolkit from outside the UI thread—this sample modifies the ImageView from the worker thread instead of the UI thread. This can result in undefined and unexpected behavior, which can be difficult and time-consuming to track down.
To fix this problem, Android offers several ways to access the UI thread from other threads. Here is a list of methods that can help:
For example, you can fix the above code by using the View.post(Runnable) method:
public void onClick(View v) {
new Thread(new Runnable() { public void run() { final Bitmap bitmap = loadImageFromNetwork("http://example.com/image.png");
mImageView.post(new Runnable() { public void run() {
mImageView.setImageBitmap(bitmap); } }); } }).start(); }
Now this implementation is thread-safe: the network operation is done from a separate thread while the ImageView is manipulated from the UI thread.
However, as the complexity of the operation grows, this kind of code can get complicated and difficult to maintain. To handle more complex interactions with a worker thread, you might consider using a Handler in your worker thread, to process messages delivered from the UI thread. Perhaps the best solution, though, is to extend the AsyncTask class, which simplifies the execution of worker thread tasks that need to interact with the UI.
Using AsyncTask
AsyncTask allows you to perform asynchronous work on your user interface. It performs the blocking operations in a worker thread and then publishes the results on the UI thread, without requiring you to handle threads and/or handlers yourself.
To use it, you must subclass AsyncTask and implement the doInBackground() callback method, which runs in a pool of background threads. To update your UI, you should implement onPostExecute(), which delivers the result from doInBackground() and runs in the UI thread, so you can safely update your UI. You can then run the task by calling execute() from the UI thread.
For example, you can implement the previous example using AsyncTask this way:
public void onClick(View v) { new DownloadImageTask().execute("http://example.com/image.png"); }
private class DownloadImageTask extends AsyncTask<String, Void, Bitmap> {
/** The system calls this to perform work in a worker thread and
* delivers it the parameters given to AsyncTask.execute() */
protected Bitmap doInBackground(String... urls) { return loadImageFromNetwork(urls[0]); }
/** The system calls this to perform work in the UI thread and delivers
* the result from doInBackground() */
protected void onPostExecute(Bitmap result) {
mImageView.setImageBitmap(result); } }
Now the UI is safe and the code is simpler, because it separates the work into the part that should be done on a worker thread and the part that should be done on the UI thread.
You should read the AsyncTask reference for a full understanding on how to use this class, but here is a quick overview of how it works:
- You can specify the type of the parameters, the progress values, and the final value of the task, using generics
- The method
doInBackground()executes automatically on a worker thread onPreExecute(),onPostExecute(), andonProgressUpdate()are all invoked on the UI thread- The value returned by
doInBackground()is sent toonPostExecute() - You can call
publishProgress()at anytime indoInBackground()to executeonProgressUpdate()on the UI thread - You can cancel the task at any time, from any thread
Caution: Another problem you might encounter when using a worker thread is unexpected restarts in your activity due to a runtime configuration change (such as when the user changes the screen orientation), which may destroy your worker thread. To see how you can persist your task during one of these restarts and how to properly cancel the task when the activity is destroyed, see the source code for the Shelves sample application.
Thread-safe methods
In some situations, the methods you implement might be called from more than one thread, and therefore must be written to be thread-safe.
This is primarily true for methods that can be called remotely—such as methods in a bound service. When a call on a method implemented in an IBinder originates in the same process in which the IBinder is running, the method is executed in the caller's thread. However, when the call originates in another process, the method is executed in a thread chosen from a pool of threads that the system maintains in the same process as the IBinder (it's not executed in the UI thread of the process). For example, whereas a service's onBind() method would be called from the UI thread of the service's process, methods implemented in the object that onBind() returns (for example, a subclass that implements RPC methods) would be called from threads in the pool. Because a service can have more than one client, more than one pool thread can engage the same IBinder method at the same time. IBinder methods must, therefore, be implemented to be thread-safe.
Similarly, a content provider can receive data requests that originate in other processes. Although the ContentResolver and ContentProvider classes hide the details of how the interprocess communication is managed, ContentProvider methods that respond to those requests—the methods query(), insert(), delete(), update(), and getType()—are called from a pool of threads in the content provider's process, not the UI thread for the process. Because these methods might be called from any number of threads at the same time, they too must be implemented to be thread-safe.
Interprocess Communication
Android offers a mechanism for interprocess communication (IPC) using remote procedure calls (RPCs), in which a method is called by an activity or other application component, but executed remotely (in another process), with any result returned back to the caller. This entails decomposing a method call and its data to a level the operating system can understand, transmitting it from the local process and address space to the remote process and address space, then reassembling and reenacting the call there. Return values are then transmitted in the opposite direction. Android provides all the code to perform these IPC transactions, so you can focus on defining and implementing the RPC programming interface.
To perform IPC, your application must bind to a service, using bindService(). For more information, see the Services developer guide.
