1 page.title=Painless Threading 2 parent.title=Articles 3 parent.link=../browser.html?tag=article 4 @jd:body 5 6 <p>This article discusses the threading model used by Android applications and how applications can ensure best UI performance by spawning worker threads to handle long-running operations, rather than handling them in the main thread. The article also explains the API that your application can use to interact with Android UI toolkit components running on the main thread and spawn managed worker threads. </p> 7 8 <h3>The UI thread</h3> 9 10 <p>When an application is launched, the system creates a thread called 11 "main" for the application. The main thread, also called the <em>UI 12 thread</em>, is very important because it is in charge of dispatching the 13 events to the appropriate widgets, including drawing events. 14 It is also the thread where your application interacts with running 15 components of the Android UI toolkit. </p> 16 17 <p>For instance, if you touch the a button on screen, the UI thread dispatches 18 the touch event to the widget, which in turn sets its pressed state and 19 posts an invalidate request to the event queue. The UI thread dequeues 20 the request and notifies the widget to redraw itself.</p> 21 22 <p>This single-thread model can yield poor performance unless your application 23 is implemented properly. Specifically, if everything is happening in a single 24 thread, performing long operations such as network access or database 25 queries on the UI thread will block the whole user interface. No event 26 can be dispatched, including drawing events, while the long operation 27 is underway. From the user's perspective, the application appears hung. 28 Even worse, if the UI thread is blocked for more than a few seconds 29 (about 5 seconds currently) the user is presented with the infamous "<a href="http://developer.android.com/guide/practices/design/responsiveness.html">application not responding</a>" (ANR) dialog.</p> 30 31 <p>If you want to see how bad this can look, write a simple application 32 with a button that invokes <code>Thread.sleep(2000)</code> in its 33 <a href="http://developer.android.com/reference/android/view/View.OnClickListener.html">OnClickListener</a>. 34 The button will remain in its pressed state for about 2 seconds before 35 going back to its normal state. When this happens, it is very easy for 36 the user to <em>perceive</em> the application as slow.</p> 37 38 <p>To summarize, it's vital to the responsiveness of your application's UI to 39 keep the UI thread unblocked. If you have long operations to perform, you should 40 make sure to do them in extra threads (<em>background</em> or <em>worker</em> 41 threads). </p> 42 43 <p>Here's an example of a click listener downloading an image over the 44 network and displaying it in an <a href="http://developer.android.com/reference/android/widget/ImageView.html">ImageView</a>:</p> 45 46 <pre class="prettyprint">public void onClick(View v) { 47 new Thread(new Runnable() { 48 public void run() { 49 Bitmap b = loadImageFromNetwork(); 50 mImageView.setImageBitmap(b); 51 } 52 }).start(); 53 }</pre> 54 55 <p>At first, this code seems to be a good solution to your problem, as it does 56 not block the UI thread. Unfortunately, it violates the single-threaded model 57 for the UI: the Android UI toolkit is <em>not thread-safe</em> and must always 58 be manipulated on the UI thread. In this piece of code above, the 59 <code>ImageView</code> is manipulated on a worker thread, which can cause really 60 weird problems. Tracking down and fixing such bugs can be difficult and 61 time-consuming.</p> 62 63 <p>Android offers several ways to access the UI 64 thread from other threads. You may already be familiar with some of 65 them but here is a comprehensive list:</p> 66 67 <ul> 68 <li>{@link android.app.Activity#runOnUiThread(java.lang.Runnable) Activity.runOnUiThread(Runnable)}</li> 69 <li>{@link android.view.View#post(java.lang.Runnable) View.post(Runnable)}</li> 70 <li>{@link android.view.View#postDelayed(java.lang.Runnable, long) View.postDelayed(Runnable, long)}</li> 71 <li>{@link android.os.Handler}</li> 72 </ul> 73 74 <p>You can use any of these classes and methods to correct the previous code example:</p> 75 76 <pre class="prettyprint">public void onClick(View v) { 77 new Thread(new Runnable() { 78 public void run() { 79 final Bitmap b = loadImageFromNetwork(); 80 mImageView.post(new Runnable() { 81 public void run() { 82 mImageView.setImageBitmap(b); 83 } 84 }); 85 } 86 }).start(); 87 }</pre> 88 89 <p>Unfortunately, 90 these classes and methods could also tend to make your code more complicated 91 and more difficult to read. It becomes even worse when your implement 92 complex operations that require frequent UI updates. </p> 93 94 <p>To remedy this problem, Android 1.5 and later platforms offer a utility class 95 called {@link android.os.AsyncTask}, that simplifies the creation of 96 long-running tasks that need to communicate with the user interface.</p> 97 98 <p>An <code>AsyncTask</code> equivalent is also available for applications that 99 will run on Android 1.0 and 1.1. The name of the class is <a 100 href="http://code.google.com/p/shelves/source/browse/trunk/Shelves/src/org/ 101 curiouscreature/android/shelves/util/UserTask.java">UserTask</a>. It offers the 102 exact same API and all you have to do is copy its source code in your 103 application.</p> 104 105 <p>The goal of <code>AsyncTask</code> is to take care of thread management for 106 you. Our previous example can easily be rewritten with 107 <code>AsyncTask</code>:</p> 108 109 <pre class="prettyprint">public void onClick(View v) { 110 new DownloadImageTask().execute("http://example.com/image.png"); 111 } 112 113 private class DownloadImageTask extends AsyncTask<String, Void, Bitmap> { 114 protected Bitmap doInBackground(String... urls) { 115 return loadImageFromNetwork(urls[0]); 116 } 117 118 protected void onPostExecute(Bitmap result) { 119 mImageView.setImageBitmap(result); 120 } 121 }</pre> 122 123 <p>As you can see, <code>AsyncTask</code> <em>must</em> be used by subclassing 124 it. It is also very important to remember that an <code>AsyncTask</code> 125 instance has to be created on the UI thread and can be executed only once. You 126 can read the <a 127 href="http://developer.android.com/reference/android/os/AsyncTask.html"> 128 AsyncTask documentation</a> for a full understanding on how to use this class, 129 but here is a quick overview of how it works:</p> 130 131 <ul> 132 <li>You can specify the type, using generics, of the parameters, the progress values and the final value of the task</li> 133 <li>The method <a href="http://developer.android.com/reference/android/os/AsyncTask.html#doInBackground%28Params...%29">doInBackground()</a> executes automatically on a worker thread</li> 134 <li><a href="http://developer.android.com/reference/android/os/AsyncTask.html#onPreExecute%28%29">onPreExecute()</a>, <a href="http://developer.android.com/reference/android/os/AsyncTask.html#onPostExecute%28Result%29">onPostExecute()</a> and <a href="http://developer.android.com/reference/android/os/AsyncTask.html#onProgressUpdate%28Progress...%29">onProgressUpdate()</a> are all invoked on the UI thread</li> 135 <li>The value returned by <a href="http://developer.android.com/reference/android/os/AsyncTask.html#doInBackground%28Params...%29">doInBackground()</a> is sent to <a href="http://developer.android.com/reference/android/os/AsyncTask.html#onPostExecute%28Result%29">onPostExecute()</a></li> 136 <li>You can call <a href="http://developer.android.com/reference/android/os/AsyncTask.html#publishProgress%28Progress...%29">publishProgress()</a> at anytime in <a href="http://developer.android.com/reference/android/os/AsyncTask.html#doInBackground%28Params...%29">doInBackground()</a> to execute <a href="http://developer.android.com/reference/android/os/AsyncTask.html#onProgressUpdate%28Progress...%29">onProgressUpdate()</a> on the UI thread</li><li>You can cancel the task at any time, from any thread</li> 137 </ul> 138 139 <p>In addition to the official documentation, you can read several complex examples in the source code of Shelves (<a href="http://code.google.com/p/shelves/source/browse/trunk/Shelves/src/org/curiouscreature/android/shelves/activity/ShelvesActivity.java">ShelvesActivity.java</a> and <a href="http://code.google.com/p/shelves/source/browse/trunk/Shelves/src/org/curiouscreature/android/shelves/activity/AddBookActivity.java">AddBookActivity.java</a>) and Photostream (<a href="http://code.google.com/p/apps-for-android/source/browse/trunk/Photostream/src/com/google/android/photostream/LoginActivity.java">LoginActivity.java</a>, <a href="http://code.google.com/p/apps-for-android/source/browse/trunk/Photostream/src/com/google/android/photostream/PhotostreamActivity.java">PhotostreamActivity.java</a> and <a href="http://code.google.com/p/apps-for-android/source/browse/trunk/Photostream/src/com/google/android/photostream/ViewPhotoActivity.java">ViewPhotoActivity.java</a>). We highly recommend reading the source code of <a href="http://code.google.com/p/shelves/">Shelves</a> to see how to persist tasks across configuration changes and how to cancel them properly when the activity is destroyed.</p> 140 141 <p>Regardless of whether or not you use <a href="http://developer.android.com/reference/android/os/AsyncTask.html">AsyncTask</a>, 142 always remember these two rules about the single thread model: </p> 143 144 <ol> 145 <li>Do not block the UI thread, and 146 <li>Make sure that you access the Android UI toolkit <em>only</em> on the UI thread. 147 </ol> 148 149 <p><code>AsyncTask</code> just makes it easier to do both of these things.</p> 150
