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Working with views
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an empty message. If you refer to listing 3.3, which is when the Handler was established, you can see where, when the message is received, we dismiss the ProgressDialog, populate the Adapter our ListView is using, and call setListAdapter() to update the UI. The setListAdapter()method iterates the Adapter and displays a returned View for every item. With the Activity created and set up and the Handler being used to update the Adapter with data, we now have a second screen in our application. The next thing we need to do is fill in some of the gaps surrounding working with handlers and different threads. These concepts aren t view-specific but are worth a small detour at this point, because you ll want to use these classes when you re trying to perform tasks related to retrieving and manipulating data that the UI needs a common design pattern when you re building Android applications.
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The Handler is the Swiss Army knife of messaging and scheduling operations for Android. This class allows you to queue tasks to be run on different threads and to schedule tasks using Message and Runnable objects. The Android platform monitors the responsiveness of applications and kills those that are considered nonresponsive. An Application Not Responding (ANR) event occurs when no response is received to a user input for 5 seconds. When a user interacts with your application by touching the screen, pressing a key, or the like, your application must respond. So does this mean that every operation in your code must complete within 5 seconds No, of course not, but the main UI thread does have to respond within that time frame. To keep the main UI thread snappy, any long-running tasks, such as retrieving data over the network, getting a large amount of data from a database, or performing complicated or time-consuming calculations, should be performed in a separate Thread, apart from the main UI Thread. Getting tasks into a separate thread and getting results back to the main UI thread is where the Handler and related classes come into play. When a Handler is created, it s associated with a Looper. A Looper is a class that contains a MessageQueue and that processes Message or Runnable objects that are sent via the Handler. When we used a Handler in listings 3.3 and 3.4, we created a Handler with a noargument constructor. With this approach, the Handler is automatically associated with the Looper of the currently running thread, typically the main UI thread. The main UI thread, which is created by the process of the running application, is an instance of a HandlerThread. A HandlerThread is an Android Thread specialization that provides a Looper. The key parts involved in this arrangement are depicted in figure 3.5. When you re implementing a Handler, you ll have to provide a handleMessage(Message m) method. This method is the hook that lets you pass messages. When you create a new Thread, you can then call one of several sendMessage methods on Handler from within that thread s run method, as our examples and figure 3.5
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demonstrate. Calling sendMessage puts your MainUIThread message on the MessageQueue, which the (HandlerThread) Looper services. Handler myHandler = new Handler() { Along with sending messages into hanpublic void handleMessage (Message m) { updateUIHere(); dlers, you can also send Runnable objects } }; directly, and you can schedule things to be new Thread() { run at different times in the future. You send public void run() { messages and you post runnables. Each of doStuff(); Message m = myHandler.obtainMessage(); these concepts supports methods such as Bundle b = new Bundle(); b.putString("key", "value"); sendEmptyMessage(int what), which we ve m.setData(b); already used, and the counterparts sendmyHandler.sendMessage(m); } EmptyMessageAtTime(int what, long time) }.start(); and sendEmptyMessageDelayed(int what, long delay). After your Message is in the Looper queue, it s processed either as soon as possiMessageQueue ble or according to the requested schedule or delay indicated when it was sent or posted. You ll see more of Handler and Message in other examples throughout the book, and Figure 3.5 Using the Handler class with we ll cover more detail in some instances, but separate threads, and the relationship between HandlerThread, Looper, and the main point to remember when you see MessageQueue these classes is that they re used to communicate between threads and for scheduling. Getting back to our RestaurantFinder application and more view-oriented topics, we next need to elaborate on the ReviewAdapter used by our RestaurantFinder ReviewList screen after it s populated with data from a Message. This adapter returns a custom View object for each data element it processes.
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