What is the purpose of Looper and how to use it?
What is Looper?
Looper is a class which is used to execute the Messages(Runnables) in a queue. Normal threads have no such queue, e.g. simple thread does not have any queue. It executes once and after method execution finishes, the thread will not run another Message(Runnable).
Where we can use Looper class?
If someone wants to execute multiple messages(Runnables) then he should use the Looper class which is responsible for creating a queue in the thread.For example, while writing an application that downloads files from the internet, we can use Looper class to put files to be downloaded in the queue.
How it works?
There is prepare() method to prepare the Looper. Then you can use loop() method to create a message loop in the current thread and now your Looper is ready to execute the requests in the queue until you quit the loop.
Here is the code by which you can prepare the Looper.
class LooperThread extends Thread { public Handler mHandler; @Override public void run() { Looper.prepare(); mHandler = new Handler() { @Override public void handleMessage(Message msg) { // process incoming messages here } }; Looper.loop(); } }
You can better understand what Looper is in the context of GUI framework. Looper is made to do 2 things.
1) Looper transforms a normal thread, which terminates when its run() method return, into something run continuously until Android app is running, which is needed in GUI framework (Technically, it still terminates when run() method return. But let me clarify what I mean in below).
2) Looper provides a queue where jobs to be done are enqueued, which is also needed in GUI framework.
As you may know, when an application is launched, the system creates a thread of execution for the application, called “main”, and Android applications normally run entirely on a single thread by default the “main thread”. But main thread is not some secret, special thread. It's just a normal thread similar to threads you create with new Thread() code, which means it terminates when its run() method return! Think of below example.
public class HelloRunnable implements Runnable { public void run() { System.out.println("Hello from a thread!"); } public static void main(String args[]) { (new Thread(new HelloRunnable())).start(); }}Now, let's apply this simple principle to Android apps. What would happen if an Android app runs on normal thread? A thread called "main" or "UI" or whatever starts your application, and draws all UI. So, the first screen is displayed to users. So what now? The main thread terminates? No, it shouldn’t. It should wait until users do something, right? But how can we achieve this behavior? Well, we can try with Object.wait() or Thread.sleep(). For example, main thread finishes its initial job to display first screen, and sleeps. It awakes, which means interrupted, when a new job to do is fetched. So far so good, but at this moment we need a queue-like data structure to hold multiple jobs. Think about a case when a user touches screen serially, and a task takes longer time to finish. So, we need to have a data structure to hold jobs to be done in first-in-first-out manner. Also, you may imagine, implementing ever-running-and-process-job-when-arrived thread using interrupt is not easy, and leads to complex and often unmaintainable code. We'd rather create a new mechanism for such purpose, and that is what Looper is all about. The official document of Looper class says, "Threads by default do not have a message loop associated with them", and Looper is a class "used to run a message loop for a thread". Now you can understand what it means.
To make things more clear, let's check the code where main thread is transformed. It all happens in ActivityThread class. In its main() method, you can find below code, which turns a normal main thread into something what we need.
public final class ActivityThread { ... public static void main(String[] args) { ... Looper.prepareMainLooper(); Looper.loop(); ... }}and Looper.loop() method loop infinitely and dequeue a message and process one at a time:
public static void loop() { ... for (;;) { Message msg = queue.next(); // might block if (msg == null) { // No message indicates that the message queue is quitting. return; } ... msg.target.dispatchMessage(msg); ... }}So, basically Looper is a class that is made to address a problem that occurs in GUI framework. But this kind of needs can also happen in other situation as well. Actually it is a pretty famous pattern for multi threads application, and you can learn more about it in "Concurrent Programming in Java" by Doug Lea(Especially, chapter 4.1.4 "Worker Threads" would be helpful). Also, you can imagine this kind of mechanism is not unique in Android framework, but all GUI framework may need somewhat similar to this. You can find almost same mechanism in Java Swing framework.
Looper allows tasks to be executed sequentially on a single thread. And handler defines those tasks that we need to be executed. It is a typical scenario that I am trying to illustrate in this example:
class SampleLooper extends Thread {@Overridepublic void run() { try { // preparing a looper on current thread // the current thread is being detected implicitly Looper.prepare(); // now, the handler will automatically bind to the // Looper that is attached to the current thread // You don't need to specify the Looper explicitly handler = new Handler(); // After the following line the thread will start // running the message loop and will not normally // exit the loop unless a problem happens or you // quit() the looper (see below) Looper.loop(); } catch (Throwable t) { Log.e(TAG, "halted due to an error", t); } }}Now we can use the handler in some other threads(say ui thread) to post the task on Looper to execute.
handler.post(new Runnable(){public void run() {//This will be executed on thread using Looper. }});On UI thread we have an implicit Looper that allow us to handle the messages on ui thread.