Develop Reference

Executing operations on UI thread after delay

I want to implement some operations on UI thread after a delay of a few seconds and have tried this approach -
final Handler handler1 = new Handler();
final Runnable r = new Runnable() {
public void run() {
// operations to do
}
};
runOnUiThread(new Runnable() {
#Override
public void run() {
handler1.postDelayed(r, 1000);
}
});
Here I have two runnable objects, so my question is that are the operations I'm performing here being executed in UI thread or another thread because I don't directly execute the operations in Runnable object of the UI thread. Also if this is not the right approach to execute operations in UI thread after a delay, please suggest any modifications required.

When you post a Runnable with a Handler, the Handler executes it on whatever Thread created that Handler.
Handler's default constructor new Handler() is a synonym of new Handler(Looper.myLooper()). That might mean that the Handler will execute Runnables on the main thread, but only if the instantiation happened on the main Thread.
Either way, what you're doing is redundant. The runOnUiThread() is useless. Just change the Handler's constructor:
final Handler handler1 = new Handler(Looper.getMainLooper());
handler1.postDelayed(r, 1000);

How can I jump from a Handler started from a service to the service's thread?

I use a Handler in my Service class to do some background work, but when it's done, I must execute some code on the Service's thread. Here's my code:
I create a handler in the Service's onCreate method.
HandlerThread handlerThread = new HandlerThread(getPackageName());
handlerThread.start();
Looper looper = handlerThread.getLooper();
mHandler = new Handler(looper);
How I use the Handler. This method is getting called dozens of times during a session.
mHandler.post(new Runnable() {
#Override
public void run() {
// It takes a few seconds to execute this method,
// so it must be running on a separate thread.
Object o = superMethod();
// However, this MUST be called from the Service's thread.
useObject(o);
}
};)
So how can I get back to the Service's thread?

Service itself runs on main thread here you want switch from Handler Thread(Background thread) to service(main Thread).
Handler handler = new Handler(getMainLooper());
handler .post(new Runnable() {
#Override
public void run() {
// do your main thread task here
}
});

What happens if a Handler posts a message to a thread after Looper.prepare() but before Looper.loop() has been called?

Consider the following snippet:
Looper.prepare();
handler = new Handler() {
#Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
getLooper().quitSafely();
}
};
for(int i = 0; i < urls.size(); i++) {
useCaseProvider.get().execute(callback, handler, urls.get(i), threadPool);
}
Looper.loop();
//Continue processing the results of all the use cases after the
//loop has been asked to terminated via the handler
A little background: I'm doing some processing on the UI thread where I will need to ping a large about of devices and do something with the result. I need to perform the requests in parallel to be efficient.
Question: If one of these use cases somehow executed fast enough and made a callback before I was able to hit Looper.loop(); would the message be queued or just lost? Callbacks are being posted back to this thread by the handler posting a runnable to the original thread.

Assuming you have invoked Looper.prepare() prior to your useCaseProvider delivering results, you should be fine.
If Looper.prepare was not called you should be seeing RuntimeException being thrown.
The Looper object is tied to a thread local which hosts the message queue. The Looper.prepare function will construct this message queue at which point you can begin queuing up messages. Once you fire Looper.loop() that's when those pending messages will begin to execute.
Looking at the snippet, I'm not too sure how things are tied together.
Generally you want to construct a looper like this:
private static final class MyThread extends Thread {
private Handler mHandler;
#Override
public void run() {
Looper.prepare();
mHandler = new Handler() {
#Override
public void handleMessage(Message msg) {
// handle message
}
};
Looper.loop();
}
public Handler getHandler() {
return mHandler;
}
}
I'm assuming your thread pool is then a pool of MyThread threads, each of which have their own Looper. The thread pool should initialize your threads so once you deliver a Runnable to be executed by your thread, the run() method should have the Looper initialized.
On the other hand, if you wish to associate your Handler with a particular looper (ie. you are not constructing the Handler within a thread like above) then you should be passing the Looper thread in to the constructor like:
Handler h = new Handler(myLooperThread);
If you don't specify that, then the handler uses the thread in which it was created to grab that thread's Looper from the ThreadLocal object.
Lastly if your intentions are to have messages delivered on the Handler which is associated with the UI thread then you should not be concerned about calling Looper.prepare or Looper.loop. This is handled by the Activity.

When to use handler.post() & when to new Thread()

I'm wondering when should I use handler.post(runnable); and when should I use
new Thread(runnable).start();
It is mentioned in developers documentation for Handler:
Causes the Runnable r to be added to the message queue. The runnable
will be run on the thread to which this handler is attached.
Does this mean if I write in the onCreate() of Activity class:
Handler handler = new Handler();
handler.post(runnable);
then runnable will be called in a separate thread or in the Activity's thread?

You should use Handler.post() whenever you want to do operations on the UI thread.
So let's say you want to change a TextView's text in the callback. Because the callback is not running on the UI thread, you should use Handler.post().
In Android, as in many other UI frameworks, UI elements (widgets) can be only modified from UI thread.
Also note that the terms "UI thread" and "main thread" are often used interchangeably.
Edit: an example of the long-running task:
mHandler = new Handler();
new Thread(new Runnable() {
#Override
public void run () {
// Perform long-running task here
// (like audio buffering).
// You may want to update a progress
// bar every second, so use a handler:
mHandler.post(new Runnable() {
#Override
public void run () {
// make operation on the UI - for example
// on a progress bar.
}
});
}
}).start();
Of course, if the task you want to perform is really long and there is a risk that user might switch to some another app in the meantime, you should consider using a Service.

To answer you specific question:
Does this mean if in the onCreate of Activity class I write:
Handler handler = new Handler() hanlder.post(runnable); then, runnable
will be called in a separate thread or on the Activity's thread?
No it won't be. The Runnable will be called on the Main Thread itself.
Handler is simply used for posting a message to the thread to which it is attached (where its is created).
It does not create a thread on its own.
In your example, you created a Handler in the main Thread (that where Activity.OnCreate() is called) and hence any message posted on such a Handler will be run on the Main Thread only.

Example is jacked:
mHandler = new Handler();
new Thread(new Runnable(){
#Override
public void run () {
mHandler.post(new Runnable() {
#Override
public void run () {
mUiView.setX(x);
}
});
}
}).start();
Alternatively you can skip the handler and use the post method on the view directly:
new Thread(new Runnable(){
#Override
public void run () {
mUiView.post(new Runnable() {
#Override
public void run () {
mUiView.setX(x);
}
});
}
}).start();
This is a good post that outlines the difference: What exactly does the post method do?

use handler.post() when you want to post the code (usually from background thread) to the main thread. Yea, POST,just like you, post a letter to someone. With the help of handler the code will be executed ASAP i.e. almost immediately.

Handler-Looper implementation in Android

I have Activity with Handler (UI thread)
I start new Thread and make handler.post(new MyRunnable()) - (new work thread)
Android documentation said about post method: "Causes the Runnable r to be added to the message queue. The runnable will be run on the thread to which this handler is attached."
Handler attached to UI thread.
How android can run runnable in the same UI thread without new thread creation?
Is new thread will be created using Runnable from handler.post()?
Or it's only run() method will be called from Runnable subclass?

Here's a rough pseudroidcode example of how to use handlers - I hope it helps :)
class MyActivity extends Activity {
private Handler mHandler = new Handler();
private Runnable updateUI = new Runnable() {
public void run() {
//Do UI changes (or anything that requires UI thread) here
}
};
Button doSomeWorkButton = findSomeView();
public void onCreate() {
doSomeWorkButton.addClickListener(new ClickListener() {
//Someone just clicked the work button!
//This is too much work for the UI thread, so we'll start a new thread.
Thread doSomeWork = new Thread( new Runnable() {
public void run() {
//Work goes here. Werk, werk.
//...
//...
//Job done! Time to update UI...but I'm not the UI thread! :(
//So, let's alert the UI thread:
mHandler.post(updateUI);
//UI thread will eventually call the run() method of the "updateUI" object.
}
});
doSomeWork.start();
});
}
}

Handler attached to UI thread.
Correct.
How android can run runnable in the same UI thread without new thread creation?
Any thread, including the main application ("UI") thread, can call post() on Handler (or on any View, for that matter).