CODEWITHSUNDEEP
androidandroid-handlerandroid-looperandroid-threadandroid-threading
Mena
Mena

Reputation: 3415

What is the difference between these methods for running code on UI Thread?

There are different methods posted on the web on how to run code on the UI thread. They all accomplish the same task, however, I really want to know the difference between these methods.

Method 1:

new Handler(Looper.getMainLooper()).post(new Runnable() {
    @Override
    public void run() {
        // Code here will run in UI thread
    }
});

Method 2:

new Handler().post(new Runnable() {
    @Override
    public void run() {
        // Code here will run in UI thread
    }
});

Method 3:

 runOnUiThread(new Runnable() {
     @Override
     public void run() {
     // Code here will run in UI thread
     }
 });

Upvotes: 2

Views: 859

Answers (3)

Son Truong
Son Truong

Reputation: 14203

In Android, a Thread might have one Looper or MessageQueue. Handler is used to send Message or post Runnable to MessageQueue of a Thread, and it must always be associated with a Looper or a MessageQueue of a Thread.

Method 1

new Handler(Looper.getMainLooper()).post(new Runnable() {
    @Override
    public void run() {
        // Code here will run in UI thread
    }
});

When open an app, Android create a new thread (called main thread or UI thread) with a Looper and MessageQueue, this thread is used to render UI and process input events from users.

The above code is create a Handler and associated with Looper of UI thread, so the runnable will be queued to MessageQueue of UI thread, and will be executed later.

Method 2

new Handler().post(new Runnable() {
    @Override
    public void run() {
        // Code here will run in UI thread
    }
});

Creating a Handler and associated with Looper of current thread, there are 3 cases:

  • If this code is executed on UI thread, then the runnable will be queued to MessageQueue of UI thread and will be executed later.
  • If this code is executed on a background thread, if this thread has a Looper, then the runnable will be queued to MessageQueue of background thread and will be executed later.
  • If this code is executed on a background thread and this thread has no Looper, then an exception will be thrown.

Method 3

runOnUiThread(new Runnable() {
     @Override
     public void run() {
     // Code here will run in UI thread
     }
});

runOnUiThread is just a utility method of Activity, it used when you want to execute some code on UI thread. The logic behind this method is if current thread is UI thread, then execute it immediately, otherwise used Handler to post a message to MessageQueue of UI thread (like method 1).

Upvotes: 2

Gabe Sechan
Gabe Sechan

Reputation: 93708

Method 1 will always work.

Method 2 will only work if you're already on the UI thread- the new Handler without a Looper parameter creates a Handler to the current thread (and fails if there is no Looper on the current thread).

Method 3 needs to be done in an Activity or called on an Activity object, as runOnUiThread is a function of Activity. But under the hood it will do the same as 1 (although probably keeps a single Handler around to be more efficient, rather than always new-ing one).

Upvotes: 2

user10507913
user10507913

Reputation:

All methods works like this:

Method 1 looping handler if loop exist

Method 2 handler can works in all activities if not private or wanted

Method 3 handler can work only in current activity

Upvotes: 0

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