javaandroidmultithreadingandroid-handler

Running code in main thread from another thread

In an Android service, I have created thread(s) for doing some background task.

I have a situation where a thread needs to post a certain task on the main thread's message queue, for example a Runnable.

Is there a way to get the Handler of the main thread and post a Message/Runnable to it from my other thread?

Solution

  • NOTE: This answer has gotten so much attention, that I need to update it. Since the original answer was posted, the comment from @dzeikei has gotten almost as much attention as the original answer. So here are 2 possible solutions:

    1. If your background thread has a reference to a Context object:

    Make sure that your background worker threads have access to a Context object (can be the Application context or the Service context). Then just do this in the background worker thread:

    // Get a handler that can be used to post to the main thread
Handler mainHandler = new Handler(context.getMainLooper());

Runnable myRunnable = new Runnable() {
    @Override 
    public void run() {....} // This is your code
};
mainHandler.post(myRunnable);

    2. If your background thread does not have (or need) a Context object

    (suggested by @dzeikei):

    // Get a handler that can be used to post to the main thread
Handler mainHandler = new Handler(Looper.getMainLooper());

Runnable myRunnable = new Runnable() {
    @Override 
    public void run() {....} // This is your code
};
mainHandler.post(myRunnable);