java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare();

android multithreading toast
Amar picture Amar · Jun 29, 2013 · Viewed 138.9k times · Source

I have an Android app running a thread. I want a Toast message to show with a message.

When I do this, I get the below exception:

Logcat trace:

FATAL EXCEPTION: Timer-0 
 java.lang.RuntimeException: Can't create handler inside thread that has not 
    called Looper.prepare()

 at android.os.Handler.<init>(Handler.java:121)
 at android.widget.Toast$TN.<init>(Toast.java:322)
 at android.widget.Toast.<init>(Toast.java:91)
 at android.widget.Toast.makeText(Toast.java:238)

Is there a work around for pushing Toast messages from threads to the User Interface?

Answer

Eric Leschinski picture Eric Leschinski · Sep 22, 2013

I got this exception because I was trying to make a Toast popup from a background thread.
Toast needs an Activity to push to the user interface and threads don't have that.
So one workaround is to give the thread a link to the parent Activity and Toast to that.

Put this code in the thread where you want to send a Toast message:

parent.runOnUiThread(new Runnable() {
    public void run() {
        Toast.makeText(parent.getBaseContext(), "Hello", Toast.LENGTH_LONG).show();
    }
});

Keep a link to the parent Activity in the background thread that created this thread. Use parent variable in your thread class:

private static YourActivity parent;

When you create the thread, pass the parent Activity as a parameter through the constructor like this:

public YourBackgroundThread(YourActivity parent) {
    this.parent = parent;
}

Now the background thread can push Toast messages to the screen.

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