正在从附加有后台线程的处理程序中显示Toast。 不确定如何发生？

Question

I used HandlerThread and then used its looper to create a new Handler so that it can run operations on a non-UI thread. In the runnable which is posted to the handler, I added Toast messages to be displayed. I expected that to cause a problem as you can't touch UI components from the non-UI thread, but it still works and that toast is still being shown. Can anyone explain why toast is being displayed from the non-UI thread?

 //Inside a Fragment class
    private Handler handler;
    private HandlerThread mHandlerThread = null;

    public void startHandlerThread() {
        mHandlerThread = new HandlerThread("HandlerThread");
        mHandlerThread.start();
        handler = new Handler(mHandlerThread.getLooper());
    }


    private Runnable submitRunnable = new Runnable() {
        @Override
        public void run() {
            //do some long running operations here
            //Thread.sleep(2000);

            //Check whether currentLooper is the Main thread looper
            boolean isUiThread = Build.VERSION.SDK_INT >= Build.VERSION_CODES.M
                    ? Looper.getMainLooper().isCurrentThread()
                    : Thread.currentThread() == Looper.getMainLooper().getThread();

            if (isUiThread) {
                // You are on the UI thread
                Log.d("Thread", "Main thread");
            } else {
                // You are on the non-UI thread
                Log.d("Thread", "Not Main thread"); //This will be printed
            }

            Toast.makeText(getContext(), "toast is shown", Toast.LENGTH_SHORT).show();
        }
    };

    submitButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                handler.post(submitRunnable);
            }
        });

I checked Toast.java and saw that the looper initializes itself with Looper.myLooper().

 if (looper == null) {
            // Use Looper.myLooper() if looper is not specified.
            looper = Looper.myLooper();
        }

From the doc :

myLooper(): Return the Looper object associated with the current thread.

And the currentThread is the HandlerThread, not the main thread. Hence, I am unable to understand how the toast is being displayed from the non-UI thread, or if it is something plain simple I am missing to see.

Answer 1

For showing Toast you used getContext() as a context.

getContext() - Returns the context the view is currently running in. Usually the currently active Activity.

While you are using fragment so it will take activity context where fragment will resides in activity.

That's why Toast shown.