如果片段事务未完成，应用程序将崩溃

Question

I have put one button on click of that there is a fragment transaction from one to another fragment. also with fragment transaction, I put the Handler to delay the transaction for 5 seconds. Now the issue is If I press back button before the fragment transaction get completed my app gets crashed after those 5 seconds get completed. Below is mine fragment transaction code and logcat error.

Fragment Transaction Code:

Handler handler = new Handler();
            Runnable runnable = new Runnable() {
                @Override
                public void run() {
                    //Second fragment after 5 seconds appears
                    FragmentManager fragmentManager = getActivity().getSupportFragmentManager();
                    FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
                    OtpVerificationFragment otpVerificationFragment = new OtpVerificationFragment();
                    fragmentTransaction.replace(R.id.fragmentContainerDashboard, otpVerificationFragment);
                    fragmentTransaction.commit();
                }
            };
            handler.postDelayed(runnable, 5000);

Logcat Error:

java.lang.NullPointerException: Attempt to invoke virtual method 'androidx.fragment.app.FragmentManager androidx.fragment.app.FragmentActivity.getSupportFragmentManager()' on a null object reference.

Answer 1

The problem is you do not remove the callbacks while pressing the back button.

private Handler handler = new Handler();
private Runnable runnable = new Runnable() {
        @Override
        public void run() {
            //Second fragment after 5 seconds appears
                    FragmentManager fragmentManager = getActivity().getSupportFragmentManager();
                    FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
                    OtpVerificationFragment otpVerificationFragment = new OtpVerificationFragment();
                    fragmentTransaction.replace(R.id.fragmentContainerDashboard, otpVerificationFragment);
                    fragmentTransaction.commit();

        }
    };

Your postDelayed call when you need to start the delay:

handler.postDelayed(runnable, 5000);

Remove the callback while pressing back button as below:

handler.removeCallbacks(runnable);

Answer 2

It is a good practice, and I'd say even a requirement, to clear Handler from all running tasks when you are leaving a Fragment or an Activity .

You should override onDetach method and clear all handlers from callbacks and messages:

@Override
public void onDetach() {
    super.onDetach()
    handler.removeCallbacksAndMessages(null)
}

And also you should make sure that the contents of runnable you passed is executed only if the current fragment is still attached to its context and is not being removed from backstack.

Runnable runnable = new Runnable() {
            @Override
            public void run() {
                //Second fragment after 5 seconds appears

                if (!isAdded() || isRemoving()) {
                    // Avoid execution
                    return;
                }

                FragmentManager fragmentManager = getActivity().getSupportFragmentManager();
                FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
                OtpVerificationFragment otpVerificationFragment = new OtpVerificationFragment();
                fragmentTransaction.replace(R.id.fragmentContainerDashboard, otpVerificationFragment);
                fragmentTransaction.commit();
            }
        };