Android导航架构组件-系统的后退按钮退出应用

Question

I am making a demo using android's navigation architecture component . I have an activity and two fragments .

In the activity's xml, I have added the following code-

<fragment
    android:id="@+id/main_container"
    android:name="androidx.navigation.fragment.NavHostFragment"
    android:layout_width="@dimen/match_constraints"
    android:layout_height="@dimen/match_constraints"
    app:defaultNavHost="true"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    app:navGraph="@navigation/nav_graph" />

And then I have set the first fragment as the start destination. In the first fragment, on a button's click I have done following -

NavHostFragment.findNavController(this).navigate(R.id.action_firstFragment_to_secondFragment)

And it works fine. Now when I press system's back button, ideally I should go back to first fragment, but the app exits. What am I doing wrong?

Answer 1

I suppose, this is occurring in your code just because you might be handling the onBackPressed() functionality somewhere in your BaseActivity . Please look carefully.

I hope, this helps you.

Answer 2

    /** orrerride activity onBackPressed() **/

private boolean doubleBackToExitPressedOnce = false;
    @RequiresApi(api = Build.VERSION_CODES.M)
    @Override
    public void onBackPressed() {
        int start = Navigation.findNavController(this, R.id.nav_host_fragment).getCurrentDestination().getId();
        if (start == R.id.nav_home) {
            if (doubleBackToExitPressedOnce) {
                super.onBackPressed();
                return;
            }
            this.doubleBackToExitPressedOnce = true;
            Toast.makeText(MainActivity.this, "Press back again to exits", Toast.LENGTH_SHORT).show();

            new Handler().postDelayed(new Runnable() {
                @Override
                public void run() {
                    doubleBackToExitPressedOnce = false;
                }
            }, 2000);
        } else {
            super.onBackPressed();
        }
    }