Android开发-Fragment返回键监听-AndroidStudio

@Override
public void onResume() {
super.onResume();

getView().setFocusableInTouchMode(true);
getView().requestFocus();
getView().setOnKeyListener(new View.OnKeyListener() {
@Override
public boolean onKey(View v, int keyCode, KeyEvent event) {
if (event.getAction() == KeyEvent.ACTION_UP && keyCode == KeyEvent.KEYCODE_BACK){
// handle back button

return true;
}
return false;
}
});
}

@Override
public void onResume() {
super.onResume();
System.out.println("TwoFragment onResume");

getView().setFocusableInTouchMode(true);
getView().requestFocus();
getView().setOnKeyListener(new View.OnKeyListener() {
@Override
public boolean onKey(View v, int keyCode, KeyEvent event) {
if (event.getAction() == KeyEvent.ACTION_UP && keyCode == KeyEvent.KEYCODE_BACK){
// handle back button
Toast.makeText(mainActivity, "TwoFragment onResume Listener", Toast.LENGTH_SHORT).show();
mainActivity.showOneFragment(); // 调用Activity的方法，从Fragment2切换到Fragment1
return true;
}
return false;
}
});
}