How to handle back button using view pager?

Question

I am having two fragments Frag A and Frag B which are rendered using view pager . If user has swiped from A to B then presses back button(when in B) then user should go to A instead of coming out of view pager . How can we achieve this ?

Adding transaction to backstack does not seem to help .

Thanks

Solution 1

You have to override the onKeyDown() method in the Activity.

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK) {
        myViewPager.setCurrentItem(0, true);
        return true;
    } else {
       return super.onKeyDown(keyCode, event);
    }
}

This will capture the "back" button press event and send the user to the first item in the ViewPager.

There is also ViewPager#getCurrentItem() which can be used to go back only if the user swiped. An implementation like this:

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK && myViewPager.getCurrentItem() == 1) {
        myViewPager.setCurrentItem(0, true);
        return true;
    } else {
       return super.onKeyDown(keyCode, event);
    }
}

would send the user back to the previous page only if the user was on the next page. Else, it will register the "back" normally and the user will exit as expected.

You may want the case in which the user goes back to the previous page (assuming you have more than two pages). In which case, you want to implement the ViewPager.OnPageChangeListener() and save the position every time onPageSelected(int position) is called. If you implement them in a stack, then you can pop them off every time the user presses "back". Once the stack is empty, exit the app.

EDIT

As correctly stated in the comments by Jade Byfield, a slightly easier way to do this would be to use onBackPressed() in the Activity. This will only work in API levels 5+.

@Override
public void onBackPressed() {
  if(i_dont_want_to_leave) {
     myViewPager.setCurrentItem(0, true);
  } else {
    super.onBackPressed(); // This will pop the Activity from the stack.
  }
}

Solution 2

On the Android Docs page

https://developer.android.com/training/animation/screen-slide.html#viewpager

I found an example of handling ViewPager sliding and there's a method

@Override
public void onBackPressed() {
  if (mPager.getCurrentItem() == 0) {
    super.onBackPressed();
  }
  else {
    mPager.setCurrentItem(mPager.getCurrentItem() - 1);
  }
}

This selects a previous page or exits the current activity.

Comments

[Jade Byfield]

Could he also override onBackPressed? Or is that only for earlier APIs?

[DeeV]

Using onKeyDown() works on every platform API level 1+. Also, unless I've been using it wrong all along, onBackPressed won't prevent the Activity from being popped from the stack. If you return true in onKeyDown(), then that will be the only thing that happens and the event will stop cascading.

[DeeV]

Wait. Just had a brain dump. Yes. He can use onBackPressed() if he wants to target API 5+. Just don't call the super.onBackPressed() unless you're absolutely sure you want to exit.