How do I get the back button to go to a certain Fragment in horizontal scrolling? Exits app right now

问题

the title pretty much explains it.

I have horizontal scrolling set up, the first screen has buttons to the other Fragments as well as the whole horizontal scroll system. What I would like is for the user to be able to press the back button when on one of these fragments and for the app to return to the first screen with all the buttons.

From there I want the back button to be an AlertDialog asking the user if they would like to exit the app. At the moment this is what is happening (On all Fragments when you press the back button the AlertDialog I created pops up).

I've looked into Fragment transactions and "addToBackStack()" but I don't know how to implement it. I've looked at the dev guide and certain questions on this site but getting one or two lines of code doesn't help in implementing it.

I have a FragmentActivity with a FragmentPagerAdapter set up and each Fragment has its own Java file. I have 5 Fragments that are all called in the FragmentActivity and FragmentPagerAdapter.

I don't think I need to show you guys any of my code for the moment since it's all set up in the normal manner. Please let me know if you do though.

The bit of code I found on other questions and one in particular was the following:

FragmentTransaction tx = fragmentManager.beginTransation();
tx.replace( R.id.fragment, new MyFragment() ).addToBackStack( "tag" ).commit();

It's a bit hard to go on just that though.

I would really appreciate your help.

EDIT: my code removed - wasn't needed.

回答1:

If you use the ViewPager from your question which I answered earlier and you want to come back to the first fragment of the ViewPager when the user presses the BACK button then override the onBackPressed method like this:

@Override
public void onBackPressed() {
    if (getSupportFragmentManager().findFragmentByTag("outDialog") != null
            && ((DialogFragment) getSupportFragmentManager()
                    .findFragmentByTag("outDialog")).isVisible()) {
        // we have the out dialog visible and the user clicked back so let
        // the
        // normal events happen
        super.onBackPressed();
        return;
    }
    int currentPosition = mViewPager.getCurrentItem();
    if (currentPosition != 0) {
        // if the page the ViewPager shows isn't the first one then move it
        // to the first one
        mViewPager.setCurrentItem(0);
    } else {
        // we are at the first position already and the user wants out, so
        // annoy him with a dialog that asks him once again if he wants out.
        DialogFragment askHim = new DialogFragment();
        askHim.show(getSupportFragmentManager(), "outDialog");
        // in the dialog listener, if the user presses ok, finish the activity
    }
}

回答2:

did you try to override Activity.onBackPressed() ? I used addToBackStack() and it works well, but I have no idea whether it works with PagerAdapter. I think you can override Activity.onBackPressed() and in that method, you can check whether current page is front page or not and do whatever job you want to do.

Here are pseudo code that I think.

public void onBackPressed() {
  if( pager.getCurrentPage() == 0 ) { //I'm not sure this method exists or not. just example. :-)
      //exit code here
  } else {
      // show first page
  }
}

来源：https://stackoverflow.com/questions/14071233/how-do-i-get-the-back-button-to-go-to-a-certain-fragment-in-horizontal-scrolling