Displaying a DialogFragment is confusing me

Question

I am going over the official Android tutorial for DialogFragments. The part that slightly confuses me is:

void showDialog() {
    mStackLevel++;

    // DialogFragment.show() will take care of adding the fragment
    // in a transaction.  We also want to remove any currently showing
    // dialog, so make our own transaction and take care of that here.
    FragmentTransaction ft = getFragmentManager().beginTransaction();
    Fragment prev = getFragmentManager().findFragmentByTag("dialog");
    if (prev != null) {
        ft.remove(prev);
    }
    ft.addToBackStack(null);

    // Create and show the dialog.
    DialogFragment newFragment = MyDialogFragment.newInstance(mStackLevel);
    newFragment.show(ft, "dialog");
}  

So my confusion arises from the fact that they are using findFragmentByTag("dialog"). Nowhere is a layout XML declared that has a tag named dialog in it. In case of normal Fragmants, there are <fragment ../> tags in the layout so I can retrieve fragments with Id or tagname. Here, there isn't any.

So, what gives ? How does this work ?
Also, what if I have multiple DialogFragments ? findFragmentByTag("dialog"); will return what??

:)

Answer 1

Fragments can be attached to an Activity in two ways: statically by declaring it in the layout xml using the <fragment> tag; or programmatically using FragmentManager and FragmentTransaction. In the case of DialogFragments, you will always be using the latter.

When you attach a fragment to the Activity, such as by using DialogFragment.show(), you can give it a tag. The FragmentManager can then later find this fragment by its tag. In this case, the code is checking if the FragmentManager already has a fragment with this tag (which I believe would be the case if the dialog was already showing when showDialog() was called). If so, it removes the fragment (dismissing the dialog) before showing a new instance of it.