如何确定片段在 ViewPager 中何时可见

Question

Problem: Fragment onResume() in ViewPager is fired before the fragment becomes actually visible.

For example, I have 2 fragments with ViewPager and FragmentPagerAdapter. The second fragment is only available for authorized users and I need to ask the user to log in when the fragment becomes visible (using an alert dialog).

BUT the ViewPager creates the second fragment when the first is visible in order to cache the second fragment and makes it visible when the user starts swiping.

So the onResume() event is fired in the second fragment long before it becomes visible. That's why I'm trying to find an event which fires when the second fragment becomes visible to show a dialog at the appropriate moment.

How can this be done?

转载于:https://stackoverflow.com/questions/10024739/how-to-determine-when-fragment-becomes-visible-in-viewpager

Answer 1

UPDATE: Android Support Library (rev 11) finally fixed the user visible hint issue, now if you use support library for fragments, then you can safely use getUserVisibleHint() or override setUserVisibleHint() to capture the changes as described by gorn's answer.

UPDATE 1 Here is one small problem with getUserVisibleHint(). This value is by default true.

// Hint provided by the app that this fragment is currently visible to the user.
boolean mUserVisibleHint = true;

So there might be a problem when you try to use it before setUserVisibleHint() was invoked. As a workaround you might set value in onCreate method like this.

public void onCreate(@Nullable Bundle savedInstanceState) {
    setUserVisibleHint(false);

---

The outdated answer:

In most use cases, ViewPager only show one page at a time, but the pre-cached fragments are also put to "visible" state (actually invisible) if you are using FragmentStatePagerAdapter in Android Support Library pre-r11.

I override :

public class MyFragment extends Fragment {
    @Override
    public void setMenuVisibility(final boolean visible) {
        super.setMenuVisibility(visible);
        if (visible) {
            // ...
        }
    }
   // ...
}

To capture the focus state of fragment, which I think is the most suitable state of the "visibility" you mean, since only one fragment in ViewPager can actually place its menu items together with parent activity's items.