Null Context in setUserVisibleHint

问题

A message needs to be displayed to the user when a fragment in a ViewPager becomes visible. Currently the call is:

// Within a class that extends Fragment
@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
    super.setUserVisibleHint(isVisibleToUser);

    if (isVisibleToUser) {
        MessageUtility.displayMessage(getContext());
    }
}

MessageUtility.displayMessage() opens an alert dialog with a message.

Context is required to display the message. However, the fragment is not guaranteed to be attached by the time setUserVisibleHint() is called. Thus, the Context will be null, even though isVisiableToUser == true .

Checking isVisibleToUser && isAttached() works in theory, but setUserVisibleHint() is not provoked after isAttached() == true.

Is there some way to allow the call to displayMessage() to wait until the Fragment is attached?

回答1:

Better late than never ,I have a little hackish solution ,call setUserVisibleHint() from onActivityCreated() method.

private boolean myIsVisibleToUser;

 @Override
public void setUserVisibleHint(boolean isVisibleToUser) {
    super.setUserVisibleHint(isVisibleToUser);
    myIsVisibleToUser=isVisibleToUser;
    if (isVisibleToUser && getActivity()!=null) {
          MessageUtility.displayMessage(getContext());    
        }
}


@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);

    setUserVisibleHint(myIsVisibleToUser);

}

回答2:

It's a bit different approach but more effective solution. Add a new OnPageChangeListenr to your ViewPager.

 yourPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
            @Override
            public void onPageScrolled(int i, float v, int i2) {

            }

            @Override
            public void onPageSelected(int i) {
                // Check if i == your fragment's position and display message        
                MessageUtility.displayMessage(getContext());
            }

            @Override
            public void onPageScrollStateChanged(int i) {

            }
        });

来源：https://stackoverflow.com/questions/39105725/null-context-in-setuservisiblehint