Android: how to notify Activity when Fragments views are ready?

问题

I'm using Google Maps API V2 in an activity wih drop down navigation where the map is on the second position.

I'm adding the map pragmatically like:

mMapFragment = supportMapFragment.newInstance();
getSupportFragmentManager()
        .beginTransaction()
        .replace(R.id.placeHolder, mMapFragment, TAG_MAP)
        .commit(); 

I want to get the GoogleMap ovject, as the documentation https://developers.google.com/maps/documentation/android/map say it should be done with mMapFragment.getMap(), but it returns null.

according to http://developer.android.com/reference/com/google/android/gms/maps/SupportMapFragment.html it returns null if the Fragment hasn't gone through the onCreateView lifecycle event.

how can I know whenever the fragment is ready ?

EDIT: I found this How do I know the map is ready to get used when using the SupportMapFragment?

Overriding onActivityCreated seems like a solution, but then I'll have to instantiate the fragment through a constructor and not with newInstance(), does it make any difference ?

回答1:

My preferred method is to use a callback to get a signal from a Fragment. Also, this is the recommended method proposed by Android at Communicating with the Activity

For your example, in your Fragment, add an interface and register it.

public static interface OnCompleteListener {
    public abstract void onComplete();
}

private OnCompleteListener mListener;

public void onAttach(Context context) {
    super.onAttach(context);
    try {
        this.mListener = (OnCompleteListener)context;
    }
    catch (final ClassCastException e) {
        throw new ClassCastException(context.toString() + " must implement OnCompleteListener");
    }
}

Now implement this interface in your Activity

public class MyActivity extends FragmentActivity implements MyFragment.OnCompleteListener {
    //...

    public void onComplete() {
        // After the fragment completes, it calls this callback.
        // setup the rest of your layout now
        mMapFragment.getMap()
    }
}

Now, whatever in your Fragment signifies that it's loaded, notify your Activity that it's ready.

@Override
protected void onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    // create your fragment
    //...

    // signal that you're done and tell the Actvity to call getMap()
    mListener.onComplete();
}

EDIT 2017-12-05 onAttach(Activity activity) is deprecated, use onAttach(Context context) instead. Code above adjusted.

回答2:

In addition to Kirk's answer: since public void onAttach(Activity activity) is deprecated you can now simply use:

@Override
public void onAttach(Context context) {
    super.onAttach(context);

    Activity activity;

    if (context instanceof Activity){

        activity=(Activity) context;

        try {
            this.mListener = (OnCompleteListener)activity;
        } catch (final ClassCastException e) {
            throw new ClassCastException(activity.toString() + " must implement OnCompleteListener");
        }
    }
}

The rest remains the same... One might want to use (Fragment sender) as a parameter though and always pass this.

回答3:

I'm not sure I'm completely understanding your issue, but I have a similar setup where I'm using a navigation drop down menu. Here's what works for me:

1.) Load the fragment from an xml file and call setupMapIfNeeded()

SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager()
                .findFragmentById(R.id.basicMap);

setUpMapIfNeeded();

Here is the xml file for reference:

<?xml version="1.0" encoding="utf-8"?>
<fragment xmlns:android="http://schemas.android.com/apk/res/android"
  android:id="@+id/basicMap"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  class="com.google.android.gms.maps.SupportMapFragment" />

2.) Then setup the map (for details of isGoogleMapsInstalled(), see this question)

    private void setUpMapIfNeeded() 
    {
    // Do a null check to confirm that we have not already instantiated the map.
    if (mMap == null) 
    {
        // Try to obtain the map from the SupportMapFragment.
        mMap = ((SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.basicMap)).getMap();
        // Check if we were successful in obtaining the map.
        if(isGoogleMapsInstalled())
        {
            if (mMap != null) 
            {
                mMap.setOnCameraChangeListener(getCameraChangeListener());
                mMap.setInfoWindowAdapter(new MyCustomInfoWindowAdapter(this));
            }
        }
        else
        {
            MapConstants.showDialogWithTextAndButton(this, R.string.installGoogleMaps, R.string.install, false, getGoogleMapsListener());
        }
    }
    }

3.) Make sure you also call setUpMapIfNeeded() from onResume():

public void onResume()
{
    super.onResume();

    setUpMapIfNeeded();
}

回答4:

If you want to do it without any listener:

Add Fragment with a TAG

 supportFragmentManager
                .beginTransaction()
                .add(R.id.pagerContainer, UniversalWebViewFragment.newInstance(UniversalWebViewFragment.YOUTUBE_SERACH_URL+"HD trailers"), 
                "UniversalWebView")
                .disallowAddToBackStack()
                .commit()

Create a public method in Hosting Activity class which you want to call after fragment is loaded. Here I am calling back a method of my fragment for example

public fun loadURL() {
        val webViewFragment = supportFragmentManager
                              .findFragmentByTag("UniversalWebView") 
                               as UniversalWebViewFragment

        webViewFragment.searchOnYoutube("Crysis Warhead")
    }

Now inside onViewCreated Method of your fragment you can simply call that public method of Host activity like this:

    (activity as HomeActivity ).loadURL()

来源：https://stackoverflow.com/questions/14999698/android-how-to-notify-activity-when-fragments-views-are-ready