为什么SupportMapFragment为null（支持回API9）

Question

我创建了一个 Github 项目来演示该问题。

git 克隆 https://github.com/anujgoyal/buggymap.git

我知道存在一些问题，但很少有人将程序提炼到这个级别。此外，我有一个硬性要求使用 android.support.v4.app.Fragment class，而不是 FragmentActivity，不是 MapFragment.

请注意，您必须为 AndroidManifest.xml

中的 com.google.android.maps.v2.API_KEY 更改 android:value

为什么 SupportMapFragment 为 NULL？

// MainActivity.java
package com.txt2lrn.buggymap;
import android.support.v7.app.ActionBarActivity;
import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.OnMapReadyCallback;
import com.google.android.gms.maps.SupportMapFragment;

public class MainActivity extends ActionBarActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment())
                    .commit();
        }
    }

    public static class PlaceholderFragment extends Fragment implements OnMapReadyCallback {

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            super.onCreateView(inflater, container, savedInstanceState);
            View rootView = inflater.inflate(R.layout.fragment_main, container, false);

            //SupportMapFragment smf0 =
            //        (SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.map);

            SupportMapFragment smf1 =
                (SupportMapFragment) getFragmentManager().findFragmentById(R.id.map);

            SupportMapFragment smf2 =
                    (SupportMapFragment)
            getActivity().getSupportFragmentManager().findFragmentById(R.id.map);

            Log.d("maps", "smf1: "+smf1); // always NULL
            Log.d("maps", "smf2: "+smf2); // always NULL
            //smf.getMapAsync(this);
            return rootView;
        }

        @Override
        public void onMapReady(GoogleMap map) {
            // code will go here once getMapAsync works
        }

    }
}

这是fragment_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity$PlaceholderFragment">

    <fragment
        android:id="@+id/map"
        class="com.google.android.gms.maps.SupportMapFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</RelativeLayout>

Answer 1

您似乎在尝试使用嵌套片段。在这种情况下，您通过 getChildFragmentManager() 返回的 FragmentManager 获取这些嵌套片段，而不是 activity.

调用的 getSupportFragmentManager()