java - 从 MainActivity 启动 fragment

Question

我有一个 fragment 类和一个 Activity 类。

这是我的 fragment 类:

public class LoginFragment extends Fragment {

    private TextView mTextDetails;

    private CallbackManager mCallbackManager;

    private FacebookCallback<LoginResult> mCallback = new FacebookCallback<LoginResult>() {

        @Override
        public void onSuccess(LoginResult loginResult) {
            AccessToken accessToken = loginResult.getAccessToken();
            Profile profile = Profile.getCurrentProfile(); //Access the profile who Is the person login in

            if(profile != null) {
                mTextDetails.setText("Welcome" + profile.getName());
            }
        }

        @Override
        public void onCancel() {

        }

        @Override
        public void onError(FacebookException e) {

        }

    };

    public LoginFragment() {

    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        FacebookSdk.sdkInitialize(getActivity().getApplicationContext());
        mCallbackManager = CallbackManager.Factory.create();
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.login, container, false);
    }

    @Override
    public void onViewCreated(View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        LoginButton loginButton = (LoginButton) view.findViewById(R.id.login_button);

        //Ask the use for permission to access friends
        loginButton.setReadPermissions("user_friends");

        //Because we work with fragments
        loginButton.setFragment(this);

        loginButton.registerCallback(mCallbackManager, mCallback);
    }

    @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        mCallbackManager.onActivityResult(requestCode, resultCode, data);
    }
}

我想从我的 MainActivity 启动这个 fragment :

public class MainActivity extends FragmentActivity {

    CallbackManager callbackManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        FacebookSdk.sdkInitialize(getApplicationContext());

        FragmentManager fm = getSupportFragmentManager();
        Fragment fragment = fm.findFragmentById(R.id.fragmentContainer);

        fm.beginTransaction().add(R.id.fragmentContainer, fragment).commit();


    }
}

当我尝试运行我的应用程序时，收到此错误消息:

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.waddapp.bryan.waddapp/com.waddapp.bryan.waddapp.MainActivity}: java.lang.NullPointerException: Attempt to write to field 'android.support.v4.app.FragmentManagerImpl android.support.v4.app.Fragment.mFragmentManager' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
        at android.app.ActivityThread.access$800(ActivityThread.java:151)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5254)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
 Caused by: java.lang.NullPointerException: Attempt to write to field 'android.support.v4.app.FragmentManagerImpl android.support.v4.app.Fragment.mFragmentManager' on a null object reference

我做错了什么？ FragmentContainer 是位于我的 Activity_main.xml 中的 RelatieLayoute 上的 ID

Answer 1

下面的代码返回一个空 fragment ，因为没有 fragment 添加到 View 中。

Fragment fragment = fm.findFragmentById(R.id.fragmentContainer);

尝试以下操作:

LoginFragment fragment = new LoginFragment();
fm.beginTransaction().add(R.id.fragmentContainer, fragment).commit();