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[英]MainActivity to Fragment and from that Fragment back to MainActivity
[英]Start fragment from MainActivity
我有一個片段類和一個活動類。
這是我的片段類:
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);
}
}
我想從我的MainActivity開始這個片段:
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();
}
}
當我嘗試運行我的應用程序時,出現以下錯誤消息:
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
我究竟做錯了什么? fragmentContainer是我的RelatieLayoute上的ID,位於我的activity_main.xml中
以下代碼返回空片段,因為沒有片段添加到視圖中。
Fragment fragment = fm.findFragmentById(R.id.fragmentContainer);
請嘗試以下操作:
LoginFragment fragment = new LoginFragment();
fm.beginTransaction().add(R.id.fragmentContainer, fragment).commit();
我發現了問題。 我忘了實例化我的片段類。
我添加了這個:
if(fragment == null) {
fragment = new LoginFragment();
fm.beginTransaction().add(R.id.fragmentContainer, fragment).commit();
}
嘗試使用支持庫片段以實現向后兼容。 擴展LoginFragment中的支持片段
import android.support.v4.app.Fragment;
public class LoginFragment extends android.support.v4.app.Fragment {
並使用supportFragmentManager進行片段交易
FragmentManager fm = getSupportFragmentManager();
Fragment fragment = fm.findFragmentById(R.id.fragmentContainer);
fm.beginTransaction().add(R.id.fragmentContainer, fragment).commit();
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