尝试从父活动访问片段视图时，为什么我得到一个空值？

Question

首先，我知道之前有人问过很多类似的问题，但没有一个回答我的问题。

我会保持简单。

我有一个名为 MainActivity 的类，其中包含以下代码：

public class MainActivity extends AppCompatActivity {
    Car myCar = new Car();
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
       getSupportFragmentManager().beginTransaction().add(R.id.buttonsContainer,myCar,"me").commitNow();
        View v = myCar.getView(); // returns null
        Button b = v.findViewById(R.id.submitButton); //throws an exception as I'm trying to execute the "findViewById" method on a null object , as the debugger say . 
        b.setText("It works !"); // it doesn't
    }
}

我有一个 fragment，其中包含一个带有以下代码的按钮：

public class Car extends Fragment {
    View view;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.car, container, false);
        return rootView;
    }
}

它只显示与其对应的布局。

这是我的 MainActivity XML 文件：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:id="@+id/buttonsContainer"
    tools:context="com.example.myapplication.MainActivity">

</LinearLayout>

这是我的fragment XML 代码：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <Button
        android:id="@+id/submitButton"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Button" />
</LinearLayout>

我想要做的基本上是我想在单击其他内容时更改按钮文本（它必须发生在父级 activity 上。

我已经知道如何在片段活动中做到这一点）。

我还想从MainActivity 而不是fragment activity 本身为该按钮设置一个onClickListner，但是每当我尝试从主activity 访问该fragment 时，我都会得到@987654338 @ 值（无论我是想获取其Activity 或View 的引用，如myCar.getActivity() 或MyCar.getView()）。

提前致谢。

Logcat：

AndroidRuntime: FATAL EXCEPTION: main
 Process: com.example.myapplication, PID: 27060
 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.myapplication/com.example.myapplication.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference
 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2665)
 at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
 at android.app.ActivityThread.-wrap12(ActivityThread.java)
 at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
 at android.os.Handler.dispatchMessage(Handler.java:102)
 at android.os.Looper.loop(Looper.java:154)
 at android.app.ActivityThread.main(ActivityThread.java:6119)
 at java.lang.reflect.Method.invoke(Native Method)
 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference
 at com.example.myapplication.MainActivity.onCreate(MainActivity.java:23)
 at android.app.Activity.performCreate(Activity.java:6679)
 at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2618)
 at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) 
 at android.app.ActivityThread.-wrap12(ActivityThread.java) 
 at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477) 
 at android.os.Handler.dispatchMessage(Handler.java:102) 
 at android.os.Looper.loop(Looper.java:154) 
 at android.app.ActivityThread.main(ActivityThread.java:6119) 
 at java.lang.reflect.Method.invoke(Native Method) 
 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

Answer 1

您的问题是，当您在代码中创建片段并添加它时，片段的视图不会被夸大，因此它不存在。如果添加 Log.v("Trace", "End of onCreate");在 onCreate 和 Log.v("Trace", "Car view inflated") 结束时；在 Car 片段的 onCreateView 中，您将看到这一点。您应该做的是更改它，以便在片段视图的膨胀内更改按钮文本。 @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { // Inflate the layout for this fragment View rootView = inflater.inflate(R.layout.fragment_car, container, false); Button b = (Button)rootView.findViewById(R.id.submitButton); b.setText("It works !"); return rootView; }

Answer 2

试试下面的汽车片段代码

public class Car extends Fragment {
    View view;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        view = inflater.inflate(R.layout.car, container, false);
        return view;
    }

    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        Button b = (Button) view.findViewById(R.id.submitButton); //throws an exception as I'm trying to execute the "findViewById" method on a null object , as the debugger say .
        b.setText("It works !"); // it doesn't
    }
}

和 MainActivity 一样

public class MainActivity extends AppCompatActivity {
    Car myCar = new Car();
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        getSupportFragmentManager().beginTransaction().add(R.id.buttonsContainer,myCar,"me").commit();
    }
}

Answer 3

显然我误解了你的问题。要从父活动访问片段，过程类似，这是您可以实现的一种方式：

MyFragment myFragment;

myFragment = (MyFragment)getSupportFragmentManager().findFragmentByTag("MyFragmentTag");
if(myFragment != null && myFragment.isAdded()) // you need to verify if the instance is actually added to the activity
    myFragment.submitButton.setText("It works");