当我在onCreate()中调用它时,findViewById()返回null

Question

我的第一个Android应用程序上的findViewById有问题.我试图调用此函数但始终返回null.

我的应用有2项活动.在第二个活动(activity_display_message)中,我有以下代码:

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

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }

    //Obtener el mensaje desde el intent en MainActivity
    Intent intent = getIntent();
    String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);

    //Buscar el textview en la vista
    TextView textView = (TextView) findViewById(R.id.texto);
    textView.setText(message);
}

这是fragment_display_message.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:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin"
tools:context="com.example.app.DisplayMessageActivity$PlaceholderFragment">

<TextView
    android:id="@+id/texto"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" />

我不知道为什么它会返回NULL并NullPointerException在我尝试执行时生成一个setText().我正在调用正确的Id并在XML上声明它.而我正打电话setContextView()给onCreate().

对不起,如果这个问题太明显了(我认为我忽略了一些明显的事情),但我是Android开发的新手.

编辑:Logcat错误:

02-10 10:25:58.960    1031-1031/com.example.app E/AndroidRuntime? FATAL EXCEPTION: main
Process: com.example.app, PID: 1031
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.app/com.example.app.DisplayMessageActivity}: java.lang.NullPointerException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
        at android.app.ActivityThread.access$800(ActivityThread.java:135)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:5017)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
        at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.NullPointerException
        at com.example.app.DisplayMessageActivity.onCreate(DisplayMessageActivity.java:35)
        at android.app.Activity.performCreate(Activity.java:5231)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)

          

编辑:

问题解决了.只需要将代码移动onCreate()到onCreateView()并编辑引用即可.

onCreate() 将是默认值.

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

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }
}

而onCreateView()上PlaceholderFragment:

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

        //Obtener el mensaje desde el intent en MainActivity
        Intent intent = getActivity().getIntent();
        String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);

        //Buscar el textview en la vista
        TextView textView = (TextView) rootView.findViewById(R.id.texto);
        textView.setText(message);

        return rootView;
    }

Answer 1

Activity findViewById()搜索活动的视图层次结构集,其中setContentView()包含给定的视图ID.您似乎要查找的视图位于片段中,而不在活动层次结构中(尚未).片段事务也不会立即发生,即使您将其添加到活动层次结构中的容器中,它也不会立即附加到那里,而是仅在处理事务时附加.

从理论上讲,可以同步执行挂起的片段事务executePendingTransactions().但是,最好只移动触及片段视图的代码onCreateView().碎片及其托管活动应相互独立,因此应避免访问碎片内部的活动.