java - Android - 如何从 FragmentActivity 访问 fragment

Question

当我试图从我的主要 Activity 中访问一个 fragment 时，我总是得到一个NullPointerException。不管我做什么。

问题是我使用了 TabsPagerAdapter 和 ViewPager 并且我不知道如何获得膨胀的 View ( fragment 的 onCreate() 方法已经返回膨胀 View )。

目标是访问 fragment 内的元素并通过单个后台线程动态隐藏或显示它，对于更多 fragment 也应该这样做。

MainActivity.java

public class MainActivity extends FragmentActivity implements
        ActionBar.TabListener
{
/* swipe view */
private ViewPager viewPager;
private TabsPagerAdapter mAdapter;
private android.app.ActionBar actionBar;

// tab titles
private String[] tabs = { "Basic", "Advanced", "Settings"};


@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    /* init swipe views */
    mAdapter = new TabsPagerAdapter(getSupportFragmentManager());
    viewPager = (ViewPager) findViewById(R.id.pager);
    viewPager.setAdapter(mAdapter);

    actionBar = getActionBar();
    actionBar.setHomeButtonEnabled(false);
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

    /* addTab returns void, how to geht my fragements and their views???*/
    for (String tabName : tabs)
        actionBar.addTab(actionBar.newTab().setText(tabName).setTabListener(this));

    viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {

        @Override
        public void onPageSelected(int position) {
            actionBar.setSelectedNavigationItem(position);
        }

        @Override
        public void onPageScrolled(int arg0, float arg1, int arg2) {}

        @Override
        public void onPageScrollStateChanged(int arg0) {}
    });
}

public void testFunction()
{
    FragmentPage1 fragmentPage1 = (FragmentPage1) getSupportFragmentManager().findFragmentById(R.layout.fragment_page1);
    GridLayout gridlayout = (GridLayout) fragmentPage1.getRootView().findViewById(R.id.adBannerBasicLayout);
    gridlayout.setVisibility(GridLayout.VISIBLE); /* THATS MY GOAL */
}

FragmentPage1.java

public class FragmentPage1 extends Fragment {

    private View rootView;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        rootView = inflater.inflate(R.layout.fragment_page1, container, false);


        /* HERE IT IS WORKING FINE, 
        but later I want to make it visible again 
        from code OUTSIDE FragmentPage1 ??? */

        GridLayout gridlayout = (GridLayout) rootView.findViewById(R.id.adBannerBasicLayout);
        gridlayout.setVisibility(GridLayout.GONE);

        return rootView;
    }    


    /* so I tried this, but also get always NullPointerException */
    public View getRootView()
    {
        return rootView;
    }
}

fragment_page1.xml

<?xml version="1.0" encoding="utf-8"?>

<ScrollView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:ads="http://schemas.android.com/apk/res-auto"
    android:id="@+id/settings_scroll_view"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    >


    <GridLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:ads="http://schemas.android.com/apk/res-auto"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="#000000"
        android:orientation="vertical"
        android:rowCount="12"
        android:columnCount="5"
        >

        <!-- Some Banner Ads I want to hide show -->           
        <!-- I want to access this from everywhere! --> 
        <GridLayout
            xmlns:android="http://schemas.android.com/apk/res/android"
            xmlns:ads="http://schemas.android.com/apk/res-auto"
            android:id="@+id/adBannerBasicLayout"
            android:orientation="vertical"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="fill"
            android:rowCount="3"
            android:columnCount="1"
            android:layout_row="0"
            android:layout_column="0"
            android:layout_columnSpan="5">

            <Space
                android:layout_width="0dp"
                android:layout_height="5dp"
                android:layout_row="0"
                android:layout_column="0"
                />

            <com.google.android.gms.ads.AdView
                android:id="@+id/adBannerBasic"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="fill"
                ads:adSize="BANNER"
                ads:adUnitId="xxxxxxxxxxxxxxxxxxxxxxxxxx"
                android:layout_row="1"
                android:layout_column="0"
                >
            </com.google.android.gms.ads.AdView>

            <Space
                android:layout_width="0dp"
                android:layout_height="5dp"
                android:layout_row="2"
                android:layout_column="0"
                />

        </GridLayout>

        <!-- more stuff... -->
  </GridLayout>
</ScrollView>

请帮帮我，我完全卡住了!

谢谢!

编辑:

testFunction() 中的第二行抛出 NullPointerException:

 GridLayout gridlayout = (GridLayout) fragmentPage1.getRootView().findViewById(R.id.adBannerBasicLayout);

因为 getSupportFragmentManager() 总是返回 null:

FragmentPage1 fragmentPage1 = (FragmentPage1) getSupportFragmentManager().findFragmentById(R.layout.fragment_page1);

Logcat 输出

java.lang.IllegalStateException: Could not execute method of the activity
            at android.view.View$1.onClick(View.java:3969)
            at android.view.View.performClick(View.java:4633)
            at android.view.View$PerformClick.run(View.java:19330)
            at android.os.Handler.handleCallback(Handler.java:733)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:157)
            at android.app.ActivityThread.main(ActivityThread.java:5356)
            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:1265)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
            at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:132)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.reflect.InvocationTargetException
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at android.view.View$1.onClick(View.java:3964)
            at android.view.View.performClick(View.java:4633)
            at android.view.View$PerformClick.run(View.java:19330)
            at android.os.Handler.handleCallback(Handler.java:733)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:157)
            at android.app.ActivityThread.main(ActivityThread.java:5356)
            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:1265)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
            at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:132)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at org.tzapp.smote.MainActivity.testFuntion(MainActivity.java:393)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at android.view.View$1.onClick(View.java:3964)
            at android.view.View.performClick(View.java:4633)
            at android.view.View$PerformClick.run(View.java:19330)
            at android.os.Handler.handleCallback(Handler.java:733)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:157)
            at android.app.ActivityThread.main(ActivityThread.java:5356)
            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:1265)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
            at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:132)
            at dalvik.system.NativeStart.main(Native Method)

Answer 1

您使用的是自动生成的 R 文件中的错误静态值!

不要使用引用 XML 资源的 R.layout.fragment_page1 您应该使用 R.id.fragment_page1 它应该是您的 R.layout.activity_main 位于/res/layout/activity_main.xml 的 XML 文件

• R.layout 引用 XML 布局文件
• R.id 引用单个 XML 节点( View 、 fragment 等)

简而言之，改变:

FragmentPage1 fragmentPage1 = (FragmentPage1) getSupportFragmentManager().findFragmentById(R.layout.fragment_page1);

收件人:

FragmentPage1 fragmentPage1 = (FragmentPage1) getSupportFragmentManager().findFragmentById(R.id.fragment_page1);

并确保将/res/layout/activity_main.xml 中的 fragment ID 设置为 R.id.fragment_page1，如下所示:

<fragment android:name="com.example.yourpackage.FragmentPage1"
              android:id="@+id/fragment_page1"
              android:layout_weight="1"
              android:layout_width="0dp"
              android:layout_height="match_parent" />