android - 抽屉导航 + Viewpager + setVisibility = InflateException/NullPointer

Question

我开发了一个 Android 应用程序，但遇到了一些问题。

在我的 MainActivity 中，我有以下组件:

• Navigation Drawer(实例化并加载许多 fragment )
• ViewPager(仅用于显示文章详细 View 的ArticleFragment)

通常情况下，ViewPager 与特定的 Activity 相关，但我真的想同时使用 Navigation Drawer 和 ViewPager。唯一的方法是在同一个 Activity 中实现这两个元素。

DrawerLayout 由抽屉导航本身和内容区域组成。在我的内容区域中，我有一个 FrameLayout 和一个 ViewPager。问题是 ViewPager 始终显示。

所以我的想法是使用 android:visibility 属性来交替显示/隐藏 FrameLayout 和 ViewPager。如果您查看我的 activity_main.xml，您会注意到 ViewPager 上的 android:visibility 设置为 invisible。

/layout/activity_main.xml

    <!-- The main content view -->
    <RelativeLayout
        android:id="@+id/content_frame"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <FrameLayout
            android:id="@+id/container"
            android:paddingTop="?android:attr/actionBarSize"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />

        <!-- ViewPager -->
        <android.support.v4.view.ViewPager xmlns:android="http://schemas.android.com/apk/res/android"
            android:id="@+id/pager"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:visibility="invisible" />

    </RelativeLayout>

    <!-- The navigation drawer -->
    <fragment android:id="@+id/navigation_drawer"
        android:layout_width="@dimen/navigation_drawer_width"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:name="NavigationDrawerFragment" />

</android.support.v4.widget.DrawerLayout>

然后:

MainActivity.java

public class MainActivity extends ActionBarActivity implements NavigationDrawerFragment.NavigationDrawerCallbacks {

    private NavigationDrawerFragment mNavigationDrawerFragment;
    private FrameLayout mContainer;

    private ViewPager mPager;
    private PagerAdapter mPagerAdapter;

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

        // Set up the navigation drawer
        mContainer = (FrameLayout) findViewById(R.id.container);
        mNavigationDrawerFragment = (NavigationDrawerFragment) getSupportFragmentManager().findFragmentById(R.id.navigation_drawer);
        mNavigationDrawerFragment.setUp(R.id.navigation_drawer, (DrawerLayout) findViewById(R.id.drawer_layout));

        // Set up the viewpager
        mPager = (ViewPager) findViewById(R.id.pager);
        mPagerAdapter = new ScreenSlidePagerAdapter(getSupportFragmentManager());
        mPager.setAdapter(mPagerAdapter);
        mPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
            @Override
            public void onPageSelected(int position) {
                invalidateOptionsMenu();
            }
        });
    }

    //-------------------------------//
    //       NAVIGATION DRAWER       //
    //-------------------------------//

    /**
     * Update the main content by replacing fragments
     */
    @Override
    public void onNavigationDrawerItemSelected(int position) {
        Fragment fragment = null;
        Boolean viewPagerArticles = false;

        switch (position) {
            case 0:
                fragment = new HomeFragment();
                break;
            case 1:
                fragment = new FooFragment();
                break;
            case 2:
                viewPagerArticles = true;

                // Hide navigation drawer container and show the viewpager
                mContainer.setVisibility(FrameLayout.GONE);
                mPager.setVisibility(ViewPager.VISIBLE);

                // NullPointerException with new this line. Why?
                // Fragment already instanciate in ScreenSlidePagerAdapter -> getItem() -> ArticleFragment.create()?
                //fragment = new ArticleFragment();
                break;
            default:
                break;
        }

        // Show the view pager and hide navigation drawer container
        // =================
        // THE BUG IS HERE
        // =================
        if (!viewPagerArticles) {
            mContainer.setVisibility(FrameLayout.VISIBLE);
            mPager.setVisibility(ViewPager.GONE);
        }

        if (fragment != null) {
            FragmentManager fragmentManager = getSupportFragmentManager();
            fragmentManager.beginTransaction()
                    .replace(R.id.container, fragment)
                    .commit();
        }
    }

    ...

    //-------------------------------//
    //           VIEWPAGER           //
    //-------------------------------//

    private class ScreenSlidePagerAdapter extends FragmentStatePagerAdapter {
        public ScreenSlidePagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            return DayFragment.create(position);
        }

        @Override
        public int getCount() { ... }
    }
    ...
}

NavigationDrawerFragment.java

http://pastebin.com/LwKZnNiB

显示ViewPager内容

在我的 HomeFragment 上，我有一个带有 onClickListener 的按钮:

mButton.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
        ((MainActivity)getActivity()).onNavigationDrawerItemSelected(2);
    }
});

当我加载 MainActivity 时出现这些错误:InflateException 和 NullPointerException

AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.android/com.example.android.ui.MainActivity}: android.view.InflateException: Binary XML file line #34: Error inflating class fragment
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
            at android.app.ActivityThread.access$600(ActivityThread.java:141)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5103)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:525)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: android.view.InflateException: Binary XML file line #34: Error inflating class fragment
            at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:713)
            at android.view.LayoutInflater.rInflate(LayoutInflater.java:755)
            at android.view.LayoutInflater.inflate(LayoutInflater.java:492)
            at android.view.LayoutInflater.inflate(LayoutInflater.java:397)
            at android.view.LayoutInflater.inflate(LayoutInflater.java:353)
            at com.android.internal.policy.impl.PhoneWindow.setContentView(PhoneWindow.java:267)
            at android.app.Activity.setContentView(Activity.java:1895)
            at android.support.v7.app.ActionBarActivity.superSetContentView(ActionBarActivity.java:216)
            at android.support.v7.app.ActionBarActivityDelegateICS.setContentView(ActionBarActivityDelegateICS.java:111)
            at android.support.v7.app.ActionBarActivity.setContentView(ActionBarActivity.java:76)
            at com.example.android.ui.MainActivity.onCreate(MainActivity.java:42)
            at android.app.Activity.performCreate(Activity.java:5133)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
            at android.app.ActivityThread.access$600(ActivityThread.java:141)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5103)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:525)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at com.example.android.ui.MainActivity.onNavigationDrawerItemSelected(MainActivity.java:117)
            at com.example.android.ui.NavigationDrawerFragment.selectItem(NavigationDrawerFragment.java:202)
            at com.example.android.ui.NavigationDrawerFragment.onCreate(NavigationDrawerFragment.java:80)
            at android.support.v4.app.Fragment.performCreate(Fragment.java:1477)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:893)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1082)
            at android.support.v4.app.FragmentManagerImpl.addFragment(FragmentManager.java:1184)
            at android.support.v4.app.FragmentActivity.onCreateView(FragmentActivity.java:291)
            at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:685)
            at android.view.LayoutInflater.rInflate(LayoutInflater.java:755)
            at android.view.LayoutInflater.inflate(LayoutInflater.java:492)
            at android.view.LayoutInflater.inflate(LayoutInflater.java:397)
            at android.view.LayoutInflater.inflate(LayoutInflater.java:353)
            at com.android.internal.policy.impl.PhoneWindow.setContentView(PhoneWindow.java:267)
            at android.app.Activity.setContentView(Activity.java:1895)
            at android.support.v7.app.ActionBarActivity.superSetContentView(ActionBarActivity.java:216)
            at android.support.v7.app.ActionBarActivityDelegateICS.setContentView(ActionBarActivityDelegateICS.java:111)
            at android.support.v7.app.ActionBarActivity.setContentView(ActionBarActivity.java:76)
            at com.example.android.ui.MainActivity.onCreate(MainActivity.java:42)
            at android.app.Activity.performCreate(Activity.java:5133)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
            at android.app.ActivityThread.access$600(ActivityThread.java:141)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5103)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:525)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
            at dalvik.system.NativeStart.main(Native Method)

一切都很好(没有错误， Activity 已加载)没有:

    // =================
    // THE BUG IS HERE
    // =================
    if (!viewPagerArticles) {
        mContainer.setVisibility(FrameLayout.VISIBLE);
        mPager.setVisibility(ViewPager.GONE);
    }

...但是它不符合我的预期，因为抽屉导航 fragment 总是隐藏(这是正常的，因为 mContainer.setVisibility(FrameLayout.GONE);。我只是想反转 mContainer/mPager 可见性。

我希望我说得足够清楚...!

有什么想法吗？

Answer 1

你明白了 NullPointerException因为你构建 NavigationDrawerFragment 的方式分段。更准确地说，在其 onCreate()你调用的方法selectItem()通过 Activity 传播(注册为监听器)到 onNavigationDrawerItemSelected()打回来。问题是 fragment 将在 setContentView() 时膨胀。被称为(在 onCreate() 中)，这意味着您将输入 onNavigationDrawerItemSelected()行前回调:

mContainer = (FrameLayout) findViewById(R.id.container);
mPager = (ViewPager) findViewById(R.id.pager);

运行，渲染那些引用 null在onNavigationDrawerItemSelected() .

要解决这个问题，您可以简单地替换当前的 <fragment />来自 Activity 的标签使用基本容器布局(例如占位符为空 FrameLayout)并添加 NavigationDrawerFragment手动输入 onCreate()在上面的行之后(绑定(bind) View 的地方)。可能有另一个更好的解决方案(或者我可以提出其他建议)，但你不太清楚你想如何处理可见性。也许您可以对此进行扩展。

编辑 2:

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

    // Set up the navigation drawer
    mContainer = (FrameLayout) findViewById(R.id.container);


    // Set up the viewpager
    mPager = (ViewPager) findViewById(R.id.pager);
    mPagerAdapter = new ScreenSlidePagerAdapter(getSupportFragmentManager());
    mPager.setAdapter(mPagerAdapter);
    mPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
        @Override
        public void onPageSelected(int position) {
            invalidateOptionsMenu();
        }
    });
    if (savedInstanceState == null) {
        NavigationDrawerFragment ndf = new NavigationDrawerFragment();
        ndf.setUp(R.id.navigation_drawer, (DrawerLayout) findViewById(R.id.drawer_layout));
        getSupportFragmentManager().beginTransaction().add(R.id.nav_drawer, ndf).commit();
    }
}

编辑 1:

//...
<!-- ViewPager -->
        <android.support.v4.view.ViewPager xmlns:android="http://schemas.android.com/apk/res/android"
            android:id="@+id/pager"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:visibility="invisible" />

    </RelativeLayout>
    <!-- The navigation drawer -->
    <FrameLayout android:id="@+id/nav_drawer"
        android:layout_width="@dimen/navigation_drawer_width"
        android:layout_height="match_parent"
        android:layout_gravity="start"
     />

</android.support.v4.widget.DrawerLayout>

并且在 onCreate()最后:

//...
mPager.setAdapter(mPagerAdapter);
        mPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
            @Override
            public void onPageSelected(int position) {
                invalidateOptionsMenu();
            }
        });
getSupportFragmentManager().beginTransaction().add(R.id.nav_drawer, new NavigationDrawerFragment()).commit();

如果上面的代码出现异常，它在同一个 onNavigationDrawerItemSelected() 中方法？