Question

我试图将折叠的工具栏添加到Fragment，但是打开Fragment时应用程序崩溃了：这是我得到的错误：

2018-10-19 18:05:38.950 13301-13301/io.github.davidwickerhf.diceroller E/AndroidRuntime: FATAL EXCEPTION: main
Process: io.github.davidwickerhf.diceroller, PID: 13301
java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
    at androidx.appcompat.app.AppCompatDelegateImpl.<init>(AppCompatDelegateImpl.java:249)
    at androidx.appcompat.app.AppCompatDelegate.create(AppCompatDelegate.java:182)
    at androidx.appcompat.app.AppCompatActivity.getDelegate(AppCompatActivity.java:520)
    at androidx.appcompat.app.AppCompatActivity.findViewById(AppCompatActivity.java:191)
    at io.github.davidwickerhf.diceroller.ProfileFragment.onCreateView(ProfileFragment.java:36)
    at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2439)
    at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManager.java:1460)
    at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1784)
    at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManager.java:1852)
    at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:802)
    at androidx.fragment.app.FragmentManagerImpl.executeOps(FragmentManager.java:2625)
    at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2411)
    at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2366)
    at androidx.fragment.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2273)
    at androidx.fragment.app.FragmentManagerImpl$1.run(FragmentManager.java:733)
    at android.os.Handler.handleCallback(Handler.java:809)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:166)
    at android.app.ActivityThread.main(ActivityThread.java:7436)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:469)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:963)

片段Java：

public class ProfileFragment extends Fragment {

    private MainActivity mMainActivity;
    private SettingsViewModel settingsViewModel;
    private SettingAdapter adapter;
    private CoordinatorLayout coordinatorLayout;
    private View fragmentView;

    private TabLayout mTabLayout;
    private ViewPager mViewPager;

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

        mMainActivity = new MainActivity();

        androidx.appcompat.widget.Toolbar mToolbar = fragmentView.findViewById(R.id.profile_toolbar);
        mMainActivity.setSupportActionBar(mToolbar);

        mViewPager = (ViewPager) fragmentView.findViewById(R.id.viewpager);
        ViewPagerAdapter adapter = new ViewPagerAdapter(mMainActivity.getSupportFragmentManager());
        adapter.AddFragment(new ProfileInfoFragment(), "Profile");
        adapter.AddFragment(new ProfileMessagingFragment(), "Messages");
        mViewPager.setAdapter(adapter);
        mTabLayout = (TabLayout) fragmentView.findViewById(R.id.tab_layout_profile);
        mTabLayout.setupWithViewPager(mViewPager);

        return fragmentView;
    }
}

片段XML：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
xmlns:app="http://schemas.android.com/apk/res-auto"
tools:context=".ProfileFragment"
android:orientation="vertical">

<com.google.android.material.appbar.AppBarLayout
    android:id="@+id/appBar"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:theme="@style/AppTheme"
    app:toolbarId="@id/profile_toolbar">

    <com.google.android.material.appbar.CollapsingToolbarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:fitsSystemWindows="true"
        app:contentScrim="?attr/colorPrimary"
        app:layout_scrollFlags="scroll|exitUntilCollapsed"
        app:titleEnabled="false">

        <ImageView
            android:id="@+id/background_image_Toolbar"
            android:layout_width="match_parent"
            android:layout_height="256dp"
            android:fitsSystemWindows="true"
            android:scaleType="centerCrop"
            app:layout_collapseMode="parallax" />

        <androidx.appcompat.widget.Toolbar
            android:layout_marginTop="24dp"
            android:id="@+id/profile_toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            app:layout_collapseMode="pin"
            app:popupTheme="@style/AppTheme" />

    </com.google.android.material.appbar.CollapsingToolbarLayout>

    <com.google.android.material.tabs.TabLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:tabGravity="fill"
        app:tabMode="fixed"
        android:id="@+id/tab_layout_profile"
        app:tabTextColor="@color/greyText"
        app:tabIndicatorColor="@color/greyText"
        android:background="#fafafa"
        android:elevation="0dp">

    </com.google.android.material.tabs.TabLayout>

</com.google.android.material.appbar.AppBarLayout>





<androidx.viewpager.widget.ViewPager
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/viewpager">

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent">













    </androidx.constraintlayout.widget.ConstraintLayout>



</androidx.viewpager.widget.ViewPager>

请帮助？

Answer 1

我猜您正在收到该错误，因为您正在实例化一个新的MainActivity对象：

 mMainActivity = new MainActivity();

对于Fragments，您可以调用getActivity()来获取托管Fragment的活动的引用。像这样：

private MainActivity mMainActivity;
....
mMainActivity = (MainActivity) getActivity();

或者，由于您正在调用通用方法（可在MainActivity上使用，因此您甚至无需强制转换为Activity

private Activity mMainActivity;
....
mMainActivity = getActivity();

``android.view.Window $ Callback android.view.Window.getCallback（）''上的Java空指针异常

1 个答案: