Question

我在android活动中有一个BottomNavigationView，它由4个menuItems组成。当我有时在这些片段之间导航时，应用程序崩溃并出现空指针异常。在我的AppCentes诊断部分中，它说崩溃是在FragmentManager内部发生的。唯一的例外是在空对象上有一个getClass（）方法调用。谁能指出造成这种现象的原因吗？附加崩溃日志。

下面介绍了用于切换片段的代码。

private void BottomNavigationItemSelected(object obj, BottomNavigationView.NavigationItemSelectedEventArgs args)
    {
        string title = "";
        if (args.Item.ItemId == Resource.Id.menu_explore)
        {
            title = Resources.GetString(Resource.String.shelf_title);
            fragment = _exploreFragment;
            _fragmentTag = "Home";
        }
        else
        {
            title = args.Item.TitleFormatted.ToString();
        }
        if (args.Item.ItemId == Resource.Id.menu_dashboard)
        {
            fragment = _redesignDashboard;
            _fragmentTag = "Dashboard";
        }
        else if (args.Item.ItemId == Resource.Id.menu_more)
        {
            fragment = _moreFragment;
            _fragmentTag = "More";
        }
        else if (args.Item.ItemId == Resource.Id.menu_report)
        {
            fragment = _reportFragment;
            _fragmentTag = "Report";
        }
        else if (args.Item.ItemId == Resource.Id.menu_downloads)
        {
            fragment = _downloadGalleryFragment;
            _fragmentTag = "Downloads";

        }
        if (fragment != null)
        {
            _toolbarTitle.Text = title;
            ToggleTitle(true);
            SupportFragmentManager.BeginTransaction().SetCustomAnimations(Resource.Animation.fab_slide_in_from_right, Resource.Animation.fab_slide_out_to_left).Replace(Resource.Id.content_frame, fragment, _fragmentTag).Commit();
        }
    }

StackTrace用于MS AppCenter中的相同操作

JniEnvironment+InstanceMethods.CallObjectMethod (Java.Interop.JniObjectReference instance, Java.Interop.JniMethodInfo method, Java.Interop.JniArgumentValue* args)
JNIEnv.CallObjectMethod (System.IntPtr jobject, System.IntPtr jmethod, Android.Runtime.JValue* parms)
FragmentTransactionInvoker.Replace (System.Int32 containerViewId, Android.Support.V4.App.Fragment fragment, System.String tag)
BookshelfActivity.BottomNavigationItemSelected (System.Object obj, Android.Support.Design.Widget.BottomNavigationView+NavigationItemSelectedEventArgs args)
BottomNavigationView+IOnNavigationItemSelectedListenerImplementor.OnNavigationItemSelected (Android.Views.IMenuItem item)
BottomNavigationView+IOnNavigationItemSelectedListenerInvoker.n_OnNavigationItemSelected_Landroid_view_MenuItem_ (System.IntPtr jnienv, System.IntPtr native__this, System.IntPtr native_item)
(wrapper dynamic-method) System.Object.26(intptr,intptr,intptr)
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference
android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:392)
android.support.v4.app.BackStackRecord.replace(BackStackRecord.java:439)
mono.android.support.design.widget.BottomNavigationView_OnNavigationItemSelectedListenerImplementor.n_onNavigationItemSelected(Native Method)
mono.android.support.design.widget.BottomNavigationView_OnNavigationItemSelectedListenerImplementor.onNavigationItemSelected(BottomNavigationView_OnNavigationItemSelectedListenerImplementor.java:30)
android.support.design.widget.BottomNavigationView$1.onMenuItemSelected(BottomNavigationView.java:182)
android.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:822)
android.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:171)
android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:973)
android.support.design.internal.BottomNavigationMenuView$1.onClick(BottomNavigationMenuView.java:95)
android.view.View.performClick(View.java:6935)
android.view.View$PerformClick.run(View.java:26211)
android.os.Handler.handleCallback(Handler.java:790)
android.os.Handler.dispatchMessage(Handler.java:99)
android.os.Looper.loop(Looper.java:164)
android.app.ActivityThread.main(ActivityThread.java:7000)
java.lang.reflect.Method.invoke(Native Method)
com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:441)
com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1408)

Answer 1

我怀疑您的碎片正在被Android垃圾收集，这导致了此问题。

您不应持有对片段的引用以切换标签。 FragmentManager具有内置的缓存机制，您可以利用该机制。

您已经在标记片段。您可以使用它来在片段管理器中查找缓存的片段，如果它为null，则创建一个新的片段：

var cachedFragment = SupportFragmentManager.FindFragmentByTag(fragmentTag);
if (cachedFragment == null){
    cachedFragment = new MyFragment(); //or whatever fragment you are trying to show
}

SupportFragmentManager
    .BeginTransaction()
    .SetCustomAnimations(Resource.Animation.fab_slide_in_from_right, Resource.Animation.fab_slide_out_to_left)
    .Replace(Resource.Id.content_frame, cachedFragment, fragmentTag)
    .AddToBackStack(null) //this caches the fragment
    .Commit();

使用BottomNavigationView在片段之间导航时出现异常崩溃

1 个答案: