使用 Xamarin Essential 选择联系人（并尝试返回）时应用程序崩溃答案

【问题标题】：App Crashing when selecting Contact (and trying to go back) using Xamarin Essential使用 Xamarin Essential 选择联系人（并尝试返回）时应用程序崩溃
【发布时间】：2021-06-03 13:47:14
【问题描述】：

我尝试使用 Xamarin Essential API 来选择并显示单个联系人。不幸的是，一旦我从模拟器中选择了一个联系人，该应用程序就会进入后台。当尝试从应用仪表板中选择应用时，它会返回到其第一页。

此外，在联系人窗口中，如果我按下返回按钮（硬件返回按钮或导航栏中的返回按钮），应用程序会崩溃。因此，在这两种方式中，我都无法访问正在显示的联系人。

令人惊讶的是，当我设置断点时，我可以看到选定的联系人进入。我可以看到所有数据。

我正在使用 MvvmCross，我的参考点是 James Montemagno 的最新 Xamarin.Essential Contact API 视频和 Contacts API Xamarin.Forms 文档。

这是我的文件：

MainActivity.cs

    //[Activity(Label = "Vary.Core", Icon = "@mipmap/icon", Theme = "@style/MainTheme", MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation | ConfigChanges.UiMode | ConfigChanges.ScreenLayout | ConfigChanges.SmallestScreenSize)]
        [Activity(Label = "Vary.Core", MainLauncher = true, Theme = "@style/MainTheme", NoHistory = true)]
        public class MainActivity : MvxFormsAppCompatActivity<Setup, Core.MvxApp, App> //MvxFormsAndroidSetup<Core.MvxApp, App> //Setup
        {
            protected override void OnCreate(Bundle savedInstanceState)
            {
                TabLayoutResource = Resource.Layout.Tabbar;
                ToolbarResource = Resource.Layout.Toolbar;
    
                base.OnCreate(savedInstanceState);
    
                Xamarin.Essentials.Platform.Init(this, savedInstanceState);
                global::Xamarin.Forms.Forms.Init(this, savedInstanceState);
                //LoadApplication(new App());
            }
            public override void OnRequestPermissionsResult(int requestCode, string[] permissions, [GeneratedEnum] Android.Content.PM.Permission[] grantResults)
            {
                Xamarin.Essentials.Platform.OnRequestPermissionsResult(requestCode, permissions, grantResults);
    
                base.OnRequestPermissionsResult(requestCode, permissions, grantResults);
            }
    
            protected override void OnResume()
            {
                base.OnResume();
    
                Xamarin.Essentials.Platform.OnResume();
            }
    
            protected override void OnPause()
            {
                base.OnPause();
            }

ContactDisplayViewModel.cs

   

  

      namespace Vary.Core.ViewModels
    {
        public class ContactDisplayViewModel : MvxViewModel
        {
            #region Private variables
            private readonly IMvxNavigationService navigationService;
            #endregion
    
            #region Properties
            private bool isSingleContactRequired;
            public bool IsSingleContactRequired
            {
                get => isSingleContactRequired;
                set
                {
                    isSingleContactRequired = value;
                    RaisePropertyChanged(() => IsSingleContactRequired);
                }
            }
    
            private string contactInformation;
            public string ContactInformation
            {
                get => contactInformation;
                set
                {
                    contactInformation = value;
                    RaisePropertyChanged(() => ContactInformation);
                }
            }
            #endregion
    
            #region Commands
            public IMvxCommand SelectSingleContactCommand => new MvxCommand(OnSelectSingleContactSelected);
            public MvxCommand DisplayAllContactsCommand => new MvxCommand(OnDisplayAllContactsSelected);
            #endregion
    
            #region Command Events
            async void OnSelectSingleContactSelected()
            {
                try
                {
                    var contact = await Contacts.PickContactAsync();
    
                    if (contact == null)
                        return;
    
                    var contactInfo = new StringBuilder();
                    contactInfo.AppendLine(contact.DisplayName);
                    contactInfo.AppendLine(contact.FamilyName);
                    contactInfo.AppendLine(contact.Emails.FirstOrDefault().ToString());
                    contactInfo.AppendLine(contact.Phones.FirstOrDefault().ToString());
    
                    IsSingleContactRequired = true;
                    ContactInformation = contactInfo.ToString();
                }
                catch(Exception ex)
                {
                    Debug.WriteLine(ex.Message);
                }
            }
    
            void OnDisplayAllContactsSelected()
            {
    
            }
            #endregion
    
            #region .Ctor
            public ContactDisplayViewModel(IMvxNavigationService navigationService)
            {
                this.navigationService = navigationService;
            }
            #endregion
    
            #region Override Methods
            public override void Prepare()
            {
                base.Prepare();
    
                IsSingleContactRequired = false;
            }
            #endregion
        }
    }

ContactDisplayView.xaml

    <Button Grid.Row="0"
                            Grid.Column="0"
                            BackgroundColor="Transparent"
                            Text="Pick A Contact"
                            Command="{Binding SelectSingleContactCommand}"/>
    
                    <Label Grid.Row="1"
                           Grid.Column="1"
                           Grid.ColumnSpan="2"
                           Text="{Binding ContactInformation}"
                           TextColor="DarkOrange"
                           IsVisible="{Binding IsSingleContactRequired}"/>

在尝试从任何此类本机/系统定义的应用程序导航回 Xamarin.Forms 应用程序时，是否有人遇到过这种崩溃，或者一般情况下遇到任何崩溃问题？

这是应用程序输出：

    [Mono] Assembly Loader probing location: '/Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/sdks/out/android-x86-release/lib/Xamarin.Forms.Platform.Android.AppLinks.exe'.
[Mono] Assembly Loader probing location: '//Facades/Xamarin.Forms.Platform.Android.AppLinks.exe'.
[Mono] Requesting loading reference 13 (of 22) of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.Forms.Platform.Android.dll
[Mono] Loading reference 13 of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.Forms.Platform.Android.dll asmctx DEFAULT, looking for Xamarin.AndroidX.Legacy.Support.Core.UI, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
[Mono] Assembly Ref addref Xamarin.Forms.Platform.Android[0xe64638e0] -> Xamarin.AndroidX.Legacy.Support.Core.UI[0xe6461de0]: 2
[Mono] Requesting loading reference 3 (of 4) of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.AndroidX.Legacy.Support.Core.UI.dll
[Mono] Loading reference 3 of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.AndroidX.Legacy.Support.Core.UI.dll asmctx DEFAULT, looking for Xamarin.AndroidX.DrawerLayout, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
[Mono] Requesting loading reference 2 (of 4) of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.AndroidX.Legacy.Support.Core.UI.dll
[Mono] Loading reference 2 of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.AndroidX.Legacy.Support.Core.UI.dll asmctx DEFAULT, looking for Java.Interop, Version=0.1.0.0, Culture=neutral, PublicKeyToken=84e04ff9cfb79065
[Mono] Assembly Ref addref Xamarin.AndroidX.Legacy.Support.Core.UI[0xe6461de0] -> Java.Interop[0xe649f180]: 21
[Mono] DllImport searching in: '__Internal' ('(null)').
[Mono] Searching for 'java_interop_jnienv_call_static_int_method_a'.
[Mono] Probing 'java_interop_jnienv_call_static_int_method_a'.
[Mono] Found as 'java_interop_jnienv_call_static_int_method_a'.
[Mono] DllImport searching in: '__Internal' ('(null)').
[Mono] Searching for 'java_interop_jnienv_call_nonvirtual_int_method_a'.
[Mono] Probing 'java_interop_jnienv_call_nonvirtual_int_method_a'.
[Mono] Found as 'java_interop_jnienv_call_nonvirtual_int_method_a'.
Thread started:  #12
[HostConnection] HostConnection::get() New Host Connection established 0xc0ada410, tid 17005
[HostConnection] HostComposition ext ANDROID_EMU_CHECKSUM_HELPER_v1 ANDROID_EMU_native_sync_v2 ANDROID_EMU_native_sync_v3 ANDROID_EMU_native_sync_v4 ANDROID_EMU_dma_v1 ANDROID_EMU_direct_mem ANDROID_EMU_host_composition_v1 ANDROID_EMU_host_composition_v2 ANDROID_EMU_YUV420_888_to_NV21 ANDROID_EMU_YUV_Cache ANDROID_EMU_async_unmap_buffer GL_OES_EGL_image_external_essl3 GL_OES_vertex_array_object GL_KHR_texture_compression_astc_ldr ANDROID_EMU_host_side_tracing ANDROID_EMU_gles_max_version_3_0 
[OpenGLRenderer] Failed to choose config with EGL_SWAP_BEHAVIOR_PRESERVED, retrying without...
[eglCodecCommon] setVertexArrayObject: set vao to 0 (0) 0 0
[EGL_emulation] eglCreateContext: 0xe650a220: maj 3 min 0 rcv 3
[EGL_emulation] eglMakeCurrent: 0xe650a220: ver 3 0 (tinfo 0xbe49d190)
[Gralloc3] mapper 3.x is not supported
[HostConnection] createUnique: call
[HostConnection] HostConnection::get() New Host Connection established 0xbe2518a0, tid 17005
[HostConnection] HostComposition ext ANDROID_EMU_CHECKSUM_HELPER_v1 ANDROID_EMU_native_sync_v2 ANDROID_EMU_native_sync_v3 ANDROID_EMU_native_sync_v4 ANDROID_EMU_dma_v1 ANDROID_EMU_direct_mem ANDROID_EMU_host_composition_v1 ANDROID_EMU_host_composition_v2 ANDROID_EMU_YUV420_888_to_NV21 ANDROID_EMU_YUV_Cache ANDROID_EMU_async_unmap_buffer GL_OES_EGL_image_external_essl3 GL_OES_vertex_array_object GL_KHR_texture_compression_astc_ldr ANDROID_EMU_host_side_tracing ANDROID_EMU_gles_max_version_3_0 
[eglCodecCommon] allocate: Ask for block of size 0x1000
[eglCodecCommon] allocate: ioctl allocate returned offset 0x3ff24c000 size 0x2000
[Mono] Requesting loading reference 8 (of 9) of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Vary.Core.Android.dll
[Mono] Loading reference 8 of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Vary.Core.Android.dll asmctx DEFAULT, looking for Xamarin.Forms.Platform, Version=2.0.0.0, Culture=neutral, PublicKeyToken=null
[Mono] Assembly Ref addref Vary.Core.Android[0xe64538c0] -> Xamarin.Forms.Platform[0xe64639a0]: 3
[Mono] DllImport searching in: '__Internal' ('(null)').
[Mono] Searching for 'java_interop_jnienv_call_nonvirtual_float_method_a'.
[Mono] Probing 'java_interop_jnienv_call_nonvirtual_float_method_a'.
[Mono] Found as 'java_interop_jnienv_call_nonvirtual_float_method_a'.
[EGL_emulation] eglMakeCurrent: 0xe650a220: ver 3 0 (tinfo 0xbe49d190)
[eglCodecCommon] setVertexArrayObject: set vao to 0 (0) 1 0
[Mono] Requesting loading reference 11 (of 22) of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.Forms.Platform.Android.dll
[Mono] Loading reference 11 of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.Forms.Platform.Android.dll asmctx DEFAULT, looking for Xamarin.AndroidX.Core, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
[Mono] Assembly Ref addref Xamarin.Forms.Platform.Android[0xe64638e0] -> Xamarin.AndroidX.Core[0xe6461900]: 7
[Mono] Requesting loading reference 16 (of 22) of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.Forms.Platform.Android.dll
[Mono] Loading reference 16 of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.Forms.Platform.Android.dll asmctx DEFAULT, looking for Xamarin.AndroidX.SwipeRefreshLayout, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
[Mono] Assembly Ref addref Xamarin.Forms.Platform.Android[0xe64638e0] -> Xamarin.AndroidX.SwipeRefreshLayout[0xe6463040]: 2
[Mono] Requesting loading reference 3 (of 4) of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.AndroidX.SwipeRefreshLayout.dll
[Mono] Loading reference 3 of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.AndroidX.SwipeRefreshLayout.dll asmctx DEFAULT, looking for Xamarin.AndroidX.Core, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
[Mono] Assembly Ref addref Xamarin.AndroidX.SwipeRefreshLayout[0xe6463040] -> Xamarin.AndroidX.Core[0xe6461900]: 8
[Mono] Requesting loading reference 2 (of 4) of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.AndroidX.SwipeRefreshLayout.dll
[Mono] Loading reference 2 of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.AndroidX.SwipeRefreshLayout.dll asmctx DEFAULT, looking for Java.Interop, Version=0.1.0.0, Culture=neutral, PublicKeyToken=84e04ff9cfb79065
[Mono] Assembly Ref addref Xamarin.AndroidX.SwipeRefreshLayout[0xe6463040] -> Java.Interop[0xe649f180]: 22
[Mono] DllImport searching in: '__Internal' ('(null)').
[Mono] Searching for 'java_interop_jnienv_call_static_void_method_a'.
[Mono] Probing 'java_interop_jnienv_call_static_void_method_a'.
[Mono] Found as 'java_interop_jnienv_call_static_void_method_a'.
[Mono] DllImport searching in: '__Internal' ('(null)').
[Mono] Searching for 'java_interop_jnienv_call_float_method_a'.
[Mono] Probing 'java_interop_jnienv_call_float_method_a'.
[Mono] Found as 'java_interop_jnienv_call_float_method_a'.
2021-03-05 03:03:56 [TRACE] (MvxForms) PresentationAttribute not found for ContactDisplayViewModel. Assuming ContentPage presentation
2021-03-05 03:03:56 [TRACE] (MvvmCross.Logging.MvxLog) No view model association found for candidate view MainActivity
2021-03-05 03:03:56 [TRACE] (MvvmCross.Logging.MvxLog) No view model association found for candidate view MainActivity
[Mono] DllImport searching in: '__Internal' ('(null)').
[Mono] Searching for 'java_interop_jnienv_is_assignable_from'.
[Mono] Probing 'java_interop_jnienv_is_assignable_from'.
[Mono] Found as 'java_interop_jnienv_is_assignable_from'.
Resolved pending breakpoint at 'ContactDisplayViewModel.cs:67,1' to void Vary.Core.ViewModels.ContactDisplayViewModel.<OnSelectSingleContactSelected>d__13.MoveNext () [0x0015b].
[Mono] Requesting loading reference 5 (of 8) of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.Essentials.dll
[Mono] Loading reference 5 of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.Essentials.dll asmctx DEFAULT, looking for System.Core, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e
[Mono] Assembly Ref addref Xamarin.Essentials[0xe64636a0] -> System.Core[0xe64a0140]: 9
[Mono] DllImport searching in: '__Internal' ('(null)').
[Mono] Searching for 'java_interop_jnienv_get_object_field'.
[Mono] Probing 'java_interop_jnienv_get_object_field'.
[Mono] Found as 'java_interop_jnienv_get_object_field'.
[EGL_emulation] eglMakeCurrent: 0xe650a220: ver 3 0 (tinfo 0xbe49d190)
[EGL_emulation] eglMakeCurrent: 0xe650a220: ver 3 0 (tinfo 0xbe49d190)
[Mono] Requesting loading reference 12 (of 22) of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.Forms.Platform.Android.dll
[Mono] Loading reference 12 of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.Forms.Platform.Android.dll asmctx DEFAULT, looking for System.Runtime.Serialization, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e
[Mono] Assembly Ref addref Xamarin.Forms.Platform.Android[0xe64638e0] -> System.Runtime.Serialization[0xe64a17c0]: 4
[Mono] DllImport searching in: 'libmono-native.so' ('./libmono-native.so').
[Mono] Searching for 'SystemNative_MkDir'.
[Mono] Probing 'SystemNative_MkDir'.
[Mono] Found as 'SystemNative_MkDir'.
[Mono] Requesting loading reference 21 (of 22) of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.Forms.Platform.Android.dll
[Mono] Loading reference 21 of /storage/emulated/0/Android/data/com.vinayakjuneja.vary/files/.__override__/Xamarin.Forms.Platform.Android.dll asmctx DEFAULT, looking for System.Xml, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e
[Mono] Assembly Ref addref Xamarin.Forms.Platform.Android[0xe64638e0] -> System.Xml[0xe64a16a0]: 7
[System] A resource failed to call close. 
[System] A resource failed to call close.

【问题讨论】：

能否提供您使用 MVVMCross 的方式。我使用INotifyPropertyChanged 的正常绑定检查了您的代码。有用。请检查屏幕截图。 imgur.com/evVRWzD 当我单击返回按钮或导航栏中的返回按钮时，没有崩溃。如果你愿意，我可以提供我的代码。
嗨@WendyZang-MSFT！我已经编辑了我的答案以包含完整的视图模型！此外，如果您也可以共享您的代码，那就太好了。谢谢:)
我会先提供我测试的代码，然后再用 MvvmCross 检查。
好吧，我尝试使用一个新项目做同样的操作，没有 mvvmcross。它就像一个魅力！我猜@WendyZang-MSFT 你怀疑 Mvvm 交叉实现是对的。现在我只需要确定实施有什么问题！如果您需要我的代码中的更多文件来分析问题，请告诉我。提前致谢！ :)
我会尽快检查并反馈。如果我需要更多详细信息，我会要求更多。

标签： c# xamarin.forms xamarin.android crash xamarin.essentials

【解决方案1】：

完整代码：

public partial class MainPage : ContentPage, INotifyPropertyChanged
{
    public Command SelectSingleContactCommand
    {
        get
        {
            return new Command(val =>
            {
                OnSelectSingleContactSelected();
            });
        }
    }
    private bool _isSingleContactRequired;
    public bool IsSingleContactRequired
    {
        get
        {
            return _isSingleContactRequired;
        }
        set
        {
            if (_isSingleContactRequired != value)
            {
                _isSingleContactRequired = value;
                NotifyPropertyChanged("IsSingleContactRequired");
            }
        }
    }



    private string _contactInformation;
    public string ContactInformation
    {
        get
        {
            return _contactInformation;
        }
        set
        {
            if (_contactInformation != value)
            {
                _contactInformation = value;
                NotifyPropertyChanged("ContactInformation");
            }
        }
    }

    public MainPage()
    {
        InitializeComponent();
        this.BindingContext = this;
    }
    async void OnSelectSingleContactSelected()
    {
        try
        {
            var contact = await Contacts.PickContactAsync();

            if (contact == null)
                return;

            var contactInfo = new StringBuilder();
            contactInfo.AppendLine(contact.DisplayName);
            contactInfo.AppendLine(contact.FamilyName);
            contactInfo.AppendLine(contact.Emails.FirstOrDefault().ToString());
            contactInfo.AppendLine(contact.Phones.FirstOrDefault().ToString());

            IsSingleContactRequired = true;
            ContactInformation = contactInfo.ToString();
        }
        catch (Exception ex)
        {
            Debug.WriteLine(ex.Message);
        }
    }
    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

截图：

https://imgur.com/evVRWzD

【讨论】：

这个案例有什么更新吗？你解决了这个问题吗？
温迪，我不知道该说什么。我一次又一次地尝试，废弃多个代码，删除和修改多个属性等，你知道问题是什么吗？ MainActivity 中的“NoHistory = true”！！！我现在想通了。因此，每当我选择联系人、文件或图像时，当我返回时，它就会崩溃。该活动的历史记录已被删除。这是一个菜鸟的错误，或者我应该说这么小的属性造成了巨大的问题！写这篇文章的同时我又哭又笑！！
Vinayak Juneja 你是个混蛋！！我追了几天这个问题，当我使用 media.plugin 选择视频或单击后退按钮时，我的 XF Android 应用程序一直在后台运行......无法弄清楚！进行各种代码更改！谢谢兄弟！

【解决方案2】：

好吧，经过几个月的零星和频繁检查，经过大量代码更改和大修，在 MainActivity 文件中发现了问题。

应该从 MainActivity.cs 中删除“NoHistory = true”，否则一旦应用程序进入后台，它会删除您的所有活动历史记录，您将无法返回！因此应用程序崩溃（或者你可以说它进入后台，当你尝试从最近打开它时，你会得到第一个活动/屏幕/视图模型）！

这么简单的解决这个缺陷，我哭笑不得！

P.S : 这解决了图像拾取、文件拾取和联系人拾取的所有崩溃问题！

干杯！

【讨论】：