java.lang.nullpoInterException：＆＃x27;尝试调用虚拟方法＆＃x27; void android.app.App.Acp.requestpermissions（java.lang.lang.string []，int）＆＃x27＆＃x27;

Question

我试图在App的类上请求read_contacts权限，但引发了例外 例外 java.lang.nullpointerexception：“尝试调用虚拟方法” void android.app.activity.requestpermissions（java.lang.string []，int）'null对象引用

{
    public App(SignalrMessenger signalrMessenger, ChatServicesSettings chatServicesSettings, ContactServices contactServices)
    {
        InitializeComponent();
        

        if (!IsPermissioGranted(Android.App.Application.Context, Manifest.Permission.ReadContacts))
        {
            RequestPermission(Android.App.Application.Context as Activity, new[] { Manifest.Permission.ReadContacts }).GetAwaiter().GetResult();
        }

        MauiApplicationServices.WasDatabaseUpdatedOnStartUp = contactServices.SynchronizeDbWithLocalContacts().GetAwaiter().GetResult();

        MainPage = new MainPage(signalrMessenger,chatServicesSettings);//new AppShell();
    }
    public bool IsPermissioGranted(Context ctx, string permission)
    {
        return ActivityCompat.CheckSelfPermission(ctx, permission)== Permission.Granted;
    }
    public async Task RequestPermission(Activity activity, params string[] permissions)
    {
        ActivityCompat.RequestPermissions(activity, permissions, 0);
        await Task.CompletedTask;
    }
}

'要在活动中获得许可并注入服务？由于MainActity应用程序初始化中调用了默认的CTOR

Answer 1

on Create方法将按预期触发MainActivity类中的预期。

您可以要求在mainActivity中索取权限，而不是app类。

示例代码

protected override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);

        if (ContextCompat.CheckSelfPermission(this, Manifest.Permission.Camera) == (int)Permission.Granted)
        {
            // We have permission, go ahead and use the camera.
        }
        else
        {
            ActivityCompat.RequestPermissions(this, new String[] { Manifest.Permission.Camera },0);
        }
    }

public override void OnRequestPermissionsResult(int requestCode, string[] permissions, Permission[] grantResults)
    {
         base.OnRequestPermissionsResult(requestCode, permissions, grantResults);
    }

Answer 2

您可以将Main Activity作为App Methord

步骤1中的参数传递为参数：初始化App Methord步骤2中的活动变量

     public App(SignalrMessenger signalrMessenger, ChatServicesSettings 
             chatServicesSettings, ContactServices contactServices,Activity activity)

：在App Methord

       public App(SignalrMessenger signalrMessenger, ChatServicesSettingschatServicesSettings, ContactServices contactServices,Activity activity)
{
    InitializeComponent();
    this.activity = activity;
    .........
    ........}

步骤3中创建构造函数：现在，用可缝合参数调用App Methord

    App( signalrMessenger, chatServicesSettings,contactServices,MainActivity.this)

     
    

{
public App（signalrmessenger signalrmessenger，chatservicessettings chatservicessetsings，ContactServices ContactServices）
{
InitializeComponent（）;

    if (!IsPermissioGranted(Android.App.Application.Context, Manifest.Permission.ReadContacts))
    {
        RequestPermission(Android.App.Application.Context as Activity, new[] { Manifest.Permission.ReadContacts }).GetAwaiter().GetResult();
    }

    MauiApplicationServices.WasDatabaseUpdatedOnStartUp = contactServices.SynchronizeDbWithLocalContacts().GetAwaiter().GetResult();

    MainPage = new MainPage(signalrMessenger,chatServicesSettings);//new AppShell();
}
public bool IsPermissioGranted(Context ctx, string permission)
{
    return ActivityCompat.CheckSelfPermission(ctx, permission)== Permission.Granted;
}
public async Task RequestPermission(Activity activity, params string[] permissions)
{
    ActivityCompat.RequestPermissions(activity, permissions, 0);
    await Task.CompletedTask;
}

}