c# - ui 工具包中的错误一致性错误

Question

我在访问 View Controller .cs 文件中的文本框时遇到问题

 async partial void loginUser(UIButton sender)

{

    // Show the progressBar as the MainActivity is being loade


    Console.WriteLine("Entered email : " + txtEmail.Text);


        // Create user object from entered email
        mCurrentUser = mJsonHandler.DeserialiseUser(txtEmail.Text);
        try
        {

            Console.WriteLine("Starting network check");
            // Calls email check to see if a registered email address has been entered
            if (EmailCheck(txtEmail.Text) == true)
            {
                await  CheckPassword();
            }
            else
            {
                UIAlertView alert = new UIAlertView()
                {
                    Title = "Login Alert",
                    Message = "Incorrect email or password entered"
                };
                alert.AddButton("OK");
                alert.Show();

            }


        }
        catch (Exception ex)
        {
            System.Diagnostics.Debug.WriteLine("An error has occured: '{0}'", ex);
        }

正是在这个函数中，它提示它无法访问 aynsc 方法上的文本框

    public Task CheckPassword()
    {

   return Task.Run(() =>
    {
            // Creates instance of password hash to compare plain text and encrypted passwords.
            PasswordHash hash = new PasswordHash();
            // Checks password with registered user password to confirm access to account.
            if (hash.ValidatePassword(txtPassword.Text ,mCurrentUser.password)==true)
            {
                Console.WriteLine("Password correct");


                UIAlertView alert = new UIAlertView()
                {
                    Title = "Login Alert",
                    Message = "Password Correct Loggin In"
                };
                alert.AddButton("OK");
                alert.Show();
                //insert intent call to successful login here.

            }
            else
            {
                UIAlertView alert = new UIAlertView()
                {
                    Title = "Login Alert",
                    Message = "Incorrect email or password entered"
                };
                alert.AddButton("OK");
                alert.Show();

            }
            Console.WriteLine("Finished check password");
        });
    }

错误发生在这一行:

txtPassword.Text 

错误如下:

UIKit.UIKitThreadAccessException: UIKit Consistency error: you are calling a UIKit method that can only be invoked from the UI thread.

即使密码正确，也不会显示我的密码正确。我是否必须在单独的线程上运行 UI 警报？

Answer 1

任何 UIKit 方法都必须从 UI 线程(或主线程、主队列等)调用。这确保了 UI 的一致性。 Xamarin 在 Debug模式下对所有 UIKit 方法添加检查，并在您尝试使用后台线程更改 UI 时抛出该异常。

解决方案很简单:仅从 UI 线程修改 UI。这实质上意味着，如果您使用的是前面带有“UI”的类，则您应该从 UI 线程进行操作。 (这是一个经验法则，还有其他时间在 UI 线程上)。

如何在这个神话般的 UI 线程上获取我的代码？很高兴你问了。在 iOS 中，您有几种选择:

• 当在 NSObject 的子类中时，InvokeOnMainThread 将起到作用。
• 在任何地方，CoreFoundation.DispatchQueue.MainQueue.DispatchAsync 都将始终有效。

这两个方法都只接受一个 Action，它可以是一个 lambda 或一个方法。

所以在您的代码中，如果我们添加一个 InvokeOnMainThread(因为我认为这是在您的 UIViewController 子类中)...

 public Task CheckPassword()
 {

     return Task.Run(() =>
     {
        // Creates instance of password hash to compare plain text and encrypted passwords.
        PasswordHash hash = new PasswordHash();
        // Checks password with registered user password to confirm access to account.
        InvokeOnMainThread(() => {
            if (hash.ValidatePassword(txtPassword.Text ,mCurrentUser.password)==true)
            {
                Console.WriteLine("Password correct");


                UIAlertView alert = new UIAlertView()
                {
                    Title = "Login Alert",
                    Message = "Password Correct Loggin In"
                };
                alert.AddButton("OK");
                alert.Show();
                //insert intent call to successful login here.

            }
            else
            {
                UIAlertView alert = new UIAlertView()
                {
                    Title = "Login Alert",
                    Message = "Incorrect email or password entered"
                };
                alert.AddButton("OK");
                alert.Show();

            }
        });
        Console.WriteLine("Finished check password");
    });
}