ViewModel 构建崩溃应用程序

Question

我正在尝试通过与 Notes 联系（一对多）来学习 Xamarin 表单。 我已经完成了一个带有 ContactsViewModel 绑定的 ListView，效果很好。

所以现在我正在尝试制作一个编辑上下文按钮：

public void OnEdit(object sender, EventArgs e)
{
    Contact contact = ((MenuItem)sender).CommandParameter as Contact;
    Navigation.PushAsync(new ContactPage(contact));
}

所以在这里我将我的联系人发送到带有基本条目的页面，这个 ContactPage 的构造函数：

public ContactPage(Contact contact)
{
    InitializeComponent();
    BindingContext = new ContactViewModel()
    {
        Id = contact.Id,
        Firstname = contact.Firstname,
        Lastname = contact.Lastname,
        Email = contact.Email,
        Address = contact.Address,
        Notes = contact.Notes
    };
}

但是当我点击我的编辑上下文按钮时，应用程序崩溃了，我不明白为什么。

如果您需要更深入地查看代码，这里是存储库： https://github.com/yerffeog/Contactium

感谢您的关注，我也愿意接受任何代码批评和改进。

删除参数时它可以工作，但是当我只输入一个参数时，我会崩溃。

Answer 1

检查以确保将有效值传递给页面。

public async void OnEdit(object sender, EventArgs e) { 
    Contact contact = ((MenuItem)sender).CommandParameter as Contact;
    if(contact != null) {
        var contactPage = new ContactPage(contact);
        await Navigation.PushAsync(contactPage);
    }
}

根据提供的存储库代码，

这个视图模型

public class ContactViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public ContactViewModel()
    {

    }

    /// <summary>
    /// Event when property changed.
    /// </summary>
    /// <param name="s">string</param>
    void OnPropertyChanged(string s)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(s));
    }

    /// <summary>
    /// Id of the contact.
    /// </summary>
    public int Id {
        get
        {
            return Id;
        }

        set
        {
            Id = value;
            OnPropertyChanged(nameof(Id));
        }
    }

    /// <summary>
    /// Lastname of the contact.
    /// </summary>
    public string Lastname {
        get
        {
            return Lastname;
        }

        set
        {
            Lastname = value;
            OnPropertyChanged(nameof(Lastname));
        }
    }

    /// <summary>
    /// Firstname of the contact.
    /// </summary>
    public string Firstname {
        get
        {
            return Firstname;
        }

        set
        {
            Firstname = value;
            OnPropertyChanged(nameof(Firstname));
        }
    }

    /// <summary>
    /// Email of the contact.
    /// </summary>
    public string Email {
        get
        {
            return Email;
        }

        set
        {
            Email = value;
            OnPropertyChanged(nameof(Email));
        }
    }

    /// <summary>
    /// Address of the contact.
    /// </summary>
    public string Address
    {
        get
        {
            return Address;
        }

        set
        {
            Address = value;
            OnPropertyChanged(nameof(Address));
        }
    }

    /// <summary>
    /// Notes of the contact.
    /// </summary>
    public List<Note> Notes
    {
        get
        {
            return Notes;
        }

        set
        {
            Notes = value;
            OnPropertyChanged(nameof(Notes));
        }
    }
}

上述操作将失败，因为属性引用自身，这将导致堆栈溢出并使应用程序崩溃。

为避免重复代码，使用基础创建基本视图模型

public class ViewModelBase : INotifyPropertyChanged {
    public event PropertyChangedEventHandler PropertyChanged;
    /// <summary>
    /// Event when property changed.
    /// </summary>
    /// <param name="s">string</param>
    protected void OnPropertyChanged([CallerMemberName] string member = "") {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(member));
    }
}

从那里确保视图模型具有公开属性的支持字段。

public class ContactViewModel : ViewModelBase {
    private string id;
    /// <summary>
    /// Id of the contact.
    /// </summary>
    public int Id {
        get { return id; }
        set {
            id = value;
            OnPropertyChanged();
        }
    }

    string lastname;
    /// <summary>
    /// Lastname of the contact.
    /// </summary>
    public string Lastname {
        get { return lastname; }
        set {
            lastname = value;
            OnPropertyChanged();
        }
    }

    string firstname
    /// <summary>
    /// Firstname of the contact.
    /// </summary>
    public string Firstname {
        get { return firstname; }
        set {
            firstname = value;
            OnPropertyChanged();
        }
    }

    string email;
    /// <summary>
    /// Email of the contact.
    /// </summary>
    public string Email {
        get { return email; }
        set {
            email = value;
            OnPropertyChanged();
        }
    }

    string address;
    /// <summary>
    /// Address of the contact.
    /// </summary>
    public string Address {
        get { return address; }
        set {
            address = value;
            OnPropertyChanged();
        }
    }

    string notes;
    /// <summary>
    /// Notes of the contact.
    /// </summary>
    public List<Note> Notes {
        get { return notes; }
        set {
            notes = value;
            OnPropertyChanged();
        }
    }
}