在 Udemy 课程中了解 MVVM 和 ObjectDetailViewModel

Question

我一直在为 Xamarin 做一门课程，我在 MVVM 部分，所有 xaml.cs 代码都移到 ViewModels 中。

这是课程（第 104、105、106 课）：[url]https://www.udemy.com/course/complete-xamarin-developer-course-ios-and-android[/url]

到目前为止，我已经能够理解 MVVM 的概念，但是，这门课程似乎做错了一些事情（我相信），并且以前讲座中的一些代码不在新讲座中（我们' 将暂时不理会这种烦恼），继续前进，我不确定如何正确编码。我也明白我目前正在展示一个简单的用例，所以让我们认为 Post 是一个不应该一遍又一遍地重新创建的巨大对象。

例如我有一个PostDetailPage.xaml(.cs)，在xaml中，我目前有以下（注意大部分Mode=TwoWay可能使用不正确）：

<ContentPage.Content>
    <StackLayout Margin="10">
        <Entry x:Name="experienceEntry" 
               Text="{Binding Experience, Mode=TwoWay}"
               Margin="-5"></Entry>
        <label x:name="venuename"
               text="{binding post.venuename, mode=onetime}"
               fontattributes="bold"/>
        <label x:name="categoryname"
               text="{binding post.categoryname, mode=onetime}"/>
        <label x:name="address"
               text="{binding post.address, mode=onetime}"/>
        <label x:name="coordinatelabel"
               text="{binding coordinates, mode=onetime}"/>
        <label x:name="distance"
               text="{binding post.distance, mode=onetime, stringformat='{0:0}'}"/>

        <Button Text="Update"
                x:Name="updateButton"
                Command="{Binding UpdatePostCommand}"
                CommandParameter="{Binding Post}" />

    </StackLayout>
</ContentPage.Content>

代码如下：

public partial class PostDetailPage : ContentPage
{
    PostDetailViewModel viewModel;

    public PostDetailPage(Post selectedPost)
    {
        InitializeComponent();

        viewModel = new PostDetailViewModel(selectedPost);
        BindingContext = viewModel;
    }
}

然后我有一个不完整的 PostDetailViewModel：

public class PostDetailViewModel : INotifyPropertyChanged
{
    public UpdatePostCommand UpdatePostCommand { get; set; }

    public event PropertyChangedEventHandler PropertyChanged;

    public PostDetailViewModel(Post post)
    {
        UpdatePostCommand = new UpdatePostCommand(this);
        SelectedPost = post;
    }

    //Can this become another ViewModel or ?
    private Post selectedPost;
    public Post SelectedPost
    {
        get { return selectedPost; }
        set 
        {
            selectedPost = value;
            OnPropertyChanged("Post");
        }
    }

    private string experience;
    public string Experience
    {
        get { return experience; }
        set
        {
            experience = value;
            UpdatePostObject();
            OnPropertyChanged("Experience");
        }
    }

    private void UpdatePostObject() {
        Post = new Post() {
            Experience = experience,

            //many more properties are needed here
            //seems like if the solution was to grow,
            //this would become unmanagable or something

        }
    }

    private void UpdatePostObject()
    {
        //This is used to Trigger the change on Post
        //I don't think this is the best way (hence my question(s))
        SelectedPost = new Post()
        {
            Experience = experience
        };
    }

    private void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

    public async void Update()
    {
        await App.Current.MainPage.Navigation.PushAsync(new HistoryPage());
    }
}

在课程中，讲师建议当Experience 发生更改（或与Post 相关的任何其他属性）时，我们一次又一次地重新创建Post 对象以触发CanExecute 和关联的@ 987654330@。每次更改后必须一次又一次地重新创建 Post 对象似乎是不正确的，所以最好只更新需要的对象吗？

所以，我要问（或了解）的是......

1. 我是否应该实现一个通用的PostViewModel，当Experience 发生变化时更新它，并以某种方式将ICommand 附加到它，如果是这样，这样的东西会是什么样子？当然，如果这不正确，请尽可能指出正确的方向。

2. 有没有类似OnObjectChanged 的方法可以使用？我在处理收藏时知道ObservableCollection。

如果还有什么我可以提供的，请告诉我。

谢谢 德里克

Answer 1

首先我打不开你的网址，这门课不见了。

我是否应该实现一个通用的 PostViewModel，它会在体验发生变化时更新，并以某种方式将 ICommand 附加到它，如果是这样，这样的东西会是什么样子？当然，如果这不正确，请尽可能指出正确的方向。

对于UpdatePostObject方法，你不需要new一个Post对象，如果你想为selectedPost对象设置新值，直接如下代码设置即可，

   private void UpdatePostObject()
        {
            selectedPost.Experience = experience;
            selectedPost.address= "new Adree";



            selectedPost.categoryname = "new CateGoryName";
            selectedPost.distance = "41";
            selectedPost.venuename = "new venu";

                //many more properties are needed here
                //seems like if the solution was to grow,
                //this would become unmanagable or something

           // };
        }

如果你想实现点击命令，你只需将PostDetailViewModel中的public ICommand UpdatePostCommand { get; set; }清除即可。然后在您的PostDetailViewModel 的构造函数中实现它。这是我关于PostDetailViewModel的所有代码

   public class PostDetailViewModel 
    {
        //: INotifyPropertyChanged
        public ICommand UpdatePostCommand { get; set; }

        public event PropertyChangedEventHandler PropertyChanged;

        public PostDetailViewModel(Post post)
        {
            UpdatePostCommand = new Command<Post>((key) => {


                key.categoryname = "Command categoryname";
                key.distance = "31";
                key.address = "Command address";
                key.venuename = "Command  venuename";
                key.Experience = "Command Experience";

            });






            SelectedPost = post;
        }

        //Can this become another ViewModel or ?
        private Post selectedPost;
        public Post SelectedPost
        {
            get { return selectedPost; }
            set
            {
                selectedPost = value;
             //   OnPropertyChanged("SelectedPost");
            }
        }

        private string experience;
        public string Experience
        {
            get { return experience; }
            set
            {
                experience = value;
                UpdatePostObject();
              //  OnPropertyChanged("Experience");
            }
        }

        private void UpdatePostObject()
        {
            selectedPost.Experience = experience;
            selectedPost.address= "new Adree";



            selectedPost.categoryname = "new CateGoryName";
            selectedPost.distance = "41";
            selectedPost.venuename = "new venu";

                //many more properties are needed here
                //seems like if the solution was to grow,
                //this would become unmanagable or something

           // };
        }

        //private void UpdatePostObject()
        //{
        //    //This is used to Trigger the change on Post
        //    //I don't think this is the best way (hence my question(s))
        //    SelectedPost = new Post()
        //    {
        //        Experience = experience
        //    };
        //}

        //public event PropertyChangedEventHandler PropertyChanged;

        //protected virtual void OnPropertyChanged(string propertyName)
        //{
        //    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        //}
        public async void Update()
        {
            await App.Current.MainPage.Navigation.PushAsync(new HistoryPage());
        }
    }
}

如果你想在运行时实现属性变化，你应该为你的POST模型中的所有属性实现INotifyPropertyChanged接口。

   public class Post:INotifyPropertyChanged
    {
     //   public string _venuename { get; set; }
      //  public string categoryname { get; set; }
        string _categoryname;
        public string categoryname
        {
            set
            {
                if (_categoryname != value)
                {
                    _categoryname = value;
                    OnPropertyChanged("categoryname");

                }
            }
            get
            {
                return _categoryname;
            }
        }

        //public string address { get; set; }


        string _address;
        public string address
        {
            set
            {
                if (_address != value)
                {
                    _address = value;
                    OnPropertyChanged("address");

                }
            }
            get
            {
                return _address;
            }
        }


       // public string Experience { get; set; }

        string _experience;
        public string Experience
        {
            set
            {
                if (_experience != value)
                {
                    _experience = value;
                    OnPropertyChanged("Experience");

                }
            }
            get
            {
                return _experience;
            }
        }

      //  public string distance { get; set; }

        string _distance;
        public string distance
        {
            set
            {
                if (_distance != value)
                {
                    _distance = value;
                    OnPropertyChanged("distance");

                }
            }
            get
            {
                return _distance;
            }
        }

        string _venuename;
        public string venuename
        {
            set
            {
                if (_venuename != value)
                {
                    _venuename = value;
                    OnPropertyChanged("venuename");

                }
            }
            get
            {
                return _venuename;
            }
        }


        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged(string propertyName)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

你的布局有一些错误，我改变了。

   <ContentPage.Content>
        <StackLayout Margin="10">
            <Entry x:Name="experienceEntry" 
               Text="{Binding Experience, Mode=TwoWay}"
               Margin="-5">

            </Entry>

            <Label x:Name="venuename"
               Text="{Binding SelectedPost.venuename, Mode=TwoWay}"
                   FontAttributes="Bold"
              />
            <Label x:Name="categoryname"
               Text="{Binding SelectedPost.categoryname, Mode=TwoWay}"/>
            <Label x:Name="address"
               Text="{Binding SelectedPost.address, Mode=TwoWay}"/>
            <Label x:Name="coordinatelabel"
               Text="{Binding SelectedPost.coordinates, Mode=TwoWay}"/>
            <Label x:Name="distance"
               Text="{Binding SelectedPost.distance, Mode=TwoWay, StringFormat='{0:0}'}"/>

            <Button Text="Update"
                x:Name="updateButton"
                Command="{Binding UpdatePostCommand}"
                CommandParameter="{Binding SelectedPost}" />

        </StackLayout>
    </ContentPage.Content>

这是我的布局背景代码。

   public MainPage()
        {
            InitializeComponent();
            Post selectedPost=new Post() { 
                address= "address", 
                categoryname= "categoryname", 
                distance= "21", 

                venuename= "venuename" };
            PostDetailViewModel viewModel = new PostDetailViewModel(selectedPost);
            BindingContext = viewModel;
        }
    }

是否可以使用某种类型的类似 OnObjectChanged 的​​方法？我在处理集合时知道 ObservableCollection。

在您的PostDetailViewModel 中，没有必要实现这一点。如果你想用ObservableCollection添加Post，就达到INotifyPropertyChanged，如果你Post项目会增加或减少，它会自动改变，但如果Post的值会改变，它会不变，你必须在你的Post模型中实现INotifyPropertyChanged。

这是我运行的GIF。一般情况下，它有一些默认值，如果我在Entry中输入这个值，post的这个值会被改变，如果我点击Button，post的值会被改变为另一个值

这里有一篇关于它的有用文章，你可以参考一下。 https://docs.microsoft.com/en-us/xamarin/xamarin-forms/xaml/xaml-basics/data-bindings-to-mvvm