Xamarin.Forms 可绑定属性

Posted 2023-03-12

【中文标题】Xamarin.Forms 可绑定属性

【英文标题】：Xamarin.Forms Bindable Properties

【发布时间】：2021-03-11 12:06:59

【问题描述】：

这听起来像是一个愚蠢的问题，但为什么可绑定属性是静态的？

public static readonly BindableProperty MapSpanProperty = BindableProperty.Create
            (
                propertyName: "MapSpan",
                returnType: typeof(MapSpan),
                declaringType: typeof(BindableMap),
                defaultValue: null,
                defaultBindingMode: BindingMode.TwoWay,
                propertyChanged: MapSpanPropertyChanged
            );

        public MapSpan MapSpan
        
            get
            
                return (MapSpan)GetValue(MapSpanProperty);
            
            set
            
                SetValue(MapSpanProperty, value);
            
        

我有这段代码，如果我将可绑定属性设为静态，它就可以正常工作，否则它就不起作用。如果我将此可绑定属性设为静态，这意味着，假设我同时打开了 2 个地图，如果我在其中一个上设置此值，那么这两个地图上的值将相同？

【参考方案1】：

每个可绑定属性都有一个对应的BindableProperty 类型的public static readonly 字段，该字段在同一类上公开，是可绑定属性的标识符。

您可以查看提供binable属性的控件的源代码。

例如，我使用内容视图。代码来自下面的链接。 Xamarin forms update viewmodel field from a bindable property

public partial class MyCustomControl : ContentView

    private string _text;
    public string Text
    
        get  return _text; 
        set
        
            _text = value;
            OnPropertyChanged();
        
    
    public static readonly BindableProperty TextProperty = BindableProperty.Create(
             nameof(Text),
             typeof(string),
             typeof(MyCustomControl),
             string.Empty,
             propertyChanged: (bindable, oldValue, newValue) =>
             
                 var control = bindable as MyCustomControl;
                 //var changingFrom = oldValue as string;
                 //var changingTo = newValue as string;
                 control.Title.Text = newValue.ToString();
             );
    
    public MyCustomControl()
    
        InitializeComponent();
    

ContentView 提供公共静态只读 BindableProperty。

  //
// Summary:
//     An element that contains a single child element.
//
// Remarks:
//     The following example shows how to construct a new ContentView with a Label inside.
[ContentProperty("Content")]
public class ContentView : TemplatedView

    //
    // Summary:
    //     Backing store for the Xamarin.Forms.ContentView.Content property..
    //
    // Remarks:
    //     To be added.
    public static readonly BindableProperty ContentProperty;

    public ContentView();

    //
    // Summary:
    //     Gets or sets the content of the ContentView.
    public View Content  get; set; 

    //
    // Summary:
    //     Method that is called when the binding context changes.
    //
    // Remarks:
    //     To be added.
    protected override void OnBindingContextChanged();

您可以查看 MS 文档以了解更多关于可转换属性中的静态的信息。 https://docs.microsoft.com/en-us/xamarin/xamarin-forms/xaml/bindable-properties