Xamarin 可绑定行为的 "PropertyChanged Event" 能否直接处理其视图的属性？

Question

我想用可绑定行为处理视图的属性。

是否有任何方法可以直接访问事件处理程序中连接的视图？

将 "AssociatedObject" 更改为静态会导致绑定问题。

我该怎么办？

public class CustNameBehavior : Behavior<Label>
{
    public Label AssociatedObject { get; private set; }

    public static readonly BindableProperty MyNameProperty = BindableProperty.Create(nameof(MyName), typeof(string), typeof(CustNameBehavior), null, BindingMode.OneWay, propertyChanged: OnMyNameChanged);
    private static void OnMyNameChanged(BindableObject bindable, object oldValue, object newValue)
    {
        CustNameBehavior target = (CustNameBehavior)bindable;

        if (target != null)
        {
            var value = (string)newValue;

            if (string.IsNullOrWhiteSpace(value))
            {
                AssociatedObject.Text = "Empty!";   => this is inaccessible. because of non static.
            }
            else
            {
                AssociatedObject.Text = value;   => this is inaccessible. because of non static.
            }
        }
    }

    protected override void OnAttachedTo(Label bindable)
    {
        base.OnAttachedTo(bindable);
        AssociatedObject = bindable;
        bindable.BindingContextChanged += Bindable_BindingContextChanged;
    }

    protected override void OnDetachingFrom(Label bindable)
    {
        base.OnDetachingFrom(bindable);
        bindable.BindingContextChanged -= Bindable_BindingContextChanged;
        AssociatedObject = null;
    }

    private void Bindable_BindingContextChanged(object sender, EventArgs e)
    {
        base.OnBindingContextChanged();
        BindingContext = AssociatedObject.BindingContext;
    }

    public string MyName
    {
        get => (string)GetValue(MyNameProperty);
        set
        {
            SetValue(MyNameProperty, value);
        }
    }
}

Answer 1

OnMyNameChanged 处理程序必须是静态的，因为 MyNameProperty 也是静态的（好吧，从技术上讲，这也许是可行的，但它会很脆弱并且会跳过箍）。无论如何，您希望 AssociatedObject 是非静态的，因为每个 CustNameBehavior 实例都需要一个，其他任何东西都可能不会按预期运行。

但是，您可以从OnMyNameChanged访问非静态方法和属性，因为CustNameBehavior的实例是通过bindable传递的范围。此外，此实例通过 AssociatedObject 持有对 Label 的引用。因此，您可以通过 target 访问 AssociatedObject（已经转换为 CustNameBehavior 的 bindable），因此您的方法将变为

private static void OnMyNameChanged(BindableObject bindable, object oldValue, object newValue)
{
    CustNameBehavior target = (CustNameBehavior)bindable;

    if (target != null && target.AssociatedObject != null)
    {
        var value = (string)newValue;

        if (string.IsNullOrWhiteSpace(value))
        {
            target.AssociatedObject.Text = "Empty!"; 
        }
        else
        {
            target.AssociatedObject.Text = value;
        }
    }
}

请注意 我添加了一个检查 AssociatedObject 是否为 null，因为我假设 OnMyNameChanged 在行为之前被调用被绑定。

恕我直言，如果您将实例逻辑单独移动到一个方法中会更好

private static void OnMyNameChanged(BindableObject bindable, object oldValue, object newValue)
{
    CustNameBehavior target = (CustNameBehavior)bindable;
    target?.SetLabelName((string)newValue);
}

private void SetLabelName(string name)
{
    if(AssociatedObject != null)
    {
        AssociatedObject.Text = string.IsNullOrEmpty(name) ? "Empty!" : name;
    }
}

你还必须从OnAttachedTo设置AssociatedObject.Text，因为MyNameProperty暂时不会设置，这会导致AssociatedObject.Text不正在更新。您可以为此目的重用 SetLabelName，这样，您就不必重复检查字符串是否为空的逻辑。

protected override void OnAttachedTo(Label bindable)
{
    base.OnAttachedTo(bindable);
    AssociatedObject = bindable;
    SetLabelName((string)GetValue(MyNameProperty));
    bindable.BindingContextChanged += Bindable_BindingContextChanged;
}

Xamarin 可绑定行为的 "PropertyChanged Event" 能否直接处理其视图的属性？

Can "PropertyChanged Event" of Xamarin bindable behavior directly handle property of its view?

behavior

xamarin

xamarin.forms

Xamarin 可绑定行为的 "PropertyChanged Event" 能否直接处理其视图的 属性？

Can "PropertyChanged Event" of Xamarin bindable behavior directly handle property of its view?

behavior

xamarin

xamarin.forms

Xamarin 可绑定行为的 "PropertyChanged Event" 能否直接处理其视图的属性？