Xamarin形式BindableProperty在构造函数之前更改 [英] Xamarin Forms BindableProperty Changed before constructor

问题描述

我正在通过XAML在App.xaml中添加资源.此资源是隐式样式，将分配给CustomControl. CustomControl包含一个标签.

I'm adding a resource through XAML in my App.xaml. This resource is a implicit style that will be assigned to a CustomControl. The CustomControl contains a label.

要设置此标签的TextColor，我在CustomControl上创建一个可绑定的属性，并使用隐式样式指定一个值.使用BindableProperty的PropertyChanged方法，在CustomControl内设置标签的TextColor.

To set the TextColor of this label I create a bindable property on the CustomControl and assign a value with the implicit style. Using the PropertyChanged method of the BindableProperty I set the TextColor of the Label inside my CustomControl.

<Style TargetType="CustomControl" >
     <Setter Property="InnerLabelTextColor" Value="Green" />
</Style>

-

private static void InnerLabelTextColorChanged(BindableObject bindableObject, object oldValue, object newValue)
{
    ((CustomControl)bindableObject).InnerLabel.TextColor = (Color)newValue;
}

这曾经在XF 2.3.2.127中起作用，但是当我更新到XF 2.3.4.270时，我开始在CustomControl-BindableProperty-中获得NullReferenceException- InnerLabelTextColorChanged方法.

This used to work in XF 2.3.2.127 but when I updated to XF 2.3.4.270 I started getting a NullReferenceException in the CustomControl - BindableProperty - InnerLabelTextColorChanged method.

在执行构造函数之前，将调用PropertyChanged方法.当执行PropertyChanged方法导致NullReferenceException时，我的InnerLabel为null.

The PropertyChanged method is called before executing the constructor. My InnerLabel is null when the PropertyChanged method is executed which causes the NullReferenceException.

我想知道此行为是请求的XF行为还是错误?

I was wondering if this behaviour is the requested XF behaviour or if it is a bug?

如果这是要求的行为，那么谁能提供正确的方法来处理这种情况?

If it is the requested behaviour, can anyone provide the correct way of handling this situation?

谢谢！

编辑-自定义控制代码示例

Edit - custom control code sample

public sealed class CustomControl : ContentView
{
    public static readonly BindableProperty InnerLabelTextColorProperty =
        BindableProperty.Create("InnerLabelTextColor", typeof(Color), typeof(CustomControl), Color.Black,
            BindingMode.OneWay, null, CustomControl.InnerLabelTextColorChanged, null, null, null);


    public CustomControl()
    {
        this.InnerLabel = new Label();

        this.Content = this.InnerLabel;
    }


    public Label InnerLabel { get; set; }


    public Color InnerLabelTextColor
    {
        get
        {
            return (Color)this.GetValue(CustomControl.InnerLabelTextColorProperty);
        }
        set
        {
            this.SetValue(CustomControl.InnerLabelTextColorProperty, value);
        }
    }


    private static void InnerLabelTextColorChanged(BindableObject bindableObject, object oldValue, object newValue)
    {
        ((CustomControl)bindableObject).InnerLabel.TextColor = (Color)newValue;
    }
}

推荐答案

我曾经解决过类似问题 ，唯一的不同是在基于XAML的控件中遇到了它.

I had worked on a similar issue sometime back, the only difference being it was encountered in a XAML based control.

我认为导致此问题的根本原因是(当我们使用像这样的全局隐式样式时)基本构造函数尝试设置该可绑定属性，并且由于派生构造函数仍在等待轮换，我们遇到了null参考.

I think the root cause for this problem is that (when we use an global-implicit style like that) the base constructor tries to set that bindable property, and as derived constructor is still waiting for its turn, we encounter the null reference.

解决此问题的最简单方法是使用Binding在内部子控件上设置属性(

Simplest way to resolve this would be to use Binding to set properties on inner-child controls (reference):

public CustomControl()
{
    this.InnerLabel = new Label();

    // add inner binding
    this.InnerLabel.SetBinding(Label.TextColorProperty, 
         new Binding(nameof(InnerLabelTextColor), 
                     mode: BindingMode.OneWay, 
                     source: this));  

    this.Content = this.InnerLabel;
}

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