c# - 在后面的代码中设置Xamarin.Forms绑定(bind)CommandParameter

我试图在代码中的TapGestureRecognizer上设置绑定(bind)，但我想不出正确的方法。正常工作的xaml看起来像这样...

<Grid>
    <Grid.GestureRecognizers>
        <TapGestureRecognizer Command="{Binding LaunchLocationDetailsCommand}"
                              CommandParameter="{Binding}" />
    </Grid.GestureRecognizers>
</Grid>

在C#中，看起来像这样...

var gridTap = new TapGestureRecognizer();

gridTap.SetBinding(TapGestureRecognizer.CommandProperty,
                   new Binding("LaunchLocationDetailsCommand"));

gridTap.SetBinding(TapGestureRecognizer.CommandParameterProperty,
                   new Binding(/* here's where I'm confused */));

var grid = new Grid();
grid.GestureRecognizers.Add(gridTap);

我对CommandParameterProperty的绑定(bind)感到困惑。在xaml中，这只是{Binding}，没有其他参数。如何在代码中完成？传递new Binding()或this.BindingContext似乎无效。

最佳答案

CommandProperty绑定(bind)与您所做的相同。

由于您没有传递要使用的某些属性的路径，因此CommandParameterProperty不能仅创建一个空的Binding，因为它会引发异常。

为了解决这个问题，您需要指定Adam指出的Source属性。

请注意，但是，如果您尝试分配的BindingContext是 Null，我怀疑是您的情况，这将使仍然抛出异常。

下例中的Grid将BindingContext设置为 View 模型(objGrid.BindingContext = objMyView2)。

然后，我们为命令参数创建一个新的绑定(bind)，Source指向我们的 View 模型(Source = objGrid.BindingContext)。

如果运行下面的演示，则会在Output窗口中看到一条调试消息，指示 View 模型中的属性值。

        MyView2 objMyView2 = new MyView2();
        objMyView2.SomeProperty1 = "value1";
        objMyView2.SomeProperty2 = "value2";

        objMyView2.LaunchLocationDetailsCommand_WithParameters = new Command<object>((o2)=>
        {
            LaunchingCommands.LaunchLocationDetailsCommand_WithParameters(o2);
        });

        Grid objGrid = new Grid();
        objGrid.BindingContext = objMyView2;
        objGrid.HeightRequest = 200;
        objGrid.BackgroundColor = Color.Red;

        TapGestureRecognizer objTapGestureRecognizer = new TapGestureRecognizer();
        objTapGestureRecognizer.SetBinding(TapGestureRecognizer.CommandProperty, new Binding("LaunchLocationDetailsCommand_WithParameters"));

        Binding objBinding1 = new Binding()
        {
            Source = objGrid.BindingContext
        };
        objTapGestureRecognizer.SetBinding(TapGestureRecognizer.CommandParameterProperty, objBinding1);
        //
        objGrid.GestureRecognizers.Add(objTapGestureRecognizer);

支持类(class):

MyView2 :-

public class MyView2
    : ViewModelBase
{
    public string SomeProperty1 { get; set; }
    public string SomeProperty2 { get; set; }

    public ICommand LaunchLocationDetailsCommand_WithParameters { get; set; }
}

LaunchingCommands :-

public static class LaunchingCommands
{
    public static void LaunchLocationDetailsCommand_WithParameters(object pobjObject)
    {
        System.Diagnostics.Debug.WriteLine("SomeProperty1 = " + (pobjObject as MyView2).SomeProperty1);
    }
}

ViewModelBase :-

public abstract class ViewModelBase
    : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public void RaisePropertyChanged(string pstrPropertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(pstrPropertyName));
        }
    }

}