在后面的代码中设置 Xamarin.Forms 绑定 CommandParameter

Question

我正在尝试TapGestureRecognizer在代码上设置绑定，但我无法找到正确的方法。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，它看起来像这样......

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);

我的困惑来自CommandParameterProperty. 在 xaml 中，这只是{Binding}没有其他参数。这是如何在代码中完成的？传入new Binding()或this.BindingContext似乎不起作用。

Answer 1

绑定CommandProperty与您所做的相同。

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

为了解决这个问题，您需要Source像 Adam 指出的那样指定属性。

请注意，但是如果BindingContext您尝试分配的是 Null，我怀疑在您的情况下是这样，这仍然会引发异常。

Grid下面的示例中的设置为BindingContext视图模型 ( objGrid.BindingContext = objMyView2)。

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

如果运行下面的演示，您将在Output窗口中看到一条调试消息，指示视图模型中的属性值。

        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);

支援课程：-

我的视图2：-

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

    public ICommand LaunchLocationDetailsCommand_WithParameters { get; set; }
}

启动命令：-

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

视图模型库：-

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

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

}