Question

我在Xamarin Forms上遇到过这个问题（在Android和iOS上测试过）。

我有一个简单的页面

using System;

using Xamarin.Forms;

namespace BugTGR
{
    public class PageMain : ContentPage
    {
        public PageMain()
        {
            PageMainViewModel vm = new PageMainViewModel();
            this.BindingContext = vm;

            Label label1 = new Label{ Text = "Press with ICommand"};

            TapGestureRecognizer tgr = new TapGestureRecognizer();
            tgr.BindingContext = vm;
            tgr.SetBinding(TapGestureRecognizer.CommandProperty, "Tapped");
            label1.GestureRecognizers.Add(tgr);

            Label label2 = new Label { Text = "Press with Tapped"};
            TapGestureRecognizer tgr1 = new TapGestureRecognizer();
            tgr1.Tapped += async (object sender, EventArgs e) => {
                await DisplayAlert("Attention", "PRESSED WITH TAPPED", "Ok");
            };
            label2.GestureRecognizers.Add(tgr1);

            Content = new StackLayout
            {

                Children = {label1, label2}
            };
        }
    }
}

在这段代码中，我使用了这个ViewModel（非常简单，只有一个命令）

using System;
using System.Windows.Input;
using Xamarin.Forms;
using PropertyChanged;
namespace BugTGR
{
    [ImplementPropertyChanged]
    public class PageMainViewModel 
    {
        public PageMainViewModel()
        {

            this.Tapped = new Command(async() =>
            {
                await Application.Current.MainPage.DisplayAlert("Attention", "Pressed", "Ok");
            });

        }

        public ICommand Tapped { protected get; set;}
    }
}

然后，你怎么看，我尝试将命令绑定到TapGestureRecognizer，然后将TGR添加到标签，但如果我单击标签，则不会调用该命令。

在第二个标签（label2）中，我使用Tapped事件添加另一个TapGestureRecognizer而不绑定命令。这有效！

有人可以让我知道我做错了什么？

谢谢！的Alessandro

Answer 1

这是工作解决方案。问题是如何创建Tapped命令。以下是两种方法。调用VM的命令或事件处理程序。如果你在代码而不是在xaml中这样做我会使用vm.TappedHandler方法

namespace ButtonRendererDemo
{
    public class LabelTapPage : ContentPage
    {
        public LabelTapPage()
        {
            PageMainViewModel vm = new PageMainViewModel();
            this.BindingContext = vm;

            Label label1 = new Label { Text = "Press with ICommand" };
            TapGestureRecognizer tgr = new TapGestureRecognizer();
            label1.GestureRecognizers.Add(tgr);
            tgr.SetBinding(TapGestureRecognizer.CommandProperty, "Tapped");

            Label label2 = new Label { Text = "Press with Tapped Event" };
            TapGestureRecognizer tgr1 = new TapGestureRecognizer();
            tgr1.Tapped += async (object sender, EventArgs e) => {
                await DisplayAlert("Attention", "Tapped Event: Pressed", "Ok");
            };
            label2.GestureRecognizers.Add(tgr1);

            Label label3 = new Label { Text = "Press with TappedHandler" };
            var tapGestureRecognizer = new TapGestureRecognizer();
            tapGestureRecognizer.Tapped += async (s, e) => {
                await vm.TappedHandler();
            };
            label3.GestureRecognizers.Add(tapGestureRecognizer);


            Content = new StackLayout
            {

                Children = { label1, label2, label3 }
            };
        }
    }


    public class PageMainViewModel : INotifyPropertyChanged
    {
        ICommand tapCommand;

        public PageMainViewModel()
        {
            tapCommand = new Command(OnTapped);
        }

        public async Task TappedHandler()
        {
            await Application.Current.MainPage.DisplayAlert("Attention", "TappedHandler: Pressed", "Ok");
        }


        public ICommand Tapped
        {
            get { return tapCommand; }
        }

        async void OnTapped(object s)
        {
            await Application.Current.MainPage.DisplayAlert("Attention", "Tapped Command: Pressed", "Ok");
        }

        public event PropertyChangedEventHandler PropertyChanged;
    }

}

此外，您不需要设置tgr.BindingContext = vm;它由您的页面继承

在我发布之后，我发现有更简单的解决方案：

删除

中的“受保护”

public ICommand Tapped { get; set; }

因此页面可以访问它。就是这样:-)

可能是你的意思是让“set”受保护而不是得到？

Xamarin Forms和TabGestureRecognizer不会使用Command

1 个答案: