Question

我是xamarin.forms开发的新手，我仍然可以从网上找到的几个教程中迈出第一步。我有一个异步任务，从date.jsontest.com返回时间我有一个计时器，减少标签中的文本。我想把异步任务放在计时器中，以便它重复自己并在标签上显示时间但是我得到的不能将异步lamba转换为func

这里我的代码请帮帮我，谢谢

static async Task<string> RequestTimeAsync()
    {
        using (var client = new HttpClient())
        {
            var jsonString = await client.GetStringAsync("http://date.jsontest.com/");
            var jsonObject = JObject.Parse(jsonString);
            return jsonObject["time"].Value<string>();
        }
    }


 protected override async void OnAppearing()
    {
        base.OnAppearing();

        timeLabel.Text = await RequestTimeAsync();

        Device.StartTimer(TimeSpan.FromSeconds(1), () => { // i want the taks to be put 
//here so that it gets repeated
            var number = float.Parse(button.Text) - 1;
            button.Text = number.ToString();
            return number > 0;
        });

    }

重新加载定时器中的内容页面可以解决问题，所以如果有人可以帮助我，我们将不胜感激

Answer 1

您只需要在Task.Run（）中包装异步方法调用，例如：

Device.StartTimer(TimeSpan.FromSeconds(1), () =>
{
  Task.Run(async () =>
  {
    var time = await RequestTimeAsync();
    // do something with time...
  });
  return true;
});

Answer 2

解决

就像那个一样简单

 private async void ContinuousWebRequest()
    {
        while (_keepPolling)
        {
            timeLabel.Text = "call: "+counter+" "+ await RequestTimeAsync()+Environment.NewLine;
            // Update the UI (because of async/await magic, this is still in the UI thread!)
            if (_keepPolling)
            {
                await Task.Delay(TimeSpan.FromSeconds(5));
            }
        }
    }

    static async Task<string> RequestTimeAsync()
    {
        using (var client = new HttpClient())
        {
            var jsonString = await client.GetStringAsync("http://date.jsontest.com/");
            var jsonObject = JObject.Parse(jsonString);
            TimePage.counter++;
            return jsonObject["time"].Value<string>();
        }

    }

Answer 3

或者你可以试试这个：

static async Task<string> RequestTimeAsync()
{
    using (var client = new HttpClient())
    {
        var jsonString = await client.GetStringAsync("http://date.jsontest.com/");
        var jsonObject = JObject.Parse(jsonString);
        return jsonObject["time"].Value<string>();
    }
}


protected override async void OnAppearing()
{
    base.OnAppearing();

    var Text = await RequestTimeAsync();

    Device.StartTimer(TimeSpan.FromSeconds(1), () => { 
        // i want the taks to be put 
        //here so that it gets repeated
        var jsonDateTsk = RequestTimeAsync();
        jsonDateTsk.Wait();
        var jsonTime = jsonDateTsk.Result;
        var number = float.Parse(Text) - 1;
        var btnText = $"{number}";
        return number > 0;
    });
}

Answer 4

一个简单的时钟演示一秒计时器动作：

       Device.StartTimer(TimeSpan.FromSeconds(1), doitt);
        bool doitt()
        {
            label1.Text = DateTime.Now.ToString("h:mm:ss");
            return true;
        }

在xamarin表单中通过计时器重复异步任务

4 个答案: