在WPF应用程序中正确实现任务

Question

I'm looking for some advice on getting to grips with Task s in WPF and was wondering if anyone could have a look over my code and point out what I'm doing incorrectly?

Basically the application takes a postcode from the UI which is used to instantiate a Task service which will get Longitude/Latitude which can be accessed via the instance for use in another service or the UI itself.

I think I may have a race condition which I'm looking to correct as when ResultTextBlock.Text is set it's a zero, but stepping through instantiation I see those values set.

Any advice on Task implementation and wiring would be greatly appreciated.

Service Code

class PostcodeService
{
    string _result;
    string _postcode;

    HttpResponseMessage _response;        
    RootObject rootObject;

    public double Latitude { get; private set; }
    public double Longitude { get; private set; }        

    public PostcodeService(string postcode)
    {
        this._postcode = postcode;
        rootObject = new RootObject();
    }

    public async Task<string> GetLongLatAsync()
    {        
        using (HttpClient client = new HttpClient())
        {
            client.BaseAddress = new Uri("https://api.postcodes.io/postcodes/" + this._postcode);
            client.DefaultRequestHeaders.Accept.Clear();
            client.DefaultRequestHeaders.Accept.Add(
                new MediaTypeWithQualityHeaderValue("application/json"));  

            try
            {
                _response = await client.GetAsync(client.BaseAddress);
                if(_response.IsSuccessStatusCode)
                {
                    //cast result into model and then set long/lat properties which can then be used in the UI
                    _result = await _response.Content.ReadAsStringAsync();                     

                    rootObject = JsonConvert.DeserializeObject<RootObject>(_result);
                    Longitude = Double.Parse(rootObject.result.longitude.ToString());
                    Latitude =  Double.Parse(rootObject.result.latitude.ToString());
                }                                      
            }
            catch(Exception ex)
            {
                ex.ToString();
            }
        }

        TaskCompletionSource<string> tc = new TaskCompletionSource<string>(_result);

        return tc.ToString();
    }
}

UI Code

private void PostcodeButton_Click(object sender, RoutedEventArgs e)
{
    _clearStatus();

    if (_validatePostcode())
    {
        Task T1 = Task.Factory.StartNew(async () =>
        {
            // get long lat from api
            _postcode = new PostcodeService(PostcodeTextBox.Text);
            await _postcode.GetLongLatAsync();
        });

        //Race condition?
        ResultTextBlock.Text = _postcode.Latitude.ToString();
    }
}

Answer 1

Event handlers allow async void to be used

Reference Async/Await - Best Practices in Asynchronous Programming

private async void PostcodeButton_Click(object sender, RoutedEventArgs e) {
    _clearStatus();

    if (_validatePostcode()) {
        // get long lat from api
        _postcode = new PostcodeService(PostcodeTextBox.Text);
        await _postcode.GetLongLatAsync(); //Offload UI thread
        //Back on UI thread
        ResultTextBlock.Text = _postcode.Latitude.ToString();
    }
}

Secondly to avoid thread exhaustion, create a single HttpClient instead of creating and disposing it when you need to perform this action

Reference You're using HttpClient wrong

The design of the service should be refactored into separate concerns

public class Location {
    public Location(double lat, double lon) {
        Latitude = lat;
        Longitude = lon;
    }
    public double Latitude { get; private set; }
    public double Longitude { get; private set; }    
}

public class PostcodeService {
    private static Lazy<HttpClient> client;
    static PostcodeService() {
        client = new Lazy<HttpClient>(() => {
            HttpClient httpClient = new HttpClient();
            httpClient.BaseAddress = new Uri("https://api.postcodes.io/postcodes/");
            httpClient.DefaultRequestHeaders.Accept.Clear();
            httpClient.DefaultRequestHeaders.Accept.Add(
                new MediaTypeWithQualityHeaderValue("application/json"));
            return httpClient;
        });
    }

    public async Task<Location> GetLongLatAsync(string postcode) {}
        var response = await client.Value.GetAsync(postcode);
        if(response.IsSuccessStatusCode) {
            //cast result into model and then set long/lat properties which can then be used in the UI
            var rootObject = await response.Content.ReadAsAsync<RootObject>();
            var Longitude = Double.Parse(rootObject.result.longitude.ToString());
            var Latitude =  Double.Parse(rootObject.result.latitude.ToString());
            var result = new Location(Latitude, Longitude);
            return result;
        }
        return null;
    }
}

The event handler can now be refactored to

private async void PostcodeButton_Click(object sender, RoutedEventArgs e) {
    _clearStatus();

    if (_validatePostcode()) {
        // get long lat from api
        var service = new PostcodeService();
        var location = await service.GetLongLatAsync(PostcodeTextBox.Text); //Offload UI thread
        //Back on UI thread
        if(location != null) {

            ResultTextBlock.Text = location.Latitude.ToString();
        } else {
            //Some message here
        }

    }
}

Answer 2

Your GetLongLatAsync() method should return a string :

public async Task<string> GetLongLatAsync()
{
    using (HttpClient client = new HttpClient())
    {
        client.BaseAddress = new Uri("https://api.postcodes.io/postcodes/" + this._postcode);
        client.DefaultRequestHeaders.Accept.Clear();
        client.DefaultRequestHeaders.Accept.Add(
            new MediaTypeWithQualityHeaderValue("application/json"));

        _response = await client.GetAsync(client.BaseAddress);
        string result = null;
        if (_response.IsSuccessStatusCode)
        {
            //cast result into model and then set long/lat properties which can then be used in the UI
            result = await _response.Content.ReadAsStringAsync();

            rootObject = JsonConvert.DeserializeObject<RootObject>(_result);
            Longitude = Double.Parse(rootObject.result.longitude.ToString());
            Latitude = Double.Parse(rootObject.result.latitude.ToString());
        }
        return result;
    }
}

...and you should simply await GetLongLatAsync() in the UI:

private async void PostcodeButton_Click(object sender, RoutedEventArgs e)
{
    _clearStatus();
    if (_validatePostcode())
    {
        // get long lat from api
        _postcode = new PostcodeService(PostcodeTextBox.Text);
        string result = await _postcode.GetLongLatAsync();
        ResultTextBlock.Text = result.ToString();
    }
}

You don't need to use a TaskCompletionSource nor start a new Task to call an async method.