HttpClient 使用 Async/Await 操作获取列表

Question

我正在尝试从我之前编写的 web api 中获取列表。然后我将在 Xamarin.Forms ListView 中使用该列表。我的代码在这里：

public static class DataSource
{

    public static async Task<Restoran[]> GetRestoransAsync()
    {
        // ... Use HttpClient.
        using (HttpClient client = new HttpClient())
        using (HttpResponseMessage response = await client.GetAsync(page))
        using (HttpContent content = response.Content)
        {
            // ... Read the string.
            string result = await content.ReadAsStringAsync();

            var restorans = JsonConvert.DeserializeObject<Restoran[]>(result);
            return restorans;
        }

    }

}

我的内容页面：

public class MenuPage : ContentPage
{
    ListView listView;
    List<Restoran> restorans = new List<Restoran>();
    async Task LoadRestorans()
    {
        restorans = (await DataSource.GetRestoransAsync()).ToList();
    }

    public MenuPage(string masa)
    {
        var loadData = LoadRestorans();
        loadData.Wait();
        listView = new ListView(ListViewCachingStrategy.RecycleElement)
        {
        ItemsSource = restorans,
            ItemTemplate = new DataTemplate(() => {
                var nativeCell = new CustomCell();
                return nativeCell;
            })
        };
    }
}

但是当我调试这段代码时，“LoadRestorans()”方法在“restorans”列表初始化之前被调用。我想我不明白异步方法的心态。我该怎么办？

Answer 1

你有两个选择。

使用异步工厂方法创建页面

public class MenuPage : ContentPage {
    ListView listView;
    List<Restoran> restorans = new List<Restoran>();

    private async Task LoadRestoransAsync() {
        restorans = (await DataSource.GetRestoransAsync()).ToList();
        listView = new ListView(ListViewCachingStrategy.RecycleElement) {
            ItemsSource = restorans,
            ItemTemplate = new DataTemplate(() => {
                var nativeCell = new CustomCell();
                return nativeCell;
            })
        };
    }

    public MenuPage(string masa) {
        //...        
    }

    public static async Task<MenuPage> CreateMenuPageAsync(string masa) {
        var page = new MenuPage(masa);
        await page.LoadRestoransAsync();
        return pagel
    }
}

然后在其他异步事件处理程序中像这样使用它

var page = await MenuPage.CreateMenuPageAsync("<masa here>");

或

在OnAppearing 事件中进行。

订阅页面/视图的Appearing事件

protected override void OnAppearing() {
    this.Appearing += Page_Appearing;
}

并在实际的偶数处理程序上调用您的异步代码

private async void Page_Appearing(object sender, EventArgs e) {
    //...call async code here

    //unsubscribing from the event
    this.Appearing -= Page_Appearing;
}

整个课程看起来像这样

public class MenuPage : ContentPage {
    ListView listView;
    List<Restoran> restorans = new List<Restoran>();

    private async Task LoadRestoransAsync() {
        restorans = (await DataSource.GetRestoransAsync()).ToList();
        listView = new ListView(ListViewCachingStrategy.RecycleElement) {
            ItemsSource = restorans,
            ItemTemplate = new DataTemplate(() => {
                var nativeCell = new CustomCell();
                return nativeCell;
            })
        };
    }

    public MenuPage(string masa) {
        //...        
    }

    protected override void OnAppearing() {
        this.Appearing += Page_Appearing;
    }

    private async void Page_Appearing(object sender, EventArgs e) {
        //...call async code here
        await LoadRestoransAsync();

        //unsubscribing from the event
        this.Appearing -= Page_Appearing;
    }

}

之前发生的情况是，通过调用 .Wait()（这是一个阻塞调用），该类混合了异步调用和阻塞调用（如 .Result 和 .Wait()），这可能导致死锁。因此，为什么您在尝试测试时无法通过该方法。

Answer 2

在restorans之前调用，因为构造函数MenuPage在属性初始化之前被调用。

建议

public class MenuPage : ContentPage
{
    ListView listView;
    List<Restoran> restorans;
    async Task<Restoran[]> LoadRestorans()
    {
        return await DataSource.GetRestoransAsync();
    }

    public MenuPage(string masa)
    {
        this.restorans = LoadRestorans().GetAwaiter().GetResult().ToList();
        listView = new ListView(ListViewCachingStrategy.RecycleElement)
        {
        ItemsSource = this.restorans,
            ItemTemplate = new DataTemplate(() => {
                var nativeCell = new CustomCell();
                return nativeCell;
            })
        };
    }
}

注意：我没有编译这段代码，但根据我的理解，这应该是正确的代码。