wpf 如何在视图模型中调用异步方法
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How to call an async method in viewmodel
提问by r9s
I'm trying to learn MVVM pattern in WPF application. I wrote this async method in my viewmodel (it has to be async since I'm using HttpClient and it's methods are async):
我正在尝试在 WPF 应用程序中学习 MVVM 模式。我在我的视图模型中编写了这个异步方法（它必须是异步的，因为我使用的是 HttpClient 并且它的方法是异步的）：
public async Task<Dictionary<int, BusStop>> GetBusStops()
{
var busStopDict = new Dictionary<int, BusStop>();
var url = "my url";
using (HttpClient client = new HttpClient())
using (HttpResponseMessage response = await client.GetAsync(url))
using (HttpContent content = response.Content)
{
string data = await content.ReadAsStringAsync();
var regularExpression = Regex.Match(data, "\[(.)*\]");
var result = regularExpression.Groups[0];
var json = JValue.Parse(result.ToString());
var jsonArray = json.ToArray();
foreach (var a in jsonArray)
{
// irrelevant logic
busStopDict.Add(nr, bs);
}
}
return busStopDict;
}
This methods returns a dictionary filled with bus stops (my model). I would like to bind this dictionary with combobox in view, but I can't get it work, because I can't call this async method in constructor of my viewmodel and I have no idea where can I call it. Do you have any suggestions?
这个方法返回一个充满公交车站的字典（我的模型）。我想在视图中将这个字典与组合框绑定，但我无法让它工作，因为我无法在我的视图模型的构造函数中调用这个异步方法，我不知道我可以在哪里调用它。你有什么建议吗？
回答by NtFreX
I would not recommend to write logic in your viewmodel constructor. Instead I would create an Loaded event trigger in your view also to ensure you do not interfere with the loading procedure of the view.
我不建议在您的视图模型构造函数中编写逻辑。相反，我会在您的视图中创建一个 Loaded 事件触发器，以确保您不会干扰视图的加载过程。
xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
<i:Interaction.Triggers>
<i:EventTrigger EventName="Loaded">
<i:InvokeCommandAction Command="{Binding LoadedCommand}" />
</i:EventTrigger>
</i:Interaction.Triggers>
Then in your viewmodel I recomend doing the following:
然后在您的视图模型中，我建议执行以下操作：
Add the following property for your Loaded event
为您的 Loaded 事件添加以下属性
public DelegateCommand LoadedCommand { get; }
Then assign it in your constructor
然后在您的构造函数中分配它
LoadedCommand = new DelegateCommand(async () => await ExecuteLoadedCommandAsync());
Add the loaded method and call your method within it
添加加载的方法并在其中调用您的方法
private async Task ExecuteLoadedCommandAsync()
{
var busStops = await GetBusStops();
//TODO: display the busStops or do something else
}
Furthermore is adding "Async" as suffix to your asynchron methods names a good naming pattern. It enables you to quickly see which methods are asynchron. (so rename "GetBusStops" to "GetBusStopsAsync")
此外，将“Async”作为后缀添加到您的异步方法名称是一个很好的命名模式。它使您能够快速查看哪些方法是异步的。（因此将“GetBusStops”重命名为“GetBusStopsAsync”）
This is a simple DelegateCommandimplementation
这是一个简单的DelegateCommand实现
public class DelegateCommand : ICommand
{
private readonly Predicate<object> _canExecute;
private readonly Action<object> _execute;
public event EventHandler CanExecuteChanged;
public DelegateCommand(Action<object> execute)
: this(execute, null)
{
}
public DelegateCommand(Action<object> execute,
Predicate<object> canExecute)
{
_execute = execute;
_canExecute = canExecute;
}
public override bool CanExecute(object parameter)
{
if (_canExecute == null)
{
return true;
}
return _canExecute(parameter);
}
public override void Execute(object parameter)
{
_execute(parameter);
}
public void RaiseCanExecuteChanged()
{
if( CanExecuteChanged != null )
{
CanExecuteChanged(this, EventArgs.Empty);
}
}
}
When using this implementation you need to change your initialising of the DelegateCommandin your viewmodel constructor to the following
使用此实现时，您需要将DelegateCommand视图模型构造函数中的初始化更改为以下内容
LoadedCommand = new DelegateCommand(async (param) => await ExecuteLoadedCommandAsync());
回答by Stephen Cleary
You should use asynchronous data binding(I have an entire article on the subject).
您应该使用异步数据绑定（我有一篇关于该主题的整篇文章）。
Using NotifyTaskfrom my Mvvm.Async library, it could look like this:
使用NotifyTask从我Mvvm.Async库，它可能是这样的：
public async Task<Dictionary<int, BusStop>> GetBusStopsAsync() { ... }
public NotifyTask<Dictionary<int, BusStop>> BusStops { get; }
MyViewModelConstructor()
{
BusStops = NotifyTask.Create(() => GetBusStopsAsync());
}
Then your view can model-bind to BusStops.Resultto get the dictionary (or null if it isn't retrieved yet), and also data-bind to BusStops.IsNotCompleted/BusStops.IsFaultedfor busy spinners / error indicators.
然后您的视图可以模型绑定到BusStops.Result以获取字典（如果尚未检索，则为 null），并且还可以将数据绑定到BusStops.IsNotCompleted/BusStops.IsFaulted用于繁忙的微调器/错误指示器。
回答by Fruchtzwerg
Start your async method in the constructor and define an action to be continued with like.
在构造函数中启动异步方法并定义要继续的操作。
//Constructor
public ViewModel()
{
GetBusStops().ContinueWith((BusStops) =>
{
//This anonym method is called async after you got the BusStops
//Do what ever you need with the BusStops
});
}
Don′t forget to invoke the UI thread if you want to access an property used for the View with
如果您想访问用于 View 的属性，请不要忘记调用 UI 线程
Application.Current.Dispatcher.BeginInvoke(() =>
{
//Your code here
});
回答by Kevin B Burns
I would check out AsycLazyor check out AsyncCommandsand create a async Task based "LoadCommand". You shouldn't put much logic into a contructor as it will make it tougher to debug, forces you to strongly couple and will make it very difficult to write Unit tests for your view Model. I tend to make everything lazy if I can.
我会查看AsycLazy或查看AsyncCommands并创建一个基于“LoadCommand”的异步任务。您不应该在构造函数中放入太多逻辑，因为它会使调试变得更加困难，迫使您进行强耦合，并使为您的视图模型编写单元测试变得非常困难。如果可以的话，我倾向于让一切变得懒惰。
AsyncLazy
http://blog.stephencleary.com/2012/08/asynchronous-lazy-initialization.html
AsyncLazy
http://blog.stephencleary.com/2012/08/asynchronous-lazy-initialization.html
AsyncCommand
http://mike-ward.net/2013/08/09/asynccommand-implementation-in-wpf/
AsyncCommand
http://mike-ward.net/2013/08/09/asynccommand-implementation-in-wpf/
