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POHow to test a ViewModel that loads with a BackgroundWorker?
 

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  1. POHow to test a ViewModel that loads with a BackgroundWorker?
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    <p>One of the nice things about MVVM is the testability of the ViewModel. In my particular case, I have a VM that loads some data when a command is called, and its corresponding test:</p> <pre><code>public class MyViewModel { public DelegateCommand LoadDataCommand { get; set; } private List&lt;Data&gt; myData; public List&lt;Data&gt; MyData { get { return myData; } set { myData = value; RaisePropertyChanged(() =&gt; MyData); } } public MyViewModel() { LoadDataCommand = new DelegateCommand(OnLoadData); } private void OnLoadData() { // loads data over wcf or db or whatever. doesn't matter from where... MyData = wcfClient.LoadData(); } } [TestMethod] public void LoadDataTest() { var vm = new MyViewModel(); vm.LoadDataCommand.Execute(); Assert.IsNotNull(vm.MyData); } </code></pre> <p>So that is all pretty simple stuff. However, what I would really like to do is load the data using a <code>BackgroundWorker</code>, and have a 'loading' message displayed on screen. So I change the VM to:</p> <pre><code>private void OnLoadData() { IsBusy = true; // view is bound to IsBusy to show 'loading' message. var bg = new BackgroundWorker(); bg.DoWork += (sender, e) =&gt; { MyData = wcfClient.LoadData(); }; bg.RunWorkerCompleted += (sender, e) =&gt; { IsBusy = false; }; bg.RunWorkerAsync(); } </code></pre> <p>This works fine visually at runtime, however my test now fails because of the property not being loaded immediately. Can anyone suggest a good way to test this kind of loading? I suppose what I need is something like:</p> <pre><code>[TestMethod] public void LoadDataTest() { var vm = new MyViewModel(); vm.LoadDataCommand.Execute(); // wait a while and see if the data gets loaded. for(int i = 0; i &lt; 10; i++) { Thread.Sleep(100); if(vm.MyData != null) return; // success } Assert.Fail("Data not loaded in a reasonable time."); } </code></pre> <p>However that seems really clunky... It works, but just feels dirty. Any better suggestions?</p> <hr> <p><strong>Eventual Solution</strong>:</p> <p>Based on David Hall's answer, to mock a BackgroundWorker, I ended up doing this fairly simple wrapper around <code>BackgroundWorker</code> that defines two classes, one that loads data asynchronously, and one that loads synchronously.</p> <pre><code> public interface IWorker { void Run(DoWorkEventHandler doWork); void Run(DoWorkEventHandler doWork, RunWorkerCompletedEventHandler onComplete); } public class AsyncWorker : IWorker { public void Run(DoWorkEventHandler doWork) { Run(doWork, null); } public void Run(DoWorkEventHandler doWork, RunWorkerCompletedEventHandler onComplete) { var bg = new BackgroundWorker(); bg.DoWork += doWork; if(onComplete != null) bg.RunWorkerCompleted += onComplete; bg.RunWorkerAsync(); } } public class SyncWorker : IWorker { public void Run(DoWorkEventHandler doWork) { Run(doWork, null); } public void Run(DoWorkEventHandler doWork, RunWorkerCompletedEventHandler onComplete) { Exception error = null; var args = new DoWorkEventArgs(null); try { doWork(this, args); } catch (Exception ex) { error = ex; throw; } finally { onComplete(this, new RunWorkerCompletedEventArgs(args.Result, error, args.Cancel)); } } } </code></pre> <p>So then in my Unity configuration, I can use SyncWorker for testing, and AsyncWorker for production. My ViewModel then becomes:</p> <pre><code>public class MyViewModel(IWorker bgWorker) { public void OnLoadData() { IsBusy = true; bgWorker.Run( (sender, e) =&gt; { MyData = wcfClient.LoadData(); }, (sender, e) =&gt; { IsBusy = false; }); } } </code></pre> <p>Note that the thing i have marked as <code>wcfClient</code> is actually a Mock in my tests too, so after the call to <code>vm.LoadDataCommand.Execute()</code> I can also validate that <code>wcfClient.LoadData()</code> was called.</p>
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    1. USCodingWithSpike
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    1. COadding a CoROutine supports as in Caliburn could help too.
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      1. POHow to test a ViewModel that loads with a BackgroundWorker?
      1. USFelice Pollano
    2. CO"defines two classes, one that loads data asynchronously, and one that loads synchronously." Wow, that pattern sounds [like something I've seen before...](http://msdn.microsoft.com/en-us/library/system.threading.synchronizationcontext.aspx)
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      1. USWill
    3. CO@Will - Funny you mention that. After posting my edit, I found part of the Prism guide that talks about using the `SynchronizationContext` but the way it was written up only 1/2 made sense to me, and didn't really mention testing. I'll have to find a good example of using it and see if I can substitute that in instead.
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