{"id":33,"date":"2010-03-31T20:27:00","date_gmt":"2010-03-31T20:27:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/davidrickard\/2010\/03\/31\/using-the-dispatcher-with-mvvm\/"},"modified":"2010-03-31T20:27:00","modified_gmt":"2010-03-31T20:27:00","slug":"using-the-dispatcher-with-mvvm","status":"publish","type":"post","link":"https:\/\/engy.us\/blog\/2010\/03\/31\/using-the-dispatcher-with-mvvm\/","title":{"rendered":"Using the Dispatcher with MVVM"},"content":{"rendered":"

When writing an MVVM application, you want to separate from the UI. However you also need to make sure that UI updates happen on the UI thread. Changes made through INotifyPropertyChanged get automatically marshaled to the UI thread, so in most cases you’ll be fine. However, when using INotifyCollectionChanged (such as with an ObservableCollection), these changes are not<\/em> marshaled to the UI thread.<\/p>\n

This means that unless you modify your collection on the UI thread, you’ll get a cross threading error. Thus the problem. We’re in the ViewModel and we don’t have access to the Dispatcher. That’s on a View object. How do we update the collection on the UI thread?<\/p>\n

I came up with a small static class to help with this:<\/p>\n

<\/span><\/pre>\n
public<\/span> static<\/span> class<\/span> DispatchService<\/span>\n{\n    public<\/span> static<\/span> void<\/span> Invoke(Action<\/span> action)\n    {\n        Dispatcher<\/span> dispatchObject = Application<\/span>.Current.Dispatcher;\n        if<\/span> (dispatchObject == null<\/span> || dispatchObject.CheckAccess())\n\t{\n            action();\n        }\n        else<\/span>\n        {\n            dispatchObject.Invoke(action);\n        }\n    }\n}<\/pre>\n
When your ViewModel gives the DispatchService actions, it can run them on the correct thread. Which is, in the case of unit tests, always the current thread. A call to the DispatchService may look like this:<\/p>\n
DispatchService<\/span>.Invoke(() =>\n{\n    this<\/span>.MyCollection.Add(\"new value\"<\/span>);\n});<\/pre>\n
The use of a simple lambda statement makes it a relatively lightweight way of updating on the correct thread.<\/p>\n","protected":false},"excerpt":{"rendered":"
When writing an MVVM application, you want to separate from the UI. However you also need to make sure that UI updates happen on the UI thread. Changes made through INotifyPropertyChanged get automatically marshaled to the UI thread, so in most cases you’ll be fine. However, when using INotifyCollectionChanged (such as with an ObservableCollection), these … <\/p>\n
Continue reading “Using the Dispatcher with MVVM”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"spay_email":"","jetpack_publicize_message":"","jetpack_is_tweetstorm":false},"categories":[1],"tags":[],"jetpack_featured_media_url":"","jetpack_publicize_connections":[],"jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/pahBcK-x","jetpack-related-posts":[{"id":79,"url":"https:\/\/engy.us\/blog\/2018\/10\/03\/wpf-databinding-using-dynamicdata\/","url_meta":{"origin":33,"position":0},"title":"WPF Databinding using DynamicData","date":"October 3, 2018","format":false,"excerpt":"My project is MVVM and I had been using ReactiveList as my go-to observable collection for viewmodel properties. But ReactiveUI deprecated ReactiveList in 8.6.1. So I needed to get on to the new recommended library: DynamicData. But there is no direct drop-in replacement you can do like ObservableCollection <-> ReactiveList.\u2026","rel":"","context":"With 2 comments","img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":120,"url":"https:\/\/engy.us\/blog\/2020\/01\/01\/implementing-a-custom-window-title-bar-in-wpf\/","url_meta":{"origin":33,"position":1},"title":"Implementing a Custom Window Title Bar in WPF","date":"January 1, 2020","format":false,"excerpt":"There are several good reasons for wanting custom window chrome in WPF, such as fitting in additional UI or implementing a Dark theme. 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