Trying to be MVVM compliant is not always easy.<\/p>\n
Take the following scenario:<\/p>\n
There is a model with a property “Status”. This property changes values according to some logic in the model. This property should now be presented in the view. To have correct MVVM, the ViewModel has also a property “Status”, which is bound to the view.
\nSo, now how can the model inform the viewmodel when the “Status” of the model changed?<\/p>\n
The best bet is to have a StatusChanged event on the model and let the viewmodel subscribe to it and fire the viewmodel’s own propertychanged for it’s “Status” property. This can get quite tedious so I created a small helper: PropertyChangedProxy<\/p>\n
For this, the model and the viewmodel need to implement INotifyPropertyChanged. Then on the viewmodel, you can create new a new instance of the PropertyChangedProxy for each property of the model where you want the viewmodel to be notified, in a typesafe way.<\/p>\n
Such an instance would look like this:<\/p>\n
var statusPropertyChangedProxy = new PropertyChangedProxy<MainModel, StatusType>(\r\n\t_model, m => m.Status,\r\n\tnewValue => OnPropertyChanged(\"Status\"));<\/pre>\nSo as you see, you just need to give the source (which is the model), the property of the source (as a nice expression) and an action, that should be called when the source’s property changes (which in this case is just to fire a OnPropertyChanged on the ViewModel itself).<\/p>\n
Here’s a complete small example of a model and viewmodel, where the viewmodel is automatically notified when the model’s “Status” changes so that the viewmodel can forward this to the view:<\/p>\n
public class MyModel : INotifyPropertyChanged\r\n{\r\n\tprivate string _status;\r\n\tpublic string Status\r\n\t{\r\n\t\tget { return _status; }\r\n\t\tset { _status = value; OnPropertyChanged(); }\r\n\t}\r\n\r\n\t\/\/ Default INotifyPropertyChanged\r\n\tpublic event PropertyChangedEventHandler PropertyChanged;\r\n\tprotected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)\r\n\t{\r\n\t\tvar handler = PropertyChanged;\r\n\t\tif (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));\r\n\t}\r\n}\r\n\r\npublic class MyViewModel : INotifyPropertyChanged\r\n{\r\n\tpublic string Status\r\n\t{\r\n\t\tget { return _model.Status; }\r\n\t}\r\n\r\n\tprivate PropertyChangedProxy<MyModel, string> _statusPropertyChangedProxy;\r\n\tprivate MyModel _model;\r\n\tpublic MyViewModel(MyModel model)\r\n\t{\r\n\t\t_model = model;\r\n\t\t_statusPropertyChangedProxy = new PropertyChangedProxy<MyModel, string>(\r\n\t\t\t_model, myModel => myModel.Status, s => OnPropertyChanged(\"Status\")\r\n\t\t);\r\n\t}\r\n\r\n\t\/\/ Default INotifyPropertyChanged\r\n\tpublic event PropertyChangedEventHandler PropertyChanged;\r\n\tprotected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)\r\n\t{\r\n\t\tvar handler = PropertyChanged;\r\n\t\tif (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));\r\n\t}\r\n}<\/pre>\nHere’s the source of the PropertyChangedProxy:<\/p>\n
\/\/\/ <summary>\r\n\/\/\/ Proxy class to easily take actions when a specific property in the \"source\" changed\r\n\/\/\/ <\/summary>\r\n\/\/\/ Last updated: 20.01.2015\r\n\/\/\/ <typeparam name=\"TSource\">Type of the source<\/typeparam>\r\n\/\/\/ <typeparam name=\"TPropType\">Type of the property<\/typeparam>\r\npublic class PropertyChangedProxy<TSource, TPropType> where TSource : INotifyPropertyChanged\r\n{\r\n\tprivate readonly Func<TSource, TPropType> _getValueFunc;\r\n\tprivate readonly TSource _source;\r\n\tprivate readonly Action<TPropType> _onPropertyChanged;\r\n\tprivate readonly string _modelPropertyname;\r\n\r\n\t\/\/\/ <summary>\r\n\t\/\/\/ Constructor for a property changed proxy\r\n\t\/\/\/ <\/summary>\r\n\t\/\/\/ <param name=\"source\">The source object to listen for property changes<\/param>\r\n\t\/\/\/ <param name=\"selectorExpression\">Expression to the property of the source<\/param>\r\n\t\/\/\/ <param name=\"onPropertyChanged\">Action to take when a property changed was fired<\/param>\r\n\tpublic PropertyChangedProxy(TSource source, Expression<Func<TSource, TPropType>> selectorExpression, Action<TPropType> onPropertyChanged)\r\n\t{\r\n\t\t_source = source;\r\n\t\t_onPropertyChanged = onPropertyChanged;\r\n\t\t\/\/ Property \"getter\" to get the value\r\n\t\t_getValueFunc = selectorExpression.Compile();\r\n\t\t\/\/ Name of the property\r\n\t\tvar body = (MemberExpression)selectorExpression.Body;\r\n\t\t_modelPropertyname = body.Member.Name;\r\n\t\t\/\/ Changed event\r\n\t\t_source.PropertyChanged += SourcePropertyChanged;\r\n\t}\r\n\r\n\tprivate void SourcePropertyChanged(object sender, PropertyChangedEventArgs e)\r\n\t{\r\n\t\tif (e.PropertyName == _modelPropertyname)\r\n\t\t{\r\n\t\t\t_onPropertyChanged(_getValueFunc(_source));\r\n\t\t}\r\n\t}\r\n}<\/pre>\n","protected":false},"excerpt":{"rendered":"Trying to be MVVM compliant is not always easy. Take the following scenario: There is a model with a property “Status”. This property changes values according to some logic in the model. This property should now be presented in the view. To have correct MVVM, the ViewModel has also a property “Status”, which is bound … <\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[4,3],"tags":[],"class_list":{"0":"entry","1":"post","2":"publish","3":"author-roemer","4":"post-402","6":"format-standard","7":"category-csharp","8":"category-programming"},"acf":[],"views":5273,"_links":{"self":[{"href":"http:\/\/dev.flauschig.ch\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/402"}],"collection":[{"href":"http:\/\/dev.flauschig.ch\/wordpress\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/dev.flauschig.ch\/wordpress\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/dev.flauschig.ch\/wordpress\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/dev.flauschig.ch\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=402"}],"version-history":[{"count":0,"href":"http:\/\/dev.flauschig.ch\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/402\/revisions"}],"wp:attachment":[{"href":"http:\/\/dev.flauschig.ch\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=402"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/dev.flauschig.ch\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=402"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/dev.flauschig.ch\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=402"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}