c# - Windows 10 Universal App:PropertyChanged Event is always null ListView -

i have windows 10 universal app listview in xaml , observablecollection<contact>which bound listview via itemssource="{x:bind messenger.contacts}". want listview update text of specific bound (also via x:bind) property of listview element if property changed. contactclass implements inotifypropertychanged, public , in every property setter fire notifypropertychanged correct property name.

 public class contact : inotifypropertychanged     {         public event propertychangedeventhandler propertychanged;          protected string name;          public string name         {                          {                 return this.name;             }             set             {                 this.name= value;                 notifypropertychanged("name");             }         } protected virtual void notifypropertychanged(string propertyname)         {             propertychangedeventhandler handler = this.propertychanged;              if (handler != null)             {                 handler(this, new propertychangedeventargs(propertyname));             }         }  } 

but listview never updated , propertychanged event null. in other questions have read, listview automatically subscribe. why event null? how can force update of specific listview item?

the problem related x:bind text property of textblock. default binding mode of x:bind onetime. need change oneway desired behavior. can read more change in blog: http://iwindroid.me/binding-and-performance/