Request main thread Refresh UI in iOS

Only the main thread in iOS can immediately refresh the UI. If you are listening for asynchronous messages, triggering a callback function, or calling an asynchronous method, requesting that the UI be refreshed creates a problem with thread blocking and latency.

Can be   [OBJC] view plain copy if  ([Nsthread ismainthread])    {        [self.downloadmapbtn setimage:[uiimage imagenamed:@ "Download_map.png"]  forState:UIControlStateNormal];       [self.downloadMapBtn  setneedsdisplay];  }   else   {       dispatch_sync ( Dispatch_get_main_queue (), ^{           //update ui  in UI thread here           [ self.downloadmapbtn setimage:[uiimage imagenamed:@ "Download_map.png"] forState: uicontrolstatenormal];           [self.downloadMapBtn  setneedsdisplay];             });  }  
This method is used to send messages to the main thread and to refresh.

Related reading

Http://www.ios-developer.net/iphone-ipad-programmer/development/threads/updating-ui-controls-on-background-threads

Http://www.raywenderlich.com/4295/multithreading-and-grand-central-dispatch-on-ios-for-beginners-tutorial