xamarin.forms xaml can't generate event handler

In PCL, if you type OnClick vs2017's intellisense will tell you to generate Event Handler.
But, when i use .net standard 2.0 library project, after installed Xamarin.Forms nuget package, i type OnClick there is nothing, i have to first manual type Event Name and then go into cs file manual create Event Handler.(Visual studio Version 15.5.4)