webview navigation event is firing all the time in ios

I have simple xamarin.Forms application, it contains just a webview and ActivityNavigator, I am doing the following:
When "Navigation" event is fire, I make the indicator visible and running, and hide the webview, then when the "Navigated" event is fired I make the indicator unVisible and I show the webview.
This behavior is prefect on android devices, but on ios the "navigation" event fires frequently, and the "navigated" event fires only once and never fire again.

any ideas?