How to get the width and height of a frame view for xamarin forms for iOS?

I have implemented a gradient background color for frame and grid view.Its perfectly working fine in android but in iOS i have to mention the width and height static.
mainView = new UIView(new CGRect(0, 0, viewWidth, viewHeight));

i need to get the appended frame height and width but not the default screen size UIScreen.MainScreen.Bounds.Width.