How to Understand the '!' in the End of IBOutlet Properties in Swift

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Let us see the follow code:

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@IBOutlet var mapView: MKMapView!

As we all know, the mapView is a variable from Interface Builder, but why there is a ! behind the property, let’s analyze it.

Firstly, Let’s ask what is the meaning of ! behind of variable?

It means the variable is an optional type, further, it is an implicityly unwrapped optional which we don’t need unwrap and use directly.

Then, why we need an optional type here?

Because of a rule. We know the initializer function in Swift must initialize all of properties which are not optional type and Swift doesn’t “know” that Interface Builder is supplying the views at run time.