In the simulator, everything is ok. However, in live preview mode of canvas, no keyboard pops up when I tap in the textfield. Did I miss anything? Or need more codes in preview?Thanks in advance.

In swift language guide-"Opaque Types"-"Differences Between Opaque Types and Protocol Types", it said: "Another problem with this approach is that the shape transformations don’t nest. The result of flipping a triangle is a value of type Shape, and the protoFlip(:) function takes an argument of some type that conforms to the Shape protocol. However, a value of a protocol type doesn’t conform to that protocol; the value returned by protoFlip(:) doesn’t conform to Shape. This means code like protoFlip(protoFlip(smallTriange)) that applies multiple transformations is invalid because the flipped shape isn’t a valid argument to protoFlip(:)" I can't understand why it said"a value of a protocol type doesn’t conform to that protocol; the value returned by protoFlip(:) doesn’t conform to Shape". The return value must conform to Shape. It's a fundamental requirement by function signature, isn't it?