Swift First Glimpse----grammar advanced

Default binding (Optional binding self-empty)

By following the type variable, plus? To enable the default binding to nil

var Window:uiwindow?

That is, if the window assignment is incorrect, the value of window is nil.

This is different from the principle of objective-c in Arc mode, which has niltype in swift and the inverse operation is value (unwrapping optionals).

Value (unwrapping optionals)

can use! Take out the non-null value in the window variable (which is already the default binding), and be sure to take a value for the non-null type, otherwise it will report an execution error.

Default Chain

Using unowned references to resolve circular references

Swift First Glimpse----grammar advanced