双方括号在swift中意味着什么？

Question

the below is the sample code in swift.

var loadedMessages = [[Message]]()

Message is a custom class. i'm not sure what [[Message]] () is doing.

Answer 1

It is specifying that your variable loadedMessages is an array of arrays that contains Message objects. A JSON representation of loadedMessages might look like:

loadedMessages: [
  [ <Message>, <Message>, <Message> ],
  [ <Message>, <Message>, <Message> ]
]

A quick Playground implementation of something similar can give you a pretty good introspection of the situation:

var foo = [[String]]()
foo.append(["bar"])
foo[0][0] // reveals "bar"

Answer 2

It means it's an array of arrays of messages. Think of it in terms of whatever appears between the square brackets being an array of that, and this can include another array.

Alternatively, if you were to write out without the “shorthand” array syntax, it would be Array<Array<Messages>>() .