$0 是传递给闭包的第一个参数.$1 是第二个参数，等等.你展示的那个闭包是以下的简写:

$0 is the first parameter passed into the closure. $1 is the second parameter, etc. That closure you showed is shorthand for:

let sortedNumbers = numbers.sort { (firstObject, secondObject) in 
    return firstObject > secondObject
}