let tmp = [8,5,6,4,9,7,1]

1.

func backwards(val:Int,val2:Int) -> Bool {
    return val > val2
}
let reversed = tmp.sorted(by: backwards)

2.参数名称缩写

let reversed = tmp.sorted(by: {$0 > $1})

3.

let reversed = tmp.sorted(by: {(val, val2) -> Bool in
    return val > val2
})

4.

let backwards = {(val:Int, val2:Int) -> Bool in
    return val > val2;
}
let reversed = tmp.sorted(by: backwards)

5.运算符函数

let reversed = tmp.sorted(by: >)

6.尾随闭包（2的尾随闭包写法）

let reversed = tmp.sorted() {$0 > $1}

7.如果函数只需要闭包表达式一个参数，当您使用尾随闭包时，您甚至可以把()省略掉

let reversed = tmp.sorted {$0 > $1}

8. 3的尾随闭包写法

let reversed = tmp.sorted() {(val, val2) -> Bool in
    return val > val2
}

9. 3的尾随闭包的缩略写法

let reversed = tmp.sorted {(val, val2) -> Bool in
    return val > val2
}

结果：

print(reversed)

[9, 8, 7, 6, 5, 4, 1]

PS:
1 和 4 的写法不适用于尾随闭包