Use let to make a constant and var to make a variable. The value of a constant doesn’t need to be known at compile time,but you must assign it a value exactly once. This means you can use constants to name a value that you determine once but use in many places.

让我们考虑一下课程：

class A {
    var a = [String]()
}

由于数组a是可变的,因此它是通过var定义的.但是如果我们考虑B类,A的实例是属性呢？

class B {
    let b = A()
}

即使b是可变的,让关键字也可以,因为引用不会被改变.另一方面,var也可以,因为b的内容可以改变.我应该在这个例子中选择什么 – let或var？