Why use optional binding?

问题

I am aware that this question is here, but it only partially answers my question and I cannot comment on the answer so I'm forced to post here.

What is the difference between optional binding and just simply using ?. My understanding is when you use ? to unwrap a variable, if it contains a nil value then the code in which it is used isn't run. (Please correct me if this is not the case.)

回答1:

You use optional binding (if let) if you have a block of code that you only want to run if the variable is not nil.

You use optional chaining (the ?) only when accessing the properties/methods of an optional variable.

But there are situations where optional chaining is not possible (i.e. you're not accessing a property/method of the optional variable, but rather using that variable for other purposes). For example

// let's assume `data` is a `NSData?` optional

if let imageData = data {
    let image = UIImage(data: imageData)

    // now do something with `image`
}

We do this because in this context, we can't use optional chaining, and using forced unwrapping (e.g. let image = UIImage(data: data!)) would crash if data was nil.

来源：https://stackoverflow.com/questions/33769366/why-use-optional-binding