Swift: unwrapping cause swift compile slowly

6
votes

In func tableView(tableView: UITableView!, cellForRowAtIndexPath indexPath: NSIndexPath!) -> UITableViewCell!{

this line of code makes swift compile extremely slow:

cell!.detailTextLabel.text = child.year! + " " + child.make! + " " + child.model!

It takes me 1 min and 44 secs to build the project if I have this line of code. And 99% of the time it stuck at "Compiling Swift source files". If I change this line to

cell!.detailTextLabel.text = " "//child.year! + " " + child.make! + " " + child.model!

It only take me 5 or 6 sec to build the project. I would like to know why this line of code will cause so much time compiling.

In my Child model, they are declared as :

var name:String?
var year:String?
var make:String?
var model:String?

and the init:

init(name:String!, ... ,year:String!, make:String!, model:String!, ...){
        self.name = name
       ...
        self.year = year
        self.make = make
        self.model = model
}

The part I construct a child:

Child(name:cName,...,year:cYear,make:cMake, model:cModel,...)
2
iosuitableviewswift
You should not ask two questions in one. The slow compilation issue is a question. When unwrapping is necessary is a different question! If you ask the second question separately I'll try to answer it separately. – matt
@matt Thanks, see my second question here.stackoverflow.com/questions/24844495/… – fuiiii

2 Answers

4
votes

Yes, I filed a bug report (17585851) on this slow compilation issue, and you should do the same; the more clear use cases Apple is sent, the better. My slow code was several occurrences of this form:

let title = obj.valueForProperty(MPMediaItemPropertyTitle) as? String
self.titles += title ? title! : ""

(which, as you can see, is doing nil testing / unwrapping). It was cumbersome but not difficult for me to work around the problem by doing the same thing in a different way, and you should do likewise. But file that bug report first!

0
votes

You need to do unwrapping when your types are declared as optionals. In swift an optional type is a type that may or may not have a value. For example i could declare a string as:

var collegeName : String?

The "?" at the end of type declaration shows that the collegeName may or may not have a value and in order to get that value you will have to unwrap it in order to get its value by using ! operator.

As far as i can tell in your case the name field is declared as a string while year , make and model are declared as optional strings that is why you need to unwrap those fields to get the value out of them.

You can also declared a type using "!" operator like:

var collegeName : String!

This means that this is an optional string but it is automatically unwrapped for you so you don't need to use the "!" afterwards to unwrap this optional.