Ephemera Ephemera - 5 months ago 45
Swift Question

Using dispatch_async with self in Swift

I have run into this problem a few times while porting Objective-C code to Swift. Say I have the following code:

dispatch_async(dispatch_get_main_queue()) {
self.hostViewController?.view.addSubview(self.commandField)
}


This will result in an error, underlining the entire
dispatch_async
call, offering:

Could not find member 'addSubview'


I assume this is an error that has not yet been properly implemented because if I put the
addSubview
call outside the
dispatch_async
block, the project builds fine. Initially I assumed it may have something to do with capturing
self
in the block. However, inserting
[unowned self] in
results in the same error, as does
[weak self] in
(after the appropriate
!
unwrap operators have been inserted).

How can I get
dispatch_async
blocks to work in Swift that need to capture
self
?

Answer

You should condition spinning off this action on the non-nullity, not test for it after you've already initiated it:

if let hostView = self.hostViewController?.view {
    dispatch_async(dispatch_get_main_queue()) {
         hostView.addSubview(self.commandField)
    }
} else {
    // handle nil hostView 
}

You should never unwrap an optional outside of an if let, or testing it first. Doing this should also resolve yer weak self issue.