Using dispatch_async with self

Question

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?

score 65 · Accepted Answer · edited Mar 06 '17 at 10:31

65

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 {
    DispatchQueue.main.async {
         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.

edited Mar 06 '17 at 10:31

Renish Dadhaniya

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answered Jun 07 '14 at 01:50

iluvcapra

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Ah that works, thanks! So is the compiler complaining that it can't deterministically unwrap `hostController?` inside the block? Or is there something else going on here? – Ephemera Jun 08 '14 at 01:33
5

It's because the expression `self.hostViewController?.view` returns an object of type `NSView?`, *not* `NSView`. Optional chains always have to be checked and the result either has to be letted or forced. – iluvcapra Jun 08 '14 at 02:31

score 13 · Answer 2 · answered Jun 13 '16 at 23:16

13

The dispatch_async syntax has changed with Swift 3:

DispatchQueue.main.async { 
    hostView.addSubview(self.commandField)
}

answered Jun 13 '16 at 23:16

JAL

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Using dispatch_async with self

2 Answers2