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?

Renish Dadhaniya · Accepted Answer · 2017-03-06 10:31:57Z

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 6, 2017 at 10:31

Renish Dadhaniya

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answered Jun 7, 2014 at 1:50

iluvcapra

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2 Comments

Ephemera Over a year ago

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?

iluvcapra Over a year ago

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.

JAL · Accepted Answer · 2016-06-13 23:16:33Z

13

The dispatch_async syntax has changed with Swift 3:

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

answered Jun 13, 2016 at 23:16

JAL

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