Working with Objective-C blocks with Swift

Question

I'm having trouble using the Objective-C Firebase framework in a new Swift project. I'm coming from mostly a C# background so the Swift closure syntax isn't that clear yet.

Here's how the code work in Objective-C with f being the Firebase object

[f observeEventType:FEventTypeValue withBlock:^(FDataSnapshot *snapshot) {
  NSLog(@"%@ -> %@", snapshot.name, snapshot.value);
}];

XCode auto suggests this syntax, and I have yet to find a working solution.

f.observeEventType(FEventTypeValue, withBlock: ((FDataSnapshot!) -> Void)?)

I'd like assign the FDataSnapshot data to a variable as the Objective-C example is doing. Thanks

Closures in Swift are like anonymous methods in C#. For example, Action<T> translates to (T) -> Void and Func<TArg, TResult> becomes (TArg) -> TResult. — Erik
– Erik, Commented Jun 11, 2014 at 17:49

Jack · Accepted Answer · 2014-06-11 17:12:36Z

14

Here's the Swift equivalent:

f.observeEventType(FEventTypeValue, withBlock: {
    snapshot in
    println("\(snapshot.name) -> \(snapshot.value)")
})

The key here is the in keyword to assign arguments to the closure to variables

answered Jun 11, 2014 at 17:12

Jack

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1 Comment

Ace Green Over a year ago

I was wondering if you can take a look at this: gist.github.com/acegreen/05282d20eb45de8d30d5

David Berry · Accepted Answer · 2014-06-11 17:26:57Z

4

To throw in implied names and tail closures, you can use:

f.observeEventType(FEventTypeValue) {
    println("\($0.name) -> \($0.value)")
}

answered Jun 11, 2014 at 17:26

David Berry

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7 Comments

Chuck Over a year ago

Doesn't it need the last parameter name, though?

David Berry Over a year ago

No, it uses implied naming and typing as long as there's a single trailing block and you put the '{' on the same line as the ')'

Chuck Over a year ago

How does it know the name of the withBlock: parameter, though? I thought named parameters still needed to be named. Is this documented anywhere?

David Berry Over a year ago

It's somewhere in the iTunes book. Look through the section on closures, I think they call it tail closures.

David Berry Over a year ago

@AlexWayne It works for any function that ends with a closure argument, the names and types are irrelevant.

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Chuck · Accepted Answer · 2014-06-11 17:14:11Z

1

Swift blocks are interchangeable with Objective-C blocks, so it ought to be something like:

f.observeEventType(FEventTypeValue, withBlock: { 
    snapshot in 
    println("\(snapshot.name) -> \(snapshot.value)")
})

answered Jun 11, 2014 at 17:14

Chuck

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