How do I correctly pass a block handler into this function in Swift?

Question

After reading about Swift Closures and trying to use the same syntax to pass a anonymous function into the handler I cannot get it to compile. How can I correctly re-create the following functioning objective c code in Swift?

Here is the objective c code that I am trying to re-create in swift

 [self.motionManager
     startAccelerometerUpdatesToQueue:[[NSOperationQueue alloc] init]
     withHandler:^(CMAccelerometerData *data, NSError *error)
     {

         dispatch_async(dispatch_get_main_queue(),
                        ^{

                            float xx = data.acceleration.x;
                            float yy = -data.acceleration.y;
                            float angle = atan2(yy, xx);

                            self.dropitBehavior.gravity.angle = angle;
                         });
     }
 ];

Here are few failed attempts at re-creating the code in swift:

self.motionManager.startAccelerometerUpdatesToQueue(NSOperationQueue(), withHandler: {
            (data: CMAccelerometerData(), error: NSError()) -> Void = {
                dispatch_async(dispatch_get_main_queue()) {
                    var xx = data.acceleration.x
                    var yy = -data.acceleration.y
                    var angle = atan2(yy, xx)

                    self.dropitBehavior.gravity.angle = angle
                }
            }
  })

self.motionManager.startAccelerometerUpdatesToQueue(NSOperationQueue(), withHandler: {
            (data: CMAccelerometerData(), error: NSError()) {
                dispatch_async(dispatch_get_main_queue()) {
                    var xx = data.acceleration.x
                    var yy = -data.acceleration.y
                    var angle = atan2(yy, xx)

                    self.dropitBehavior.gravity.angle = angle
                }
            }
 })

drewag · Accepted Answer · 2014-06-15 19:45:52Z

12

By doing CMAccelerometerData() and NSError() you are actually calling the initializers of those classes. You just need to use their types. However, because in objective-C, pointers can be nil, when you translate the types to Swift, you need to use optionals. The convention is to use Implicitly Unwrapped Optionals. Also, you separate the parameters of an anonymous closure with in not additional curly brackets:

self.motionManager.startAccelerometerUpdatesToQueue(NSOperationQueue(), withHandler: {
    (data: CMAccelerometerData!, error: NSError!) in
    // internal implementation
})

Also, because the types can be inferred from the parameter type, you don't have to even specify the types for the parameters:

self.motionManager.startAccelerometerUpdatesToQueue(NSOperationQueue(), withHandler: {
    (data, error) in
    // internal implementation
})

Also, if a block is the last parameter to a method / function call, you can define it outside of the parenthesis:

self.motionManager.startAccelerometerUpdatesToQueue(NSOperationQueue()) {
    (data, error) in
    // internal implementation
}

This way you don't need the closing ) after the closure.

That creates the final version with your internal implementation:

self.motionManager.startAccelerometerUpdatesToQueue(NSOperationQueue()) {
    (data, error) in
    dispatch_async(dispatch_get_main_queue()) {
        var xx = data.acceleration.x
        var yy = -data.acceleration.y
        var angle = atan2(yy, xx)

        self.dropitBehavior.gravity.angle = angle
    }
}

edited Jun 15, 2014 at 19:45

answered Jun 15, 2014 at 19:24

drewag

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7 Comments

Nearpoint Over a year ago

Thanks for the answer. Although I am getting an error 'Missing argument for parameter #2 in call' which highlights the dispatch_async line

drewag Over a year ago

@nearpoint Sorry, I was missing the opening curly bracket of the async block. I updated my answer

Nearpoint Over a year ago

Ah thanks so much this answer is extremely helpful. I will reference it many times

Nearpoint Over a year ago

ah and knowing about the third point (if the closure if the last argument you can put it outside the parentheses) makes me understand better why dispatch_async syntax is the way it is

drewag Over a year ago

@nearpoint Yup, the syntax can look somewhat strange with that until you get used to it

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