Swift.... Class method vs. Instance method - swift

Question

Thanks in advance for help!!
I'm trying to call a func from within my Class and I keep getting an error saying that:
Missing parameter for argument #1.............Read a few posts saying it's an instance vs class problem? I don't get it..I'm calling the method from within the Class??? There has to be an instance of the class if the method is being called????? right? Here is my code...Thanks
import Foundation
import Parse
class TestViewController {
let photos = getWallImages() //-----This is the line requesting an argument
func getWallImages() -> [WallPost] {
let query = WallPost.query()!
query.findObjectsInBackgroundWithBlock { objects, error in
if error == nil {
if let objects = objects as? [WallPost] {
return objects
println("We have \(objects.count)")
}
} else if let error = error {
println(error)
}
}
}
}

Answer 1

So the "crime" you are committing is the fact that the method is applied in an instance of the class and not as a class method. The function is expecting a self parameter (a reference to the instance). That explains the error message.
Now to fix that you have two quick options:
1. Make it a class function and call it that way too:
class TestViewController {
let photos = TestViewController.getWallImages()
class func getWallImages() -> [WallPost] {
// mumbo jumbo
}
}
This approach is problematic in case you would want to do some instance specific operations, because class func is static method and doesn't provide you with some of the object benefits.
2. Instantiate the object you are calling the method on:
class TestViewController {
let photos = TestViewController().getWallImages()
func getWallImages() -> [WallPost] {
// mumbo jumbo
}
}
This approach isn't correct with your given structure - it doesn't make sense to instantiate another view controller, but if you take the method and put it in a separate class, maybe it would then make sense.
Then of course you have multiple other ways of changing your code to make it work. Maybe you could initialize it with lazy parameter, maybe you could initialize it in the init method. Whatever suits you best. My answer is simply explaining where you've gone wrong.

Answer 2

There are a few ways you can set your property appropriately. You can make getWallImages() a type method:
class TestViewController {
let photos = TestViewController.getWallImages()
class func getWallImages() -> [WallPost] {
....
}
}
Or, you can keep your method an instance method and set your property upon initialization:
class TestViewController {
let photos: [WallPost]!
init() {
super.init()
photos = getWallImages()
}
func getWallImages() -> [WallPost] {
....
}
}

Answer 3

If you're asking a question you should reduce your code to a minimum, discarding unnecessary details.
You probably want something like this:
class MyClass {
let x = MyClass.getStuff()
static func getStuff() -> Int {
return 0
}
}
However your method getWallImages() can't do something like this, because it's returning the result asynchronous, which means you get the result much later after the function has returned.
You could do something like this though (this is how I'd be doing it):
class MyClass {
var x : Int? {
didSet {
if let x = x {
// Do something with x, here it's assigned
} else {
// x was set to nil, something failed
}
}
}
init() {
getStuffAsynchronous()
}
func getStuffAsynchronous() {
// Do your query stuff here, assign x to your objects to invoke the didSet
x = 0
// If it fails somehow, assign set x to nil
// if fail {
// x = nil
// }
}
}