Calling a function in a view controller from another view controller - swift

Question

Here is the code with the delegate process suggested...
in main view controller...
protocol FilterDelegate: class {
func onRedFilter()
func onGreenFilter()
func onBlueFilter()
func onUnfiltered()
}
class ViewController: UIViewController, FilterDelegate, UIImagePickerControllerDelegate, UINavigationControllerDelegate {
----
// Increase red color level on image by one.
func onRedFilter() {
}
override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
if segue.identifier == "filterSegue" {
let dest = segue.destinationViewController as! CollectionViewController
dest.filterDelegate = self
}
}
in collection view controller...
var filterDelegate: FilterDelegate?
func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath) {
print("Cell \(indexPath.row) selected")
guard let filterDelegate = filterDelegate else {
print("Filter delegate wasn't set!")
return
}
switch indexPath.row {
case 0:
filterDelegate.onRedFilter()
case 1:
filterDelegate.onGreenFilter()
case 2:
filterDelegate.onBlueFilter()
case 3:
filterDelegate.onUnfiltered()
default:
print("No available filter.")
}
Right now...the code stops at the guard block and prints the error message. The switch block is not executed on any press of a cell.

Answer 1

Your theory in your second last sentence is correct - when you call storyboard.instantiateViewControllerWithIdentifier in the "child" view controller, you are actually creating an entirely new instance of your main view controller. You are not getting a reference to the existing main view controller, which is why the methods you're calling are not having any effect.
There are several ways to achieve what you're trying to do, including the delegate pattern or using closures. Here's a sketch of what it could look like using a delegate protocol:
protocol FilterDelegate: class {
func onRedFilter()
func onGreenFilter()
func onBlueFilter()
func onUnfiltered()
}
class MainViewController: UIViewController, FilterDelegate {
// implement these as required
func onRedFilter() { }
func onGreenFilter() { }
func onBlueFilter() { }
func onUnfiltered() { }
// when we segue to the child view controller, we need to give it a reference
// to the *existing* main view controller
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if let dest = segue.destination as? ChildViewController {
dest.filterDelegate = self
}
}
}
class ChildViewController: UIViewController {
var filterDelegate: FilterDelegate?
func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath) {
// ...
guard let filterDelegate = filterDelegate else {
print("Filter delegate wasn't set!")
return
}
switch indexPath.row {
case 0:
filterDelegate.onRedFilter()
case 1:
filterDelegate.onGreenFilter()
case 2:
filterDelegate.onBlueFilter()
case 3:
filterDelegate.onUnfiltered()
default:
print("No available filter.")
}
}
}
Another option would be to provide closures on ChildViewController for every function on MainViewController that the child needs to call, and set them in prepareForSegue. Using a delegate seems a bit cleaner though since there are a bunch of functions in this case.