Writing a Custom Setter for my Variable - Swift - ios

Question

I am trying to run a Setter on a property when it changes, that will then also affect the logic in that class:
Here is how it used to look in Objective - C:
#property (strong, nonatomic) NSFetchedResultsController *fetchedResultsController;
Set Method for fetchedResultsController:
- (void)setFetchedResultsController:(NSFetchedResultsController *)newFetchResultsContrller
{
NSFetchedResultsController *oldfetchResultsController = _fetchedResultsController;
if (newFetchResultsContrller != oldfetchResultsController) {
_fetchedResultsController = newFetchResultsContrller;
newFetchResultsContrller.delegate = self;
if ((!self.title || [self.title isEqualToString:oldfetchResultsController.fetchRequest.entity.name]) && (!self.navigationController || !self.navigationItem.title)) {
self.title = newFetchResultsContrller.fetchRequest.entity.name;
}
if (newFetchResultsContrller) {
[self performFetch];
} else {
[self.tableView reloadData];
}
}
}
Here is my attempt in Swift. However I cannot get the newFetchResultsController, thus cannot complete rest. Is this the right way of setting a property in swift ? How can I do the same principle as above shown in Objective C in Swift?
var fetchedResultsController:NSFetchedResultsController! {
willSet {
println("Set fetchedResultsController Called")
self.fetchedResultsController.delegate = self
performFetch()
}
}

Answer 1

UPD:
Today with Swift 3.0 your logic can be reimplemented, you may have single plain stored property with only didSet value observer specified like this:
var fetchedResultsController:NSFetchedResultsController? {
didSet(oldfetchResultsController) {
// If value BEFORE it was set is not identical to value AFTER it was set.
if oldfetchResultsController !== fetchedResultsController {
// Let's assume there is such method.
if fetchedResultsController.isBadAndNotSuitable == false {
// Do your stuff here.
} else {
// If you want to override value that was set
// without observers being called, you can just set
// your property inside didSet like:
fetchedResultsController = nil
// Or, if you don't want to change value:
fetchedResultsController = oldFetchedResultsController
// this won't cause willSet to fire and your property
// will preserve its value.
// It's ok to override observers in subclasses, because
// super's implementation will be called as well before.
}
}
}
}
As it were in 2014:
You may write it like this:
var _fetchedResultsController:NSFetchedResultsController? = nil
var fetchedResultsController:NSFetchedResultsController? {
get {
return _fetchedResultsController
}
set (aNewValue) {
if (_fetchedResultsController != aNewValue)
{
_fetchedResultsController = aNewValue
}
}
}
With this approach you can prevent value to be set. So, for example, if there was a property on NSFetchedResultsController called isBadAndNotSuitable you could write:
set (aNewValue) {
if (aNewValue.isBadAndNotSuitable)
{
NSLog("Don't set, it's bad")
}
else
{
_fetchedResultsController = aNewValue
}
}

Answer 2

Inside willSet you have a hidden var called newValue that you can use like you are doing in obj-c, like this:
willSet {
if newValue != fetchedResultsController {
//do stuff
}
}
This newValue variable contains the value you just tried to assign to the property.