I viewed some Stanford iOS development classes on Youtube, and I found something that's not clear to me.
In the lecture the professor explains how to create custom views and custom data source classes, and the code is the following:
FaceView.swift
protocol FaceViewDataSource: class {
// some stuff here
}
class FaceView: UIView {
// some uninteresting properties here
weak var dataSource: FaceViewDataSource?
// other stuff here
}
HappinessViewController.swift
class HappinessViewController: UIViewController, FaceViewDataSource {
#IBOutlet weak var faceView: FaceView!
// other stuff here
}
The professor said that the dataSource property must be declared as a weak property to avoid retain cycles between the view and the view controller.
My question is: why do we have a retain cycle if we declare the dataSource property as strong? Since the outlet property is weak, isn't the retain cycle already avoided?
No, it's not. See the description below.
View controller keeps strong reference to his view.
View controller's view (not view controller) keeps strong reference to faceView.
FaceView keeps strong reference to View controller.
Related
I have a UICollectionView in my class declared as
#IBOutlet weak var artworkCollectionView: UICollectionView!
Inside this class there is one delegate method called by two other View Controllers, one of these VC is a pop up, the other one is a normal VC.
The delegate method gets some data from the database and then updates the collection view calling inside a closure:
self.artworkCollectionView.reloadData()
When the delegate method is called by the pop up VC, then all works great. BUT when the delegate method is called by the normal VC when it gets to self.artworkCollectionView.reloadData() it gets the infamous Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value.
I have checked all the references to the cell reuseIdentifier and all is correct. I suspect that since the UICollectionView is declared as weak var, when I go from the current class to the pop up and then the pop up calls the delegate methods, the reference is not lost, but when I go from the current class to the normal VC and then the normal VC calls the delegate method the reference to my weak var is lost and so it is "seen" as nil.
#IBOutlet weak var artworkCollectionView: UICollectionView!
override func viewDidLoad() {
super.viewDidLoad()
// Set up
artworkCollectionView.dataSource = self
artworkCollectionView.delegate = self
artworkCollectionView.isUserInteractionEnabled = true
artworkCollectionView.allowsSelection = true
artworkCollectionView.register(UINib(nibName:
"MyCollectionViewCell", bundle: nil),
forCellWithReuseIdentifier: "cell")
}
// delegate method
func reloadCollections() {
retrieveAlbumRatings { (isAlbum) in
if isAlbum {
self.retrieveAlbumData(completion: { (isFinished) in
if isFinished {
// Reload collection views
self.artworkCollectionView.reloadData()
}
})
}
}
}
If I am right, my question is: how can I give weak var artworkCollectionView: UICollectionView! a STRONG reference so that it does not get lost in the flow from the current class to the normal VC and back?
EDIT: here is what I tried so far:
Remove “weak” from the outlet declaration so making it: #IBOutlet var artworkCollectionView: UICollectionView!
But I got the same error
I passed artworkCollectionView to the normal VC via override performSegue and then passed it back as an argument of the delegate method. This does not give me the fatal error but also it does not reload the UICollectionView because I think that anyway the weak reference to the UICollectionView outlet is lost.
Thanks for your help (disclaimer: I am pretty new to Swift..)
Inside this class there is one delegate method called by two other
View Controllers, one of these VC is a pop up, the other one is a
normal VC.
The delegate method gets some data from the database and then updates
the collection view calling inside a closure:
self.artworkCollectionView.reloadData()
The flow appears to be that you have a VC containing the code above, the VC can either open a pop-up or just do a standard push segue to the "normal VC".
You want some operation to occur in either the pop-up VC or normal VC, load some data and then when the user is directed back to the originating VC, the UICollectionView is updated with that data.
Your problems are the following:
I passed artworkCollectionView to the normal VC via override
performSegue and then passed it back as an argument of the delegate
method. This does not give me the fatal error but also it does not
reload the UICollectionView because I think that anyway the weak
reference to the UICollectionView outlet is lost.
You shouldn't be passing anything around like this in most cases unless you have a really good reason to do so (I don't see one).
You want a separation of concerns here. You have to think carefully about what you wanjt to pass between VCs to avoid making weird dependencies between them. I wouldn't pass outlets for multiple reasons, the first being that you now have to keep track of the outlet in multiple VCs if you ever decide to change it. The second being that it requires too much mental gymnastics to keep track of the state of the outlet since it's being passed around everywhere. The outlets are also only guaranteed to be set at certain phases of the lifecycle. For example if you retrieve the destination VC from the segue in prepareForSegue:sender: and attempt to reference the outlets at that time, they will all be nil because they haven't been set yet.
These are all good reasons why the VC that contains the code above should be the one (and the only one) maintaining control over what gets displayed in artworkCollectionView and when. The problem here is how you're approaching this, rather than having the pop-up or normal VC call the delegate method or doing weird things like passing outlets from one VC to another, just pass the data around instead.
The simplest example is:
The pop-up VC and normal VC call some code to actually fetch the
data.
Then depending on how you actually segued to the pop-up VC or
normal VC from original VC, use either parentViewController or
presentingViewController to get the reference to the original VC.
Set the data into the original VC through that reference.
Dismiss the pop-up VC or normal VC if necessary (depends on your specific app, maybe you want the user to push a UIButton to dismiss instead of doing it for them).
When the original VC comes back into view, add some code to a lifecycle method like
viewWillAppear to have it load the contents of the data into the
UICollectionView at that time.
I see no reason why you should be passing any outlets outside of the original VC that should be the one managing it.
Short answer: Don't do that. You should treat a view controller's views as private. You should add a method to your view controller that other objects call to tell it to reload it's collection view.
The longer answer is that your view controller's collection view should stick around as long as the view controller is on-screen. It sounds like you don't have a very strong understanding of object lifecycle and how ARC works. You should read up on that and do some exercises until you understand it better.
Try something like this:
//Make artworkCollectionView a normal weak var, not implicitly unwrapped.
//You'll need to change your other code to unwrap it every time you use it.
#IBOutlet weak var artworkCollectionView: UICollectionView?
...
func reloadCollections() {
retrieveAlbumRatings { (isAlbum) in
if isAlbum {
//The construct `[weak self]` below is called a capture list
self.retrieveAlbumData(completion: { [weak self] (isFinished) in
guard let weakSelf = self else {
print("self is nil");
return
}
}
if isFinished {
// Reload collection views
guard let collectionView = weakSelf.artworkCollectionView else {
print("collectionView is nil!")
return
}
collectionView.reloadData()
})
}
}
}
I want to make a delegate in separate class like:
class MapViewAnnotationDelegate: NSObject, SomeDelegate { //...}
and then in my controller I want to assign:
someView.delegate = MapViewAnnotationDelegate()
but the delegate is nil... how to achieve such effect? I read something about strong in Objective-C but as far as I happen to know the strong is default in Swift.
What you're doing is fine, but you need something to hold onto your instance. delegate properties by tradition are weak, so they don't ensure that the object is retained. Something else must retain it. The most common solution is to add a property to the class that owns someView, and assign your MapViewAnnotationDelegate to that property before assigning it as the delegate. That way it won't be deallocated as long as the containing objet lives. But anything that retains it is ok.
Your current code currently does this:
Create MapViewAnnotationDelegate
Assign it to someView.delegate
Note that there are no strong references to MapViewAnnotationDelegate (since someView.delegate is a weak reference)
Destroy MapViewAnnotationDelegate
Set someView.delegate to nil
One way this may look would be like this:
class Owner {
let mapViewAnnotationDelegate = MapViewAnnotationDelegate()
let someView: ...
init() {
someView.delegate = mapViewAnnotationDelegate
}
}
In this configuration, Owner (which is the owner of someView) holds onto the delegate for its lifetime.
I was using this example which explains the use of NSNotification.
In my case, I have a UIViewController in which I have a UITableView. To this tableview I am assigning a dataSource and delegate programatically by instatiating my UITableViewController. So far I have not declared any inits, and thus have been using the simple init() to initialize my UITableViewController. (This UITableViewController is not on the StoryBoard).
class foo: UIViewController{
#IBOutlet weak var fooTable: UITableView!
var fooTableViewController = MyTableViewController()
override func viewDidLoad(){
super.viewDidLoad()
fooTable.delegate = fooTableViewController
fooTable.dataSource = fooTableViewController
}
}
class MyTableViewController: UITableViewController {
override func viewDidLoad(){
super.viewDidLoad()
NSNotificationCenter.defaultCenter().addObserver(self, selector: "notificationReceived", name: "TEST_NOTIFICATION", object: nil)
}
}
If I try to add the observer in viewDidLoad() of the UIViewController, it does not work.
So my question is: Does using the NSNotification require the usage of init(coder aDecoder: NSCoder)? If so, then what is the correct way to initialize using this init in swift? How should I be instantiating MyTableViewController in my UIViewController instance foo?
viewDidLoad is only called when the view of a view controller is loaded - in the code you're showing you create a table view controller subclass, assign it as the datasource and delegate of another table view (confusing, as it will already be the datasource and delegate of its own table view), but never actually do anything with the table view controller's view.
This means that viewDidLoad will not be called.
You should probably be adding your table view controller's tableView as a subview and also adding it as a child view controller so that rotation and appearance events are forwarded properly.
Note that the question and answer are nothing whatsoever to do with notification centers or Swift, but just about understanding the view controller lifecycle.
If you want a separate object to act as a datasource and delegate for your table view, great idea, but don't use a UITableViewController subclass. Just create a plain object which conforms to the data source and/or delegate protocols.
I have a label in a second viewController in Swift, and I want change this between my firstViewController. I try this with prepareForSegue: also with ChildView and ParentView and accessing to label since parentView.. But I get error..
What is the correct form to can make this?
Try declared secondVIew:
class ViewController: UIViewController {
var v = View2Controller()
#IBAction func but(sender : AnyObject) {
v.label2.text = "newText" //Here get the error EXC_BAD_INSTRUCTION
}
...
class View2Controller: UIViewController {
#IBOutlet var label2 : UILabel
Thanks!
The more code you provide the easier it is to get answers.
In your case, you are initializing a band new View2Controller. Since label2 is an IBOutlet it expects data from a nib file. Since it didn't get any of this data, label2 is going to be nil hence why you get a EXC_BAD_INSTRUCTION crash.
You can access the root view controller of a navigation controller because navigation controllers are special in that they have their own stack and maintain their own view hierarchy. This is why you have to push and pop view controllers in a navigation controller. This also allows child controllers to maintain a reference to its parent controller.
The proper solution for your situation would be to use protocols. Otherwise give View2Controller a property and reference to ViewController then make changes to ViewController through that property.
I'm trying to replicate the Stanford Matchismo game from "Developing ios7 apps for iphone and ipad" in iTunesU in Swift.
On page 77 of the 3rd lecture slides, it shows using an IBOutletCollection which isn't an option on Swift. The Swift doc example shows one example that has an array of IBOutlet, but I can't figure out how to make Interface Builder connect multiple outlets to the same IBOutlet/IBOutlet Array.
Has anyone figured out how to do this yet?
I know that I can create 12 outlets and deal with it that way, but I'd like to make this work as closely as possible to the example in the lecture slides.
Update: This works properly in Xcode now - "Outlet Collection" is one of the connection options in Interface Builder, which creates something that looks like:
#IBOutlet var labelCollection: [UILabel]!
While we're waiting for a fix, you can approximate this using a computed property. Let's say my view has five UILabels that I want in a collection. I still have to declare each one, but then I also declare a computed property that collects them:
class MyViewController {
#IBOutlet var label1 : UILabel
#IBOutlet var label2 : UILabel
#IBOutlet var label3 : UILabel
#IBOutlet var label4 : UILabel
#IBOutlet var label5 : UILabel
var labels: UILabel![] { return [label1, label2, label3, label4, label5] }
Kind of annoying, but from then on we can treat the labels property as if it were an IBOutletCollection, and won't have to change the rest of our code once the bug is fixed:
override func viewDidLoad() {
super.viewDidLoad()
for (index, item) in enumerate(self.labels) {
item.text = "Label #\(index)"
}
}
Use:
#IBOutlet var lineFields: [UITextField]!
Then control-drag from UITextField elements to lineFields in order.
#IBOutlet var buttons : [UIView]!
then drag it from the connections inspector in the interface builder or whatever metod you usually use for that
EDIT
This was fixed in a later Beta release of Swift - there's now in
IBCollection option in the interface builder.
For early Beta releases of Swift:
I came across the same problem: in the release notes of Beta 2 you find the following statement:
Interface Builder does not support declaring outlet collections in Swift classes
I solved this the following way (easy to customize):
class CardGameViewController: UIViewController {
#lazy var cardButtons : UIButton[] = {
var tempBtn: UIButton[] = []
for v:AnyObject in self.view.subviews {
if v is UIButton {
tempBtn.append(v as UIButton)
}
}
return tempBtn
}()
...
Basically, it loops through all the subviews and checks if one is a UIButton. In that case it gets added to a temporary array. This temporary array is then used to lazy instantiate the cardButtons array. For all details, check: Matchismo: Objective-C to Swift
Follow steps to create an array of outlets and connect it with IB Elements:
Create an array of IBOutlets
Add multiple UIElements (Views) in your Storyboard ViewController interface (As shown in below snapshot)
Select ViewController (In storyboard) and open connection inspector
There is option 'Outlet Collections' in connection inspector (You will see an array of outlets there)
Connect if with your interface elements
-
class ViewController2: UIViewController {
#IBOutlet var collection:[UIView]!
override func viewDidLoad() {
super.viewDidLoad()
}
}
I got this working in Xcode seed 3 using this syntax
#IBOutlet strong var views: NSArray?
See my discussion here: https://stackoverflow.com/a/24686602/341994
What #machine said seems to be the current state (XCode 7.1) with iOS 9 bindings.
The key is to drag them all in order.
Use the first item to control+drag into the controller code and then change the Outlet type to collection. After the from the controller code file drag the outlet point onto each of the screen controls one by one in order (as #machine says)