I have ViewController that contains a container. In this container I have a UIPageViewController.
How can I be notified in the mainVC that a page has changed ?
Mine is not the most modern approach, but it works for me.
In the child view controller I have a reference to the parent via:
weak var parentDelegate: ParentEventControllerDelegate?
and I can even update information in the parent using:
addEntry()
By creating a protocol that each can use:
#objc protocol ParentEventControllerDelegate {
func eventsForIndex(index: Int) -> [EKEvent]
func itemsForIndex(index: Int) -> [Item]
}
#objc protocol ChildEventControllerDelegate {
func addEntry()
}
Related
Let us consider the situation when you have ContentView and DestinationView. Both of them depend on some shared data, that typically lies inside the #ObservedObject var viewModel, that you pass from parent to child either via #EnvironmentObject or directly inside init().
The DestinationView in this case wants to enrich the viewModel by fetching some additional content inside .onAppear.
In this case, when using NavigationLink you might encounter the situation when the DestinationView gets into an update loop when you fetching content, as it also updates the parent view and the whole structure is redrawn.
When using the List you explicitly set the row's ids and thus view is not changed, but if the NavigationLink is not in the list, it would update the whole view, resetting its state, and hiding the DestinationView.
The question is: how to make NavigationLink update/redraw only when needed?
In SwiftUI the update mechanism compares View structs to find out whether they need to be updated, or not. I've tried many options, like making ViewModel Hashable, Equatable, and Identifiable, forcing it to only update when needed, but neither worked.
The only working solution, in this case, is making a NavigationLink wrapper, providing it with id for equality checks and using it instead.
struct NavigationLinkWrapper<DestinationView: View, LabelView: View>: View, Identifiable, Equatable {
static func == (lhs: NavigationLinkWrapper, rhs: NavigationLinkWrapper) -> Bool {
lhs.id == rhs.id
}
let id: Int
let label: LabelView
let destination: DestinationView // or LazyView<DestinationView>
var body: some View {
NavigationLink(destination: destination) {
label
}
}
}
Then in ContentView use it with .equatable()
NavigationLinkWrapper(id: self.viewModel.hashValue,
label: myOrdersLabel,
destination: DestinationView(viewModel: self.viewModel)
).equatable()
Helpful tip:
If your ContentView also does some updates that would impact the DestinationView it's suitable to use LazyView to prevent Destination from re-initializing before it's even on the screen.
struct LazyView<Content: View>: View {
let build: () -> Content
init(_ build: #autoclosure #escaping () -> Content) {
self.build = build
}
var body: Content {
build()
}
}
P.S: Apple seems to have fixed this issue in iOS14, so this is only iOS13 related issue.
In swift 5 and UIKit on clicking apns I have been pushing to the desired view controller by fetching the top view controller and push the desired view controller over top view controller.
But in SwiftUI I am not able to do that. How can I navigate from apns in SwiftUI.
I don't have to set desired controller as window root controller. I want to push the desired controller over the top view controller. I just want to push to a controller from apns click.
I am getting apns click in this delegate method:
func userNotificationCenter(_ center: UNUserNotificationCenter, didReceive response: UNNotificationResponse, withCompletionHandler completionHandler: #escaping () -> Void) {
}
You can add your own method in SceneDelegate. Create SceneDelegate object set its window rootViewController as follows.
extension SceneDelegate{
func loginViewController() {
let loginView = LoginView()
let sceneDelegate = UIApplication.shared.connectedScenes.first?.delegate as?
SceneDelegate
sceneDelegate?.window?.rootViewController = UIHostingController(rootView:
loginView)
}
}
And call method like this:
func userNotificationCenter(_ center: UNUserNotificationCenter, didReceive response: UNNotificationResponse, withCompletionHandler completionHandler: #escaping () -> Void) {
//Call method here
let sceneDelegate = UIApplication.shared.connectedScenes.first?.delegate as? SceneDelegate
sceneDelegate?.loginViewController()
}
In my app, I want to store an object in Environment to perform network queries from various views. However, during development I'd like to use a mock instead, and retrieve the payload (in JSON format) from the app's bundle. These are the steps I've taken:
Specify the protocol
typealias Token = String
protocol MyAPIConforming {
func signIn(username: String, password: String, completion: #escaping (Token?, Error?) -> Void)
}
Implement the two classes I need
class MockMyAPIManager: MyAPIConforming, ObservableObject {
func signIn(username: String, password: String, completion: #escaping (Token?, Error?) -> Void) {
// Read the JSON document from the app's bundle
}
}
class MyAPIManager: MyAPIConforming, ObservableObject {
func signIn(username: String, password: String, completion: #escaping (Token?, Error?) -> Void) {
// Perform the network request
}
}
Inject the mock object in the Environment
let myManager = MockMyAPIManager()
// Create the SwiftUI view that provides the window contents.
let contentView = LoginView().environmentObject(myManager)
Declare the #Environment object in the view
struct LoginView: View {
#EnvironmentObject var myManager: MyAPIConforming
...
I did declare myManager to be MyAPIConforming because I could be passing an object of type MockMyAPIManager or MyAPIManager.
However, in step #4 I get the following error:
Property type 'MyAPIConforming' does not match that of the 'wrappedValue' property of its wrapper type 'EnvironmentObject'
I'm not sure if the error means that there is no guarantee that the object adopting to MyAPIConforming will be also adopting the ObservableObject protocol.
So what do I need to do to store either MockMyAPIManager or MyAPIManager in the environment? Is this even possible?
Here is a possible solution
struct LoginView<T: ObservableObject & MyAPIConforming>: View {
#EnvironmentObject var myManager: T
// other code here
}
Update: usage
for content view
LoginView<MyAPIManager>().environmentObject(MyAPIManager())
and for testing/preview
LoginView<MockMyAPIManager>().environmentObject(MockMyAPIManager())
Playing around with examples out there. Found a project that had a class that was a bindableobject and it didn't give any errors. Now that Xcode 11 beta 4 is out, I'm getting the error:
Type 'UserSettings' does not conform to protocol 'BindableObject'
It has a fix button on the error which when you click on that, it adds
typealias PublisherType = <#type#>
It expects you to fill in the type.
What would the type be?
class UserSettings: BindableObject {
let didChange = PassthroughSubject<Void, Never>()
var score: Int = 0 {
didSet {
didChange.send()
}
}
}
Beta 4 Release notes say:
The BindableObject protocol’s requirement is now willChange instead of
didChange, and should now be sent before the object changes rather
than after it changes. This change allows for improved coalescing of
change notifications. (51580731)
You need to change your code to:
class UserSettings: BindableObject {
let willChange = PassthroughSubject<Void, Never>()
var score: Int = 0 {
willSet {
willChange.send()
}
}
}
In Beta 5 they change it again. This time they deprecated BindableObject all together!
BindableObject is replaced by the ObservableObject protocol from the
Combine framework. (50800624)
You can manually conform to ObservableObject by defining an
objectWillChange publisher that emits before the object changes.
However, by default, ObservableObject automatically synthesizes
objectWillChange and emits before any #Published properties change.
#ObjectBinding is replaced by #ObservedObject.
class UserSettings: ObservableObject {
#Published var score: Int = 0
}
struct MyView: View {
#ObservedObject var settings: UserSettings
}
in Xcode 11.X, I verify is fine in Xcode 11.2.1, 11.3.
BindableObject is changed to ObservableObject.
ObjectBinding is now ObservedObject.
didChange should be changed to objectWillChange.
List(dataSource.pictures, id: .self) { }
You can also now get rid of the did/willChange publisher and the .send code and just make pictures #Published
The rest will be autogenerated for you.
for example:
import SwiftUI
import Combine
import Foundation
class RoomStore: ObservableObject {
#Published var rooms: [Room]
init(rooms: [Room]) {
self.rooms = rooms
}
}
struct ContentView: View {
#ObservedObject var store = RoomStore(rooms: [])
}
ref: https://www.reddit.com/r/swift/comments/cu8cqk/getting_the_errors_pictured_below_when_try_to/
SwiftUI and Combine are two new frameworks that were announced at WWDC 2019. These two frameworks received a lot of attention at WWDC 2019, as evidenced by the number of sessions in which these technologies were featured.
SwiftUI was introduced as
a revolutionary, new way to build better apps, faster.
Combine is described as
a unified declarative framework for processing values over time
Between the initial release and now (May, 2020, Swift 5.2), there have been some changes. Anyone new to SwiftUI and Combine, who may have watched the WWDC videos, may be left with a few questions as to how the two frameworks work together.
Combine defines two interfaces: Publisher and Subscriber. A publisher sends events to subscribers. See sequence diagram below.
If you start an application in SwiftUI, and then add combine, there will be no mention of a Publisher or a Subscriber, the two main players required to use Combine. Consider this very simple sample application below.
import SwiftUI
import Combine
import SwiftUI
final class ActorViewModel: ObservableObject {
var name : String
private var imageUrl : URL?
//#Published
private (set) var image : Image = Image(systemName: "photo") {
willSet {
DispatchQueue.main.async {
self.objectWillChange.send()
}
}
}
init(name: String, imageUrl: URL?) {
self.name = name
self.imageUrl = imageUrl
self.fetchImage()
}
private func fetchImage() {
guard nil != self.imageUrl,
String() != self.imageUrl!.absoluteString else { return }
let task = URLSession.shared.dataTask(with: self.imageUrl!) { (data, response, error) in
guard nil == error , nil != response, nil != data,
let uiImage = UIImage(data: data!) else { return }
self.image = Image(uiImage: uiImage)
}
task.resume()
}
}
struct ContentView: View {
#ObservedObject var actor : ActorViewModel
var body: some View {
HStack {
actor.image
.resizable()
.aspectRatio(contentMode: ContentMode.fit)
.frame(width: 60, height: 60)
Text(actor.name)
}
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
let actor = ActorViewModel(name: "Mark Hammill",
imageUrl: URL(string: "https://m.media-amazon.com/images/M/MV5BOGY2MjI5MDQtOThmMC00ZGIwLWFmYjgtYWU4MzcxOGEwMGVkXkEyXkFqcGdeQXVyMzM4MjM0Nzg#._V1_.jpg"))
return ContentView(actor: actor)
}
}
The app preview via the canvas will look like this:
The app uses a list view to display names and images of actors. There are just two classes to consider:
ContentView -- the SwiftUI View subclass
ActorViewModel -- the source of the data for the ContentView (called a ViewModel as it performs the role of VM in MVVM)
The view has a reference to the actor object, as per the class diagram below.
Although this example is using Combine, it is not immediately apparent. There is no mention of a Publisher or a Subscriber. What is going on?
Answer: Looking at the class hierarchy fills in the missing gaps. The below class diagram explains the full picture (click on the image to see it in greater detail).
Consulting Apple's documentation provides definitions for these types:
ObservedObject: A property wrapper type that subscribes to an observable object and invalidates a view whenever the observable object changes.
ObservableObject: A type of object with a publisher that emits before the object has changed. By default an ObservableObject synthesizes an objectWillChange publisher that emits the changed value before any of its #Published properties changes.
objectWillChange: A publisher that emits before the object has changed.
PassthroughSubject: A subject that broadcasts elements to downstream subscribers. As a concrete implementation of Subject, the PassthroughSubject provides a convenient way to adapt existing imperative code to the Combine model.
First, consider what the #ObservedObject means. This is a property wrapper. A property wrapper reduces code duplication, and allows for a succinct syntax when declaring properties that hides how the property is stored and defined. In this case, the "Observed Object" is a property which observes another object.
In other words, the property is a Subscriber (from the Combine Framework). The actor is (through the use of a property wrapper) is a Subscriber, which subscribes to a Publisher, but what is the Publisher in this scenario?
The "Observable Object" is not itself the publisher, but rather has a publisher. The ActorViewModel conforms to the ObservableObject protocol. By doing so, it is provided with a publisher property called objectWillChange by an extension (which the framework provides on the ObservableObject protocol). This objectWillChange property is of type PassthroughSubject, which is a concrete type of the Publisher protocol. The passthrough subject has a property called send, which is a publisher method used to send data to any subscribers. So the property called "objectWillChange" is the Publisher.
To recap, the Subscriber is the property called actor from the ContentView class, and the Publisher is the property objectWillChange from the ActorViewModel class. What about the need for the Subscriber to Subscribe to the Publisher? The "#ObservedObject" property wrapper is itself a Subscriber, so it must subscribe to the Publisher. But how does the View find out about changes sent to the Subscriber? That is handled by the SwiftUI framework, which we never see.
Take-away: we don't need to worry about subscribing the view to the Publisher. On the other hand, we do need to worry about making sure the publisher tell the subscriber when something is about to change. When the image has been fetched from a remote server, and the data has been transformed into an image object, we call objectWillChange.send() to inform the View. Once the subscriber receives notification from the publisher that something is about to / has changed, it invalidates the view (which results in the view redrawing itself).
Summary
The way in which SwiftUI uses a ObservedObject PropertyWrapper does not on the surface give away the fact that Combine even exists in the equation. But by inspecting ObservedObject and ObservableObject, the underlying Combine framework is revealed, along with the design pattern:
subscriber --> subscribing to a publisher --> which then publishes changes --> that are received by the subscriber
References:
Blog Article
WWDC 2019 Session 204
WWDC 2019 Session 226
I am attempting to disconnect from a peripheral when ever a user leaves the current view controller. This would normally be easy by using prepare for segue in the following fashion:
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
manager.cancelPeripheralConnection(peripheral)
}
This doesnt work, however; because the view controller is inside a container view. Does anybody know how to call this method when ever the container view is change from this view controller to a different one?
You could put it in func viewWillDisappear(_ animated: Bool) or func viewDidDisappear(_ animated: Bool). Both will get called when the user navigates away from the view controller for any reason.