In SwiftUI, it is very easy to construct a list of item. For example, let say we have an array containing the names of our users, we can display these names in a list by the following code:
struct ContentView: View {
let users = ["John","Peter","Jane"]
var body: some View {
List(users){user in
Text(user)
}
}
}
In this case, we can make a list which consists of the names of all the users. However, this list of names is static, meaning that any changes to the users array will not cause the List to refresh and be updated to the latest values. To make the List to refresh whenever there is changes to the users array, we can make use of the @State
variable. With the use of @State
in front of a variable, SwiftUI will listen for any changes to that variable, and automatically render the View
again with the latest values.
struct ContentView: View {
@State var users = ["John","Peter","Jane"]
var body: some View {
List(users){user in
Text(user)
}
}
}
To see how this might be useful, let's say that we have a button that makes a network call to fetch the users information from our server, we can easily render the List with the network call result easily with the following code:
struct ContentView: View {
@State var users = ["John","Peter","Jane"]
var body: some View {
VStack{
Button(action: {
self.loadFromServer()
}){
Text("Fetch From Server")
}
List(users, id: \.self){user in
Text(user)
}
}
}
private func loadFromServer(){
//do some network call and assign the result to self.users
}
}
With the code above, when the result from the network call is ready and then assigned to self.users, SwiftUI detects changes to the variable and will redraw the List
with the updated values.
While the code above works, it is not a good idea to put our networking code in the View
Struct. View
is only responsible for rendering the UI elements onto the screen. Having networking code inside a View
Struct violates the Single Responsibility Principle. Hence, we should move the code related to networking out into separate class. However, moving networking code out of the View
Struct also means that now we are unable to directly access the @State
users variable to update the list. The solution : ObservableObject
.
We can start by creating a struct called User
with two properties: name
and id
. The reason for the need for id is to allow List
in SwiftUI to be able to unique identify and keep track of the changes to the data source.
struct User: Identifiable {
let id = UUID()
let name:String
}
Next, we can then create a class called UserContainer, and comform it to ObservableObject
.
class UsersContainer : ObservableObject{
@Published var users = [User]()
}
By marking the users variable as @Published
, it means that whenever there is any changes to the users variable, the instances of the class that are "subscribed" to it will be notified, prompting SwiftUI to re-render the View
. The final step is to connect the data source to our View
:
To do this, we will create an instance of the UsersContainer Class in the ContentView. In order to "subscribe" to be notified of any changes to the data, we have to mark it with @ObservedObject
:
struct ContentView: View {
@ObservedObject var usersContainer = UsersContainer()
var body: some View {
VStack{
List(usersContainer.users, id: \.id){user in
Text(user.name)
}
}
}
}
Now with the current arrangement, supposed that we want to make a network call to the server to retrieve the list of users, instead of doing it in the ContentView, we can now do the networking logic in the UsersContainer Class
class UsersContainer : ObservableObject{
@Published var users = [User]()
func fetchFromServer(){
//assign the result back to self.users
}
}
We can now add a button in our ContentView
to trigger the fetchFromServer() call like this:
struct ContentView: View {
@ObservedObject var usersContainer = UsersContainer()
var body: some View {
VStack{
Button(action: {
self.usersContainer.fetchFromServer()
}){
Text("Fetch From Server")
}
List(usersContainer.users, id: \.id){user in
Text(user.name)
}
}
}
}
Top comments (2)
How do you have the list view update when a property of an object in the list changes but not the list itself?
If another view updated user.name, how does the view update if there are no Published properties on the User object itself?
This helped me so so so much!!
Thank you!!!