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Transitions in SwiftUI · objc.io


Throughout our SwiftUI Workshop we frequently discover that only a few folks appear to learn about transitions, regardless that they are not very difficult and extremely helpful.

Transitions occur when a view is faraway from the view tree, or added to the view tree. Nonetheless, if you happen to’ve accomplished some SwiftUI, you’ll have observed that there isn’t a precise means so as to add views to the view tree — there isn’t a addSubview(_:). As an alternative, you may solely add and take away views by the mixture of a state change and utilizing an if assertion (or swap or ForEach). In different phrases, views are one way or the other added and eliminated for us routinely, but transitions hearth solely as soon as. Earlier than we dive into the small print of this, let’s contemplate a quite simple transition:

								struct ContentView: View {
    @State var seen = false
    var physique: some View {
        VStack {
            Toggle("Seen", isOn: $seen)
            if seen {
                Textual content("Good day, world!")
            }
        }
        .animation(.default, worth: seen)
    }
}

							

Once we run the above code we are able to see the textual content fade out and in. That is the default transition (.opacity). When the view will get inserted into the view tree, it fades in, and as soon as it will get eliminated it fades out. Be aware that if the physique executes once more, the view would not fade in once more until the situation within the if assertion adjustments.

To construct up a psychological mannequin of what is occurring, we are able to contemplate the SwiftUI view tree for the above view:

SwiftUI views are ephemeral: the physique of ContentView will get executed and from it a render tree is created. This render tree is persistent throughout view updates, and it represents the precise views on display screen. As soon as the render tree is up to date, the worth for physique then goes away. This is the render tree after the preliminary rendering:

As soon as we faucet the swap, a state change occurs and the physique of ContentView executes once more. The present render tree is then up to date. On this case, SwiftUI observed that the if situation modified from false to true, and it’ll insert our Textual content view into the render tree:

The change within the render tree is what triggers the transition. Transitions solely animate when the present transaction accommodates an animation. Within the instance above, the .animation name causes the transition to animate.

The render tree doesn’t truly exist with that identify or kind, however is solely a mannequin for understanding how SwiftUI works. We’re not utterly certain how these items are represented below the hood.

Once we change our view to have an if/else situation, issues get a bit extra fascinating. This is the code:

								struct ContentView: View {
    @State var seen = false
    var physique: some View {
        VStack {
            Toggle("Seen", isOn: $seen)
            if seen {
                Textual content("Good day, world!")
            } else {
                Picture(systemName: "hand.wave")
            }
        }
        .animation(.default, worth: seen)
    }
}

							

Once we render the preliminary view tree, it can include a VStack with a Toggle and a Textual content. As soon as the state adjustments from false to true, the textual content is changed by a picture. Within the ephemeral view tree there’s all the time both the Textual content or the Picture, by no means each. Within the render tree nevertheless, throughout the animation the tree will include each views:

As a result of we use the default transition, it appears to be like just like the textual content fades into the picture and again. Nonetheless, you may consider them as separate transitions: the textual content has a removing transition (fade out) and the picture has an insertion transition (fade in).


We’re not restricted to the default fade transition. For instance, here’s a transition that slides in from the vanguard when a view is inserted, and removes the view by scaling it down:

								let transition = AnyTransition.uneven(insertion: .slide, removing: .scale)

							

We are able to then mix it with an .opacity (fade) transition. The .mixed operator combines each transitions in parallel to get the next impact:

								let transition = AnyTransition.uneven(insertion: .slide, removing: .scale).mixed(with: .opacity)
VStack {
    Toggle("Seen", isOn: $seen)
    if seen {
        Textual content("Good day, world!")
            .transition(transition)
    } else {
        Textual content("Good day world!")
            .transition(transition)
    }
}
.animation(.default.pace(0.5), worth: seen)

							

Be aware that within the pattern above, we used a seen worth to change between the 2 Textual contents, regardless that they’re the identical. We are able to simplify the code a bit by utilizing id(_:). Each time the worth we move to id adjustments, SwiftUI considers this to be a brand new view within the render tree. Once we mix this with our data of transitions, we are able to set off a transition simply by altering the id of a view. For instance, we are able to rewrite the pattern above:

								let transition = AnyTransition.uneven(insertion: .slide, removing: .scale).mixed(with: .opacity)
VStack {
    Toggle("Seen", isOn: $seen)
    Textual content("Good day, world!")
        .id(seen)
        .transition(transition)
}
.animation(.default.pace(0.5), worth: seen)

							

Earlier than the animation, the textual content is current, and throughout the animation the newly inserted view (with id(false)) is transitioned in, and the outdated view (with id(true)) is transitioned out. In different phrases: each views are current throughout the animation:


When the builtin transitions do not cowl your wants, you too can create customized transitions. There may be the .modifier(energetic:id) transition. When a view is not transitioning, the id modifier is utilized. When a view is eliminated, the animation interpolates in between the id modifier and the energetic modifier earlier than eradicating the view utterly. Likewise, when a view is inserted it begins out with the energetic modifier initially of the animation, and ends with the id modifier on the finish of the animation.

This is an instance of a favourite button with a customized transition. This is not an ideal implementation (we might not hardcode the offsets and width of the button) but it surely does present what’s potential:

The total code is offered as a gist.


Generally when performing a transition you would possibly see surprising side-effects. In our case we had been virtually all the time in a position to resolve these by wrapping the view we’re transitioning inside a container (for instance, a VStack or ZStack). This provides some “stability” to the view tree that may assist stop glitches.

In essence, transitions aren’t very difficult. Nonetheless, reaching the consequence you need is usually a bit tough typically. To be able to successfully work with transitions you must perceive the distinction between the view tree and the render tree. And if you wish to have customized transitions, you additionally want to grasp how animations work. We cowl this in each our workshops and our ebook Considering in SwiftUI.

If your organization is occupied with a workshop on SwiftUI, do get in contact.

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