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

Transitions occur when a view is faraway from the view tree, or added to the view tree. Nevertheless, in case you’ve executed some SwiftUI, you should have observed that there is no such thing as a precise means so as to add views to the view tree — there is no such thing as a addSubview(_:). As a substitute, you’ll be able to solely add and take away views via the mixture of a state change and utilizing an if assertion (or swap or ForEach). In different phrases, views are by some means added and eliminated for us robotically, but transitions fireplace solely as soon as. Earlier than we dive into the main points 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("Hi there, world!")
            }
        }
        .animation(.default, worth: seen)
    }
}

							

Once we run the above code we will 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. Word that if the physique executes once more, the view does not fade in once more except the situation within the if assertion adjustments.

To construct up a psychological mannequin of what is taking place, we will 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. Here 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 prevailing 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 really exist with that identify or type, however is just a mannequin for understanding how SwiftUI works. We’re not utterly positive how this stuff are represented beneath the hood.

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

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

							

Once we render the preliminary view tree, it would 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 nonetheless, throughout the animation the tree will include each views:

As a result of we use the default transition, it seems to be just like the textual content fades into the picture and again. Nevertheless, you’ll be able to consider them as separate transitions: the textual content has a elimination transition (fade out) and the picture has an insertion transition (fade in).

We aren’t 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, elimination: .scale)

							

We will 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, elimination: .scale).mixed(with: .opacity)
VStack {
    Toggle("Seen", isOn: $seen)
    if seen {
        Textual content("Hi there, world!")
            .transition(transition)
    } else {
        Textual content("Hi there world!")
            .transition(transition)
    }
}
.animation(.default.velocity(0.5), worth: seen)

							

Word that within the pattern above, we used a seen worth to modify between the 2 Textual contents, though they’re the identical. We will simplify the code a bit by utilizing id(_:). At any time when the worth we go 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 will set off a transition simply by altering the id of a view. For instance, we will rewrite the pattern above:

								let transition = AnyTransition.uneven(insertion: .slide, elimination: .scale).mixed(with: .opacity)
VStack {
    Toggle("Seen", isOn: $seen)
    Textual content("Hi there, world!")
        .id(seen)
        .transition(transition)
}
.animation(.default.velocity(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 previous 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, it’s also possible to create customized transitions. There may be the .modifier(lively: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 lively modifier earlier than eradicating the view utterly. Likewise, when a view is inserted it begins out with the lively modifier firstly of the animation, and ends with the id modifier on the finish of the animation.

Here 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 attainable:

The complete code is out there as a gist.

Typically when performing a transition you may see sudden 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 sophisticated. Nevertheless, reaching the end result you need is usually a bit tough generally. As a way to successfully work with transitions it’s important to perceive the distinction between the view tree and the render tree. And whenever you wish to have customized transitions, you additionally want to grasp how animations work. We cowl this in each our workshops and our e-book Pondering in SwiftUI.

If your organization is serious about a workshop on SwiftUI, do get in contact.