Within the SwiftUI neighborhood, many individuals give you their very own model of a conditional view modifier. It lets you take a view, and solely apply a view modifier when the situation holds. It usually appears one thing like this:
extension View {
@ViewBuilder
func applyIfM: View>(situation: Bool, remodel: (Self) -> M) -> some View {
if situation {
remodel(self)
} else {
self
}
}
}
There are various weblog posts on the market with related modifiers. I feel all these weblog posts ought to include an enormous warning signal. Why is the above code problematic? Let us take a look at a pattern.
Within the following code, we’ve got a single state property myState. When it adjustments between true and false, we need to conditionally apply a body:
struct ContentView: View {
@State var myState = false
var physique: some View {
VStack {
Toggle("Toggle", isOn: $myState.animation())
Rectangle()
.applyIf(situation: myState, remodel: { $0.body(width: 100) })
}
}
}
Curiously, when working this code, the animation doesn’t look clean in any respect. In the event you look intently, you possibly can see that it fades between the “earlier than” and “after” state:
Here is the identical instance, however written with out applyIf:
struct ContentView: View {
@State var myState = false
var physique: some View {
VStack {
Toggle("Toggle", isOn: $myState.animation())
Rectangle()
.body(width: myState ? 100 : nil)
}
}
}
And with the code above, our animation works as anticipated:
Why is the applyIf model damaged? The reply teaches us loads about how SwiftUI works. In UIKit, views are objects, and objects have inherent id. Because of this two objects are equal if they’re the identical object. UIKit depends on the id of an object to animate adjustments.
In SwiftUI, views are structs — worth sorts — which signifies that they do not have id. For SwiftUI to animate adjustments, it wants to match the worth of the view earlier than the animation began and the worth of the view after the animation ends. SwiftUI then interpolates between the 2 values.
To know the distinction in conduct between the 2 examples, let us take a look at their sorts. Here is the kind of our Rectangle().applyIf(...):
_ConditionalContent<ModifiedContent<Rectangle, _FrameLayout>, Rectangle>
The outermost sort is a _ConditionalContent. That is an enum that may both comprise the worth from executing the if department, or the worth from executing the else department. When situation adjustments, SwiftUI can not interpolate between the previous and the brand new worth, as they’ve differing kinds. In SwiftUI, when you may have an if/else with a altering situation, a transition occurs: the view from the one department is eliminated and the view for the opposite department is inserted. By default, the transition is a fade, and that is precisely what we’re seeing within the applyIf instance.
In distinction, that is the kind of Rectangle().body(...):
ModifiedContent<Rectangle, _FrameLayout>
After we animate adjustments to the body properties, there aren’t any branches for SwiftUI to contemplate. It might simply interpolate between the previous and new worth and the whole lot works as anticipated.
Within the Rectangle().body(...) instance, we made the view modifier conditional by offering a nil worth for the width. That is one thing that nearly each view modifier help. For instance, you possibly can add a conditional foreground coloration by utilizing an non-obligatory coloration, you possibly can add conditional padding by utilizing both 0 or a worth, and so forth.
Observe that applyIf (or actually, if/else) additionally breaks your animations when you’re doing issues accurately on the “inside”.
Rectangle()
.body(width: myState ? 100 : nil)
.applyIf(situation) { $0.border(Coloration.crimson) }
Once you animate situation, the border won’t animate, and neither will the body. As a result of SwiftUI considers the if/else branches separate views, a (fade) transition will occur as an alternative.
There may be one more drawback past animations. Once you use applyIf with a view that incorporates a @State property, all state might be misplaced when the situation adjustments. The reminiscence of @State properties is managed by SwiftUI, primarily based on the place of the view within the view tree. For instance, think about the next view:
struct Stateful: View {
@State var enter: String = ""
var physique: some View {
TextField("My Area", textual content: $enter)
}
}
struct Pattern: View {
var flag: Bool
var physique: some View {
Stateful().applyIf(situation: flag) {
$0.background(Coloration.crimson)
}
}
}
After we change flag, the applyIf department adjustments, and the Stateful() view has a brand new place (it moved to the opposite department of a _ConditionalContent). This causes the @State property to be reset to its preliminary worth (as a result of so far as SwiftUI is worried, a brand new view was added to the hierarchy), and the consumer’s textual content is misplaced. The identical drawback additionally occurs with @StateObject.
The difficult half about all of that is that you just may not see any of those points when constructing your view. Your views look tremendous, however possibly your animations are just a little funky, otherwise you typically lose state. Particularly when the situation does not change all that usually, you may not even discover.
I’d argue that all the weblog posts that recommend a modifier like applyIf ought to have an enormous warning signal. The downsides of applyIf and its variants are by no means apparent, and I’ve sadly seen a bunch of people that have simply copied this into their code bases and have been very pleased with it (till it grew to become a supply of issues weeks later). The truth is, I’d argue that no code base ought to have this perform. It simply makes it means too straightforward to by accident break animations or state.
In the event you’re all for understanding how SwiftUI works, you can learn our e book Considering in SwiftUI, watch our SwiftUI movies on Swift Speak, or attend certainly one of our workshops.
