Within the SwiftUI group, 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, rework: (Self) -> M) -> some View {
if situation {
rework(self)
} else {
self
}
}
}
There are lots of weblog posts on the market with comparable modifiers. I believe 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, now we have a single state property myState. When it modifications between true and false, we wish 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, rework: { $0.body(width: 100) })
}
}
}
Curiously, when operating this code, the animation doesn’t look easy in any respect. Should you look carefully, you may 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 so much about how SwiftUI works. In UIKit, views are objects, and objects have inherent id. Which means that two objects are equal if they’re the identical object. UIKit depends on the id of an object to animate modifications.
In SwiftUI, views are structs — worth varieties — which implies that they do not have id. For SwiftUI to animate modifications, it wants to check 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 grasp the distinction in habits between the 2 examples, let us take a look at their varieties. 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 modifications, SwiftUI can not interpolate between the previous and the brand new worth, as they’ve differing kinds. In SwiftUI, when you’ve 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 modifications to the body properties, there aren’t any branches for SwiftUI to think about. It could actually simply interpolate between the previous and new worth and every thing 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 just about each view modifier assist. For instance, you may add a conditional foreground coloration by utilizing an non-compulsory coloration, you may add conditional padding by utilizing both 0 or a price, and so forth.
Word that applyIf (or actually, if/else) additionally breaks your animations if you end up doing issues appropriately on the “inside”.
Rectangle()
.body(width: myState ? 100 : nil)
.applyIf(situation) { $0.border(Colour.purple) }
If you animate situation, the border is not going to animate, and neither will the body. As a result of SwiftUI considers the if/else branches separate views, a (fade) transition will occur as a substitute.
There may be one more downside past animations. If you use applyIf with a view that comprises a @State property, all state shall be misplaced when the situation modifications. The reminiscence of @State properties is managed by SwiftUI, based mostly on the place of the view within the view tree. For instance, take into account 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(Colour.purple)
}
}
}
After we change flag, the applyIf department modifications, 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 person’s textual content is misplaced. The identical downside additionally occurs with @StateObject.
The tough half about all of that is that you simply won’t see any of those points when constructing your view. Your views look fantastic, however perhaps your animations are somewhat funky, otherwise you generally lose state. Particularly when the situation does not change all that always, you won’t even discover.
I might argue that all the weblog posts that counsel a modifier like applyIf ought to have a giant 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 proud of it (till it turned a supply of issues weeks later). In actual fact, I might argue that no code base ought to have this perform. It simply makes it manner too simple to by accident break animations or state.
Should you’re fascinated with understanding how SwiftUI works, you might learn our guide Considering in SwiftUI, watch our SwiftUI movies on Swift Speak, or attend one in all our workshops.
