Working with percentages in SwiftUI structure – Ole Begemann

SwiftUI’s structure primitives usually don’t present relative sizing choices, e.g. “make this view 50 % of the width of its container”. Let’s construct our personal!

Use case: chat bubbles

Take into account this chat dialog view for example of what I need to construct. The chat bubbles at all times stay 80 % as large as their container because the view is resized:

Constructing a proportional sizing modifier

1. The Format

We are able to construct our personal relative sizing modifier on high of the Format protocol. The structure multiplies its personal proposed dimension (which it receives from its dad or mum view) with the given components for width and top. It then proposes this modified dimension to its solely subview. Right here’s the implementation (the total code, together with the demo app, is on GitHub):

/// A customized structure that proposes a proportion of its
/// acquired proposed dimension to its subview.
///
/// - Precondition: should include precisely one subview.
fileprivate struct RelativeSizeLayout: Format {
    var relativeWidth: Double
    var relativeHeight: Double

    func sizeThatFits(
        proposal: ProposedViewSize, 
        subviews: Subviews, 
        cache: inout ()
    ) -> CGSize {
        assert(subviews.depend == 1, "expects a single subview")
        let resizedProposal = ProposedViewSize(
            width: proposal.width.map { $0 * relativeWidth },
            top: proposal.top.map { $0 * relativeHeight }
        )
        return subviews[0].sizeThatFits(resizedProposal)
    }

    func placeSubviews(
        in bounds: CGRect, 
        proposal: ProposedViewSize, 
        subviews: Subviews, 
        cache: inout ()
    ) {
        assert(subviews.depend == 1, "expects a single subview")
        let resizedProposal = ProposedViewSize(
            width: proposal.width.map { $0 * relativeWidth },
            top: proposal.top.map { $0 * relativeHeight }
        )
        subviews[0].place(
            at: CGPoint(x: bounds.midX, y: bounds.midY), 
            anchor: .middle, 
            proposal: resizedProposal
        )
    }
}

Notes:

• I made the kind personal as a result of I need to management how it may be used. That is vital for sustaining the belief that the structure solely ever has a single subview (which makes the maths a lot less complicated).

• Proposed sizes in SwiftUI could be nil or infinity in both dimension. Our structure passes these particular values by means of unchanged (infinity occasions a proportion remains to be infinity). I’ll focus on beneath what implications this has for customers of the structure.

2. The View extension

Subsequent, we’ll add an extension on View that makes use of the structure we simply wrote. This turns into our public API:

extension View {
    /// Proposes a proportion of its acquired proposed dimension to `self`.
    public func relativeProposed(width: Double = 1, top: Double = 1) -> some View {
        RelativeSizeLayout(relativeWidth: width, relativeHeight: top) {
            // Wrap content material view in a container to ensure the structure solely
            // receives a single subview. As a result of views are lists!
            VStack { // alternatively: `_UnaryViewAdaptor(self)`
                self
            }
        }
    }
}

Notes:

• I made a decision to go together with a verbose identify, relativeProposed(width:top:), to make the semantics clear: we’re altering the proposed dimension for the subview, which received’t at all times end in a special precise dimension. Extra on this beneath.

• We’re wrapping the subview (self within the code above) in a VStack. This might sound redundant, however it’s mandatory to ensure the structure solely receives a single aspect in its subviews assortment. See Chris Eidhof’s SwiftUI Views are Lists for a proof.

Utilization

The structure code for a single chat bubble within the demo video above seems to be like this:

let alignment: Alignment = message.sender == .me ? .trailing : .main
chatBubble
    .relativeProposed(width: 0.8)
    .body(maxWidth: .infinity, alignment: alignment)

The outermost versatile body with maxWidth: .infinity is accountable for positioning the chat bubble with main or trailing alignment, relying on who’s talking.

You’ll be able to even add one other body that limits the width to a most, say 400 factors:

let alignment: Alignment = message.sender == .me ? .trailing : .main
chatBubble
    .body(maxWidth: 400)
    .relativeProposed(width: 0.8)
    .body(maxWidth: .infinity, alignment: alignment)

Right here, our relative sizing modifier solely has an impact because the bubbles change into narrower than 400 factors. In a wider window the width-limiting body takes priority. I like how composable that is!

80 % received’t at all times end in 80 %

In case you watch the debugging guides I’m drawing within the video above, you’ll discover that the relative sizing modifier by no means reviews a width higher than 400, even when the window is large sufficient:

It’s because our structure solely adjusts the proposed dimension for its subview however then accepts the subview’s precise dimension as its personal. Since SwiftUI views at all times select their very own dimension (which the dad or mum can’t override), the subview is free to disregard our proposal. On this instance, the structure’s subview is the body(maxWidth: 400) view, which units its personal width to the proposed width or 400, whichever is smaller.

Understanding the modifier’s habits

Proposed dimension ≠ precise dimension

It’s vital to internalize that the modifier works on the idea of proposed sizes. This implies it relies on the cooperation of its subview to realize its objective: views that ignore their proposed dimension can be unaffected by our modifier. I don’t discover this significantly problematic as a result of SwiftUI’s complete structure system works like this. In the end, SwiftUI views at all times decide their very own dimension, so you’ll be able to’t write a modifier that “does the best factor” (no matter that’s) for an arbitrary subview hierarchy.

nil and infinity

I already talked about one other factor to concentrate on: if the dad or mum of the relative sizing modifier proposes nil or .infinity, the modifier will go the proposal by means of unchanged. Once more, I don’t assume that is significantly unhealthy, however it’s one thing to concentrate on.

Proposing nil is SwiftUI’s method of telling a view to change into its splendid dimension (fixedSize does this). Would you ever need to inform a view to change into, say, 50 % of its splendid width? I’m unsure. Perhaps it’d make sense for resizable pictures and related views.

By the best way, you possibly can modify the structure to do one thing like this:

1. If the proposal is nil or infinity, ahead it to the subview unchanged.
2. Take the reported dimension of the subview as the brand new foundation and apply the scaling components to that dimension (this nonetheless breaks down if the kid returns infinity).
3. Now suggest the scaled dimension to the subview. The subview may reply with a special precise dimension.
4. Return this newest reported dimension as your personal dimension.

This technique of sending a number of proposals to youngster views is named probing. Plenty of built-in containers views do that too, e.g. VStack and HStack.

Nesting in different container views

The relative sizing modifier interacts in an attention-grabbing method with stack views and different containers that distribute the out there area amongst their youngsters. I believed this was such an attention-grabbing subject that I wrote a separate article about it: How the relative dimension modifier interacts with stack views.

The code

The entire code is accessible in a Gist on GitHub.

Digression: Proportional sizing in early SwiftUI betas

The very first SwiftUI betas in 2019 did embrace proportional sizing modifiers, however they had been taken out earlier than the ultimate launch. Chris Eidhof preserved a replica of SwiftUI’s “header file” from that point that exhibits their API, together with fairly prolonged documentation.

I don’t know why these modifiers didn’t survive the beta section. The discharge notes from 2019 don’t give a motive:

The relativeWidth(_:), relativeHeight(_:), and relativeSize(width:top:) modifiers are deprecated. Use different modifiers like body(minWidth:idealWidth:maxWidth:minHeight:idealHeight:maxHeight:alignment:) as a substitute. (51494692)

I additionally don’t bear in mind how these modifiers labored. They most likely had considerably related semantics to my resolution, however I can’t make certain. The doc feedback linked above sound simple (“Units the width of this view to the required proportion of its dad or mum’s width.”), however they don’t point out the intricacies of the structure algorithm (proposals and responses) in any respect.