Working with percentages in SwiftUI structure – Ole Begemann

SwiftUI’s structure primitives typically 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

Contemplate this chat dialog view for instance of what I need to construct. The chat bubbles at all times stay 80 % as broad as their container because the view is resized:

Constructing a proportional sizing modifier

1. The Structure

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

/// A customized structure that proposes a share of its
/// obtained proposed measurement to its subview.
///
/// - Precondition: should comprise precisely one subview.
fileprivate struct RelativeSizeLayout: Structure {
    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 },
            peak: proposal.peak.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 },
            peak: proposal.peak.map { $0 * relativeHeight }
        )
        subviews[0].place(
            at: CGPoint(x: bounds.midX, y: bounds.midY), 
            anchor: .heart, 
            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 idea that the structure solely ever has a single subview (which makes the mathematics a lot easier).

• Proposed sizes in SwiftUI might be nil or infinity in both dimension. Our structure passes these particular values by way of unchanged (infinity occasions a share continues to be infinity). I’ll focus on under 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 share of its obtained proposed measurement to `self`.
    public func relativeProposed(width: Double = 1, peak: Double = 1) -> some View {
        RelativeSizeLayout(relativeWidth: width, relativeHeight: peak) {
            // 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 title, relativeProposed(width:peak:), to make the semantics clear: we’re altering the proposed measurement for the subview, which received’t at all times lead to a unique precise measurement. Extra on this under.

• We’re wrapping the subview (self within the code above) in a VStack. This may appear redundant, however it’s mandatory to ensure the structure solely receives a single factor 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 appears to be like 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 liable for positioning the chat bubble with main or trailing alignment, relying on who’s talking.

You may 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 turn 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 lead to 80 %

Should you watch the debugging guides I’m drawing within the video above, you’ll discover that the relative sizing modifier by no means experiences a width larger than 400, even when the window is broad sufficient:

It’s because our structure solely adjusts the proposed measurement for its subview however then accepts the subview’s precise measurement as its personal. Since SwiftUI views at all times select their very own measurement (which the guardian 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 conduct

Proposed measurement ≠ precise measurement

It’s vital to internalize that the modifier works on the idea of proposed sizes. This implies it depends upon the cooperation of its subview to attain its aim: views that ignore their proposed measurement can be unaffected by our modifier. I don’t discover this notably problematic as a result of SwiftUI’s whole structure system works like this. In the end, SwiftUI views at all times decide their very own measurement, so you possibly can’t write a modifier that “does the precise factor” (no matter that’s) for an arbitrary subview hierarchy.

nil and infinity

I already talked about one other factor to pay attention to: if the guardian of the relative sizing modifier proposes nil or .infinity, the modifier will cross the proposal by way of unchanged. Once more, I don’t suppose that is notably unhealthy, however it’s one thing to pay attention to.

Proposing nil is SwiftUI’s approach of telling a view to turn into its very best measurement (fixedSize does this). Would you ever need to inform a view to turn into, say, 50 % of its very best width? I’m undecided. Possibly it’d make sense for resizable pictures and related views.

By the way in which, you may 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 measurement of the subview as the brand new foundation and apply the scaling components to that measurement (this nonetheless breaks down if the kid returns infinity).
3. Now suggest the scaled measurement to the subview. The subview would possibly reply with a unique precise measurement.
4. Return this newest reported measurement as your personal measurement.

This strategy of sending a number of proposals to youngster views known as 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 fascinating approach with stack views and different containers that distribute the obtainable area amongst their youngsters. I believed this was such an fascinating matter that I wrote a separate article about it: How the relative measurement modifier interacts with stack views.

The code

The entire code is obtainable 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 reveals 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 cause:

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

I additionally don’t keep in mind how these modifiers labored. They in all probability had considerably related semantics to my resolution, however I can’t be certain. The doc feedback linked above sound easy (“Units the width of this view to the required proportion of its guardian’s width.”), however they don’t point out the intricacies of the structure algorithm (proposals and responses) in any respect.

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