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

Take into account this chat dialog view for example of what I wish to construct. The chat bubbles all the time stay 80 % as extensive 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 prime of the Structure protocol. The structure multiplies its personal proposed measurement (which it receives from its guardian view) with the given elements for width and top. It then proposes this modified measurement 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 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.rely == 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.rely == 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 sort personal as a result of I wish to management how it may be used. That is necessary 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 way of unchanged (infinity occasions a share continues to be infinity). I’ll talk about 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 share of its obtained proposed measurement 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 title, relativeProposed(width:top:), to make the semantics clear: we’re altering the proposed measurement for the subview, which gained’t all the time lead to a distinct precise measurement. Extra on this beneath.

• We’re wrapping the subview (self within the code above) in a VStack. This may appear redundant, however it’s obligatory to ensure the structure solely receives a single component in its subviews assortment. See Chris Eidhof’s SwiftUI Views are Lists for an evidence.

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 answerable 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 % gained’t all the time 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 reviews a width larger than 400, even when the window is extensive sufficient:

It is 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 all the time 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 necessary to internalize that the modifier works on the idea of proposed sizes. This implies it is determined by the cooperation of its subview to attain its aim: views that ignore their proposed measurement will probably be unaffected by our modifier. I don’t discover this notably problematic as a result of SwiftUI’s whole structure system works like this. Finally, SwiftUI views all the time decide their very own measurement, so you’ll be able to’t write a modifier that “does the appropriate 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 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 assume that is notably unhealthy, however it’s one thing to concentrate on.

Proposing nil is SwiftUI’s means of telling a view to turn into its very best measurement (fixedSize does this). Would you ever wish to inform a view to turn into, say, 50 % of its very best width? I’m unsure. Perhaps it’d make sense for resizable photographs and comparable views.

By the way in which, you could possibly 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 elements 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 distinct precise measurement.
4. Return this newest reported measurement as your personal measurement.

This technique of sending a number of proposals to baby views known as probing. A number 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 means with stack views and different containers that distribute the accessible 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 on the market in a Gist on GitHub.

Digression: Proportional sizing in early SwiftUI betas

The very first SwiftUI betas in 2019 did embody proportional sizing modifiers, however they have 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 part. The discharge notes from 2019 don’t give a purpose:

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 keep in mind how these modifiers labored. They most likely had considerably comparable semantics to my answer, however I can’t make sure. The doc feedback linked above sound easy (“Units the width of this view to the desired 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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