Working with percentages in SwiftUI format – Ole Begemann

SwiftUI’s format 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

Think about this chat dialog view for instance of what I need to construct. The chat bubbles at all times stay 80 % as huge 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 format 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 format that proposes a proportion of its
/// obtained proposed dimension 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 need to management how it may be used. That is vital for sustaining the belief that the format solely ever has a single subview (which makes the mathematics a lot easier).

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

2. The View extension

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

extension View {
    /// Proposes a proportion of its obtained 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 verify the format solely
            // receives a single subview. As a result of views are lists!
            VStack { // alternatively: `_UnaryViewAdaptor(self)`
                self
            }
        }
    }
}

Notes:

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

• We’re wrapping the subview (self within the code above) in a VStack. This may appear redundant, but it surely’s needed to verify the format solely receives a single ingredient in its subviews assortment. See Chris Eidhof’s SwiftUI Views are Lists for an evidence.

Utilization

The format 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 liable 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 % gained’t at all times end in 80 %

For those who 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 better than 400, even when the window is huge sufficient:

It is because our format 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 format’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 premise of proposed sizes. This implies it is determined by the cooperation of its subview to attain its purpose: views that ignore their proposed dimension will likely be unaffected by our modifier. I don’t discover this notably problematic as a result of SwiftUI’s complete format system works like this. Finally, SwiftUI views at all times decide their very own dimension, so you may’t write a modifier that “does the correct 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 cross the proposal by means of unchanged. Once more, I don’t assume that is notably dangerous, but it surely’s one thing to concentrate on.

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

By the way in which, you would modify the format 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 distinct precise dimension.
4. Return this newest reported dimension as your individual dimension.

This means of sending a number of proposals to little one 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 attention-grabbing approach with stack views and different containers that distribute the out there area amongst their youngsters. I assumed 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 obtainable 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 duplicate 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 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 keep in mind how these modifiers labored. They most likely had considerably comparable semantics to my answer, however I can’t make certain. The doc feedback linked above sound easy (“Units the width of this view to the desired proportion of its dad or mum’s width.”), however they don’t point out the intricacies of the format algorithm (proposals and responses) in any respect.