Working with percentages in SwiftUI format – Ole Begemann

SwiftUI’s format 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

Contemplate this chat dialog view for example of what I need to construct. The chat bubbles all the time stay 80 % as vast as their container because the view is resized:

Constructing a proportional sizing modifier

1. The Structure

We will construct our personal relative sizing modifier on prime of the Structure protocol. The format multiplies its personal proposed dimension (which it receives from its father or mother view) with the given elements for width and top. It then proposes this modified dimension to its solely subview. Right here’s the implementation (the complete code, together with the demo app, is on GitHub):

/// A customized format that proposes a share of its
/// obtained proposed dimension to its subview.
///
/// - Precondition: should include 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 non-public as a result of I need to management how it may be used. That is necessary for sustaining the idea that the format solely ever has a single subview (which makes the maths a lot easier).

• Proposed sizes in SwiftUI will be nil or infinity in both dimension. Our format passes these particular values by means of unchanged (infinity instances a share remains to be infinity). I’ll talk about 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 share 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 ensure 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 title, relativeProposed(width:top:), to make the semantics clear: we’re altering the proposed dimension for the subview, which gained’t all the time lead to a unique precise dimension. Extra on this under.

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

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 accountable 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 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 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 higher than 400, even when the window is vast 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 all the time select their very own dimension (which the father or mother 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 conduct

Proposed dimension ≠ precise dimension

It’s necessary 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 objective: views that ignore their proposed dimension shall be unaffected by our modifier. I don’t discover this notably problematic as a result of SwiftUI’s complete format system works like this. In the end, SwiftUI views all the time decide their very own dimension, so you possibly can’t write a modifier that “does the fitting 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 father or mother of the relative sizing modifier proposes nil or .infinity, the modifier will move the proposal by means of unchanged. Once more, I don’t suppose that is notably dangerous, however it’s one thing to concentrate on.

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

By the way in which, you may 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 elements to that dimension (this nonetheless breaks down if the kid returns infinity).
3. Now suggest the scaled dimension to the subview. The subview would possibly reply with a unique precise dimension.
4. Return this newest reported dimension as your personal dimension.

This means of sending a number of proposals to baby views known as probing. Numerous 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 method with stack views and different containers that distribute the out there house amongst their kids. I believed this was such an fascinating matter that I wrote a separate article about it: How the relative dimension modifier interacts with stack views.

The code

The whole code is offered 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 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 an alternative. (51494692)

I additionally don’t keep in mind how these modifiers labored. They in all probability had considerably comparable semantics to my answer, however I can’t make sure. The doc feedback linked above sound simple (“Units the width of this view to the desired proportion of its father or mother’s width.”), however they don’t point out the intricacies of the format algorithm (proposals and responses) in any respect.

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