How the relative measurement modifier interacts with stack views – Ole Begemann

I’ve another factor to say on the relative sizing view modifier from my earlier put up, Working with percentages in SwiftUI structure. I’m assuming you’ve learn that article. The next is nice to know if you wish to use the modifier in your individual code, however I hope you’ll additionally be taught some basic tidbits about SwiftUI’s structure algorithm for HStacks and VStacks.

Utilizing relative sizing inside a stack view

Let’s apply the relativeProposed modifier to one of many subviews of an HStack:

HStack(spacing: 10) {
    Coloration.blue
        .relativeProposed(width: 0.5)
    Coloration.inexperienced
    Coloration.yellow
}
.border(.major)
.body(top: 80)

What do you count on to occur right here? Will the blue view take up 50 % of the obtainable width? The reply is not any. In reality, the blue rectangle turns into narrower than the others:

It’s because the HStack solely proposes a proportion of its obtainable width to every of its kids. Right here, the stack proposes one third of the obtainable area to its first youngster, the relative sizing modifier. The modifier then halves this worth, leading to one sixth of the whole width (minus spacing) for the blue coloration. The opposite two rectangles then change into wider than one third as a result of the primary youngster view didn’t expend its full proposed width.

Order issues

Now let’s transfer the modifier to the inexperienced coloration within the center:

HStack(spacing: 10) {
    Coloration.blue
    Coloration.inexperienced
        .relativeProposed(width: 0.5)
    Coloration.yellow
}

Naively, I’d count on an equal end result: the inexperienced rectangle ought to change into 100 pt vast, and blue and yellow ought to be 250 pt every. However that’s not what occurs — the yellow view finally ends up being wider than the blue one:

I discovered this unintuitive at first, but it surely is smart if you happen to perceive that the HStack processes its kids in sequence:

1. The HStack proposes one third of its obtainable area to the blue view: (620 – 20) / 3 = 200. The blue view accepts the proposal and turns into 200 pt vast.

2. Subsequent up is the relativeProposed modifier. The HStack divides the remaining area by the variety of remaining subviews and proposes that: 400 / 2 = 200. Our modifier halves this proposal and proposes 100 pt to the inexperienced view, which accepts it. The modifier in flip adopts the dimensions of its youngster and returns 100 pt to the HStack.

3. For the reason that second subview used much less area than proposed, the HStack now has 300 pt left over to suggest to its closing youngster, the yellow coloration.

Necessary: the order by which the stack lays out its subviews occurs to be from left to proper on this instance, however that’s not all the time the case. Usually, HStacks and VStacks first group their subviews by structure precedence (extra on that under), after which order the views inside every group by flexibility such that the least versatile views are laid out first. For extra on this, see How an HStack Lays out Its Youngsters by Chris Eidhof. The views in our instance are all equally versatile (all of them can change into any width between 0 and infinity), so the stack processes them of their “pure” order.

Leftover area isn’t redistributed

By now you could find a way guess how the structure seems once we transfer our view modifier to the final youngster view:

HStack(spacing: 10) {
    Coloration.blue
    Coloration.inexperienced
    Coloration.yellow
        .relativeProposed(width: 0.5)
}

• Blue and inexperienced every obtain one third of the obtainable width and change into 200 pt vast. No surprises there.

• When the HStack reaches the relativeProposed modifier, it has 200 pt left to distribute. Once more, the modifier and the yellow rectangle solely use half of this quantity.

The tip result’s that the HStack finally ends up with 100 pt left over. The method stops right here — the HStack does not begin over in an try and discover a “higher” answer. The stack makes itself simply large enough to comprise its subviews (= 520 pt incl. spacing) and experiences that measurement to its dad or mum.

Structure precedence

We are able to use the layoutPriority view modifier to affect how stacks and different containers lay out their kids. Let’s give the subview with the relative sizing modifier a better structure precedence (the default precedence is 0):

HStack(spacing: 10) {
    Coloration.blue
    Coloration.inexperienced
    Coloration.yellow
        .relativeProposed(width: 0.5)
        .layoutPriority(1)
}

This ends in a structure the place the yellow rectangle really takes up 50 % of the obtainable area:

Rationalization:

1. The HStack teams its kids by structure precedence after which processes every group in sequence, from highest to lowest precedence. Every group is proposed the whole remaining area.

2. The primary structure group solely incorporates a single view, our relative sizing modifier with the yellow coloration. The HStack proposes all the obtainable area (minus spacing) = 600 pt. Our modifier halves the proposal, leading to 300 pt for the yellow view.

3. There are 300 pt left over for the second structure group. These are distributed equally among the many two kids as a result of every subview accepts the proposed measurement.

Conclusion

The code I used to generate the pictures on this article is offered on GitHub. I solely checked out HStacks right here, however VStacks work in precisely the identical manner for the vertical dimension.

SwiftUI’s structure algorithm all the time follows this primary sample of proposed sizes and responses. Every of the built-in “primitive” views (e.g. fastened and versatile frames, stacks, Textual content, Picture, Spacer, shapes, padding, background, overlay) has a well-defined (if not all the time well-documented) structure conduct that may be expressed as a perform (ProposedViewSize) -> CGSize. You’ll have to be taught the conduct for view to work successfully with SwiftUI.

A concrete lesson I’m taking away from this evaluation: HStack and VStack don’t deal with structure as an optimization downside that tries to search out the optimum answer for a set of constraints (autolayout type). Fairly, they type their kids in a specific manner after which do a single proposal-and-response cross over them. If there’s area leftover on the finish, or if the obtainable area isn’t sufficient, then so be it.