I’ve yet another factor to say on the relative sizing view modifier from my earlier publish, Working with percentages in SwiftUI format. I’m assuming you’ve learn that article. The next is nice to know if you wish to use the modifier in your personal code, however I hope you’ll additionally be taught some common tidbits about SwiftUI’s format 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) {
    Shade.blue
        .relativeProposed(width: 0.5)
    Shade.inexperienced
    Shade.yellow
}
.border(.major)
.body(peak: 80)

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

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

Order issues

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

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

Naively, I’d anticipate an equal outcome: the inexperienced rectangle ought to change into 100 pt large, 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 in case you perceive that the HStack processes its kids in sequence:

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

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 scale of its little one and returns 100 pt to the HStack.

3. Because the second subview used much less area than proposed, the HStack now has 300 pt left over to suggest to its ultimate little one, the yellow coloration.

Essential: the order wherein 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. Generally, HStacks and VStacks first group their subviews by format precedence (extra on that beneath), 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 Kids 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 have the option guess how the format seems once we transfer our view modifier to the final little one view:

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

• Blue and inexperienced every obtain one third of the accessible width and change into 200 pt large. 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 top 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” resolution. The stack makes itself simply large enough to include its subviews (= 520 pt incl. spacing) and stories that measurement to its mother or father.

Structure precedence

We will 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 the next format precedence (the default precedence is 0):

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

This ends in a format the place the yellow rectangle truly takes up 50 % of the accessible area:

Clarification:

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

2. The primary format group solely accommodates a single view, our relative sizing modifier with the yellow coloration. The HStack proposes all the accessible 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 format 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 photographs on this article is out there on GitHub. I solely checked out HStacks right here, however VStacks work in precisely the identical means for the vertical dimension.

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

A concrete lesson I’m taking away from this evaluation: HStack and VStack don’t deal with format as an optimization drawback that tries to seek out the optimum resolution for a set of constraints (autolayout model). Slightly, they kind their kids in a selected means after which do a single proposal-and-response move over them. If there’s area leftover on the finish, or if the accessible area isn’t sufficient, then so be it.