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Side Ratios in SwiftUI ·

One of many modifiers that all the time puzzled me a bit was .aspectRatio. How does it actually work? As soon as I figured it out, it turned out to be easier than I assumed.

One place the place we will discover out rather a lot about how SwiftUI works is SwiftUI’s .swiftinterface file. That is positioned within Xcode. Inside your Terminal, go to /Functions/, and carry out the next command:

								discover . -path "*/SwiftUI.framework*swiftinterface"


There are just a few variants of the .aspectRatio API, however all of them boil all the way down to a single implementation:

								func aspectRatio(_ aspectRatio: CGFloat?, contentMode: ContentMode) -> some View {


The variant with CGSize simply calls this methodology with dimension.width/, and .scaledToFit and .scaledToFill name this methodology with the respective content material modes and an aspectRatio of nil.

Once we name aspectRatio with a hard and fast side ratio, e.g. .aspectRatio(16/9, contentMode: .match), the side ratio implementation takes the proposed dimension, and proposes a brand new dimension to its youngster. When the content material mode is .match, it matches a rectangle with the specified side ratio contained in the proposed dimension. For instance, whenever you suggest 100×100, it is going to suggest 100×56.2 to its youngster. While you select .fill as a substitute, it is going to suggest 177.8×100 to its youngster as a substitute.

I found out this conduct by printing the proposed sizes. Extra on that under.

Maybe the commonest use of aspectRatio is mixed with a resizable picture, like so:

    .aspectRatio(contentMode: .match)


This may draw the picture to suit inside the proposed dimension. Word that we don’t specify the precise side ratio: it’s derived from the underlying picture.

Once we do not specify a hard and fast side ratio however use nil for the parameter, the side ratio modifier seems to be on the ideally suited dimension of the underlying view. This implies it merely proposes nil×nil to the underlying view, and makes use of the results of that to find out the side ratio. For instance, when the picture experiences its ideally suited dimension as 100×50, the computed side ratio is 100/50.

The method then continues like earlier than: when the view was proposed 320×480, the picture will likely be sized to 320×160 when the content material mode is ready to .match, and 960×480 when the content material mode is ready to .fill.

Determining proposed sizes

Proposed sizes aren’t a part of the general public API of SwiftUI. Regardless that you completely want to grasp how this works with a view to write efficient layouts, this is not actually documented. The one official place the place this conduct is described is within the glorious 2019 WWDC discuss Constructing Customized Views with SwiftUI.

Nonetheless, there’s a hack to do that. Contained in the interface file talked about above, I looked for “ProposedSize” and located a protocol named _ArchivableView which permits us to override sizeThatFits:

								struct MySample: _ArchivableView {
    var physique: some View {
    func sizeThatFits(in proposedSize: _ProposedSize) -> CGSize {
        return proposedSize.orDefault


We are able to now merely assemble a MySample with a facet ratio and print the end result. As an alternative of a .body, it’s also possible to use .fixedSize() to suggest nil for the width and/or top. Likewise, attempt leaving out the primary parameter and see how .aspectRatio proposes nil to determine the perfect dimension of its youngster view.

    .aspectRatio(100/50, contentMode: .fill)
    .body(width: 320, top: 480)


Sadly the width and top properties on _ProposedSize aren’t seen within the swift interface, so I had to make use of introspection to print these (and in addition add just a few helper strategies like .fairly and .orDefault). The total code is in a gist.

If you wish to study extra about how SwiftUI works, learn our e-book Considering in SwiftUI. When your organization is already constructing issues in SwiftUI — or is about to get began — contemplate reserving a SwiftUI Workshop to your staff.



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