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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 are able to discover out so much about how SwiftUI works is SwiftUI’s .swiftinterface file. That is positioned inside 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 right down to a single implementation:

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


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

After 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 little one. When the content material mode is .match, it matches a rectangle with the specified side ratio contained in the proposed dimension. For instance, while you suggest 100×100, it can suggest 100×56.2 to its little one. Whenever you select .fill as an alternative, it can suggest 177.8×100 to its little one as an alternative.

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

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

								Picture("take a look at")
    .aspectRatio(contentMode: .match)


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

After we do not specify a hard and fast side ratio however use nil for the parameter, the side ratio modifier appears on the superb 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 superb 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 usually are not a part of the general public API of SwiftUI. Regardless that you completely want to know how this works with a purpose to write efficient layouts, this is not actually documented. The one official place the place this habits is described is within the wonderful 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 side ratio and print the end result. As a substitute of a .body, it’s also possible to use .fixedSize() to suggest nil for the width and/or peak. Likewise, attempt leaving out the primary parameter and see how .aspectRatio proposes nil to determine the best dimension of its little one view.

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


Sadly the width and peak 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 complete code is in a gist.

If you wish to be taught 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 in your crew.



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