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Side Ratios in SwiftUI ·

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

One place the place we are able to discover out loads about how SwiftUI works is SwiftUI’s .swiftinterface file. That is positioned inside Xcode. Inside your Terminal, go to /Purposes/, and carry out the next command:

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


There are a number of 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 set 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 suits a rectangle with the specified side ratio contained in the proposed dimension. For instance, once you suggest 100×100, it is going to suggest 100×56.2 to its little one. Whenever you select .fill as a substitute, it is going to suggest 177.8×100 to its little one as a substitute.

I discovered this habits by printing the proposed sizes. Extra on that beneath.

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

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


It will 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.

After we do not specify a set side ratio however use nil for the parameter, the side ratio modifier appears on the excellent 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 excellent 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 shall be sized to 320×160 when the content material mode is about to .match, and 960×480 when the content material mode is about to .fill.

Determining proposed sizes

Proposed sizes usually are not a part of the general public API of SwiftUI. Although you completely want to grasp how this works in an effort to write efficient layouts, this is not actually documented. The one official place the place this habits 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 will now merely assemble a MySample with a side ratio and print the end result. As a substitute of a .body, you can too use .fixedSize() to suggest nil for the width and/or peak. Likewise, strive leaving out the primary parameter and see how .aspectRatio proposes nil to determine the perfect 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 a number of 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 ebook Considering in SwiftUI. When your organization is already constructing issues in SwiftUI — or is about to get began — think about reserving a SwiftUI Workshop on your staff.


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