Intrinsic content size in SwiftUI

When I learn SwiftUI, I try to find out how to do things I can do in UIKit. Today we going to explore one of the essential parts when you make a custom view on UIKit, intrinsicContentSize.

What is an intrinsic content size?

For those who have experienced in UIKit and Auto Layout^[1], you can skip to the next section.

Intrinsic content size is an instance property in UIView. Following is a description from Apple Documentation:

The natural size for the receiving view, considering only properties of the view itself.

It is easier to understand this property from the real examples. The natural size of UILabel is the size of the text it contains. The natural size of UIActivityIndicatorView is fixed at 20x20 points. The value from intrinsicContentSize is an amount of space the view needs for its content to display in an ideal state. The value return from intrinsicContentSize isn't final. You can override this value to whatever you want by setting width or height constraints.

When I make custom views, I always use intrinsicContentSize. In SwiftUI, it even easier to create a custom view, and Apple seems to encourage us to make one, so I want to know if SwiftUI has something equivalent or similar.

Fortunately, the answer is yes.

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.frame

To define an intrinsic content size in SwiftUI view, you need frame(minWidth:idealWidth:maxWidth:minHeight:idealHeight:maxHeight:alignment:).

func frame(minWidth: CGFloat? = nil, idealWidth: CGFloat? = nil, maxWidth: CGFloat? = nil, minHeight: CGFloat? = nil, idealHeight: CGFloat? = nil, maxHeight: CGFloat? = nil, alignment: Alignment = .center) -> some View

You can ignore most of its parameters. What we need here are idealHeight and idealWidth.

struct CustomView: View {
    var body: some View {
        Rectangle()
            .frame(idealWidth: 200, idealHeight: 200)
            .foregroundColor(Color.pink)            
    }
}

In the above example, we create a custom view with the ideal size of 200x200 points. Replace Rectangle with the view we just created.

struct ContentView: View {
    var body: some View {
        CustomView()
    }
}

Even we define ideal width and height, SwiftUI doesn't respect that, as you can see in the following screenshot:

.fixedSize

Setting view ideal size is a way to modify the view property. To enforce the ideal size, you have to use .fixedSize function.

From the Apple Document, .fixedSize fixes the view at its ideal size. This is what we need to apply the ideal size.

Fixes the view at its ideal size.

struct ContentView: View {
    var body: some View {
        CustomView()
            .fixedSize()
    }
}

There is another variation fixedSize(horizontal:vertical:) which fixes the view at its ideal size in the specified dimensions.

Fixes the view at its ideal size in the specified dimensions.

The following are the samples of fixed horizontal and vertical.

Fixed horizontal:

struct ContentView: View {
    var body: some View {
        CustomView()
            .fixedSize(horizontal: true, vertical: false)
    }
}

Fixed vertical:

struct ContentView: View {
    var body: some View {
        CustomView()
            .fixedSize(horizontal: false, vertical: true)
    }
}

Override the size

If you want to override the ideal size, you just set a view .frame without .fixedSize modifier.

CustomView()
    .frame(width: 50, height: 50)

Or if you want just one axis of ideal size, you can fix one axis and set .frame to another one.

Fixed horizontal and override height:

CustomView()
    .fixedSize(horizontal: true, vertical: false)
    .frame(height: 50)

Fixed vertical and override width:

CustomView()
    .fixedSize(horizontal: false, vertical: true)
    .frame(width: 50)

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Related Resources

SwiftUI's ViewModifier
Auto Layout Guide
intrinsicContentSize
frame(minWidth:idealWidth:maxWidth:minHeight:idealHeight:maxHeight:alignment:)
fixedSize()