This Skip framework is available at https://source.skip.tools/skip-ui.git, which can be checked out and tested with skip test once Skip is installed.

SkipUI

SwiftUI support for Skip apps.

About

SkipUI vends the skip.ui Kotlin package. It is a reimplementation of SwiftUI for Kotlin on Android using Jetpack Compose. Its goal is to mirror as much of SwiftUI as possible, allowing Skip developers to use SwiftUI with confidence.

Dependencies

SkipUI depends on the skip transpiler plugin. The transpiler must transpile SkipUI’s own source code, and SkipUI relies on the transpiler’s transformation of SwiftUI code. See Implementation Strategy for details. SkipUI also depends on the SkipFoundation and SkipModel packages.

SkipUI is part of the core SkipStack and is not intended to be imported directly. The module is transparently adopted through the translation of import SwiftUI into import skip.ui.* by the Skip transpiler.

Android Libraries

• SkipUI adds an Android dependency on Coil to implement AsyncImage.
• SkipUI includes source code from the ComposeReorderable project to implement drag-to-reorder in Lists.

Status

SkipUI - together with the Skip transpiler - has robust support for the building blocks of SwiftUI, including its state flow and declarative syntax. SkipUI also implements many of SwiftUI’s basic layout and control views, as well as many core modifiers. It is possible to write an Android app entirely in SwiftUI utilizing SkipUI’s current component set.

SkipUI is a young library, however, and much of SwiftUI’s vast surface area is not yet implemented. You are likely to run into limitations while writing real-world apps. See Supported SwiftUI for a full list of supported components and constructs. Anything not listed there is likely not yet ported.

When you want to use a SwiftUI construct that has not been implemented, you have options. You can try to find a workaround using only supported components, embed Compose code directly, or add support to SkipUI. If you choose to enhance SkipUI itself, please consider contributing your code back for inclusion in the official release.

Contributing

We welcome contributions to SkipUI. The Skip product documentation includes helpful instructions and tips on local Skip library development.

The most pressing need is to implement more core components and view modifiers. View modifiers in particular are a ripe source of low-hanging fruit. The Compose Modifier type often has built-in functions that replicate SwiftUI modifiers, making these SwiftUI modifiers easy to implement. To help fill in unimplemented API in SkipUI:

1. Find unimplemented API. Unimplemented API will either be within #if !SKIP blocks, or will be marked with @available(*, unavailable). Note that most unimplemented View modifiers are in the View.swift source file.
2. Write an appropriate Compose implementation. See Implementation Strategy below.
3. Write tests and/or playground code to exercise your component. See Tests.
4. Submit a PR.

Other forms of contributions such as test cases, comments, and documentation are also welcome!

Implementation Strategy

Code Transformations

SkipUI does not work in isolation. It depends on transformations the skip transpiler plugin makes to SwiftUI code. And while Skip generally strives to write Kotlin that is similar to hand-crafted code, these SwiftUI transformations are not something you’d want to write yourself. Before discussing SkipUI’s implementation, let’s explore them.

Both SwiftUI and Compose are declarative UI frameworks. Both have mechanisms to track state and automatically re-render when state changes. SwiftUI models user interface elements with View objects, however, while Compose models them with @Composable functions. The Skip transpiler must therefore translate your code defining a View graph into @Composable function calls. This involves two primary transformations:

1. The transpiler inserts code to sync View members that have special meanings in SwiftUI - @State, @EnvironmentObject, etc - with the corresponding Compose state mechanisms, which are not member-based. The syncing goes two ways, so that your View members are populated from Compose’s state values, and changing your View members updates Compose’s state values.
2. The transpiler turns @ViewBuilders - including View.body - into @Composable function calls.

The second transformation in particular deserves some explanation, because it may help you to understand SkipUI’s internal API. Consider the following simple example:

struct V: View {
    let isHello: Bool

    var body: some View {
        if isHello {
            Text("Hello!")
        } else {
            Text("Goodbye!")
        }
    }
}

The transpilation would look something like the following:

class V: View {
    val isHello: Bool

    constructor(isHello: Bool) {
        this.isHello = isHello
    }

    override fun body(): View {
        return ComposeView { composectx -> 
            if (isHello) {
                Text("Hello!").Compose(context = composectx)
            } else {
                Text("Goodbye!").Compose(context = composectx)
            }
            ComposeResult.ok
        }
    }

    ...
}

Notice the changes to the body content. Rather than returning an arbitrary view tree, the transpiled body always returns a single ComposeView, a special SkipUI view type that invokes a @Composable block. The logic of the original body is now within that block, and any View that body would have returned instead invokes its own Compose(context:) function to render the corresponding Compose component. The Compose(context:) function is part of SkipUI’s View API.

Thus the transpiler is able to turn any View.body - actually any @ViewBuilder - into a block of Compose code that it can invoke to render the desired content. A later section details how you can use ComposeView yourself to move fluidly between SwiftUI and Compose when writing your Android UI.

Implementation Phases

SkipUI contains stubs for the entire SwiftUI framework. API generally goes through three phases:

1. Code that no one has begun to port to Skip starts in #if !SKIP blocks. This hides it from the Skip transpiler.
2. The first implementation step is to move code out of #if !SKIP blocks so that it will be transpiled. This is helpful on its own, even if you just mark the API @available(*, unavailable) because you are not ready to implement it for Compose. An unavailable attribute will provide Skip users with a clear error message, rather than relying on the Kotlin compiler to complain about unfound API.
   • When moving code out of a #if !SKIP block, please strip Apple’s extensive API comments. There is no reason for Skip to duplicate the official SwiftUI documentation, and it obscures any Skip-specific implementation comments we may add.
   • SwiftUI uses complex generics extensively, and the generics systems of Swift and Kotlin have significant differences. You may have to replace some generics or generic constraints with looser typing in order to transpile successfully. Typing will still be enforced in user code by the Swift compiler.
   • Reducing the number of Swift extensions and instead folding API into the primary declaration of a type can make Skip’s internal symbol storage more efficient. You should, however, leave View modifiers that are specific to a given component - e.g. .navigationTitle is specific to NavigationStack - within the component’s source file.
3. Finally, we add a Compose implementation and remove any unavailable attribute.

Note that SkipUI should remain buildable throughout this process. Being able to successfully compile SkipUI in Xcode helps us validate that our ported components still mesh with the rest of the framework.

Components

Before implementing a component, familiarize yourself with SkipUI’s View protocol in Sources/View/View.swift as well as the files in the Sources/Compose directory. It is also helpful to browse the source code for components and modifiers that have already been ported. See the table of Supported SwiftUI.

The Text view exemplifies a typical SwiftUI component implementation. Here is an abbreviated code sample:

public struct Text: View, Equatable, Sendable {
    let text: String

    public init(_ text: String) {
        self.text = text
    }

    ...

    #if SKIP
    @Composable public override func ComposeContent(context: ComposeContext) {
        let modifier = context.modifier
        let font = EnvironmentValues.shared.font ?? Font(fontImpl: { LocalTextStyle.current })
        ...
        androidx.compose.material3.Text(text: text, modifier: modifier, style: font.fontImpl(), ...)
    }
    #else
    public var body: some View {
        stubView()
    }
    #endif
}

As you can see, the Text type is defined just as it is in SwiftUI. We then use an #if SKIP block to implement the composable View.ComposeContent function for Android, while we stub the body var to satisfy the Swift compiler. ComposeContent makes the necessary Compose calls to render the component, applying the modifier from the given context as well as any applicable environment values. If Text had any child views, ComposeContent would call child.Compose(context: context.content()) to compose its child content. (Note that View.Compose(context:) delegates to View.ComposeContent(context:) after performing other bookkeeping operations, which is why we override ComposeContent rather than Compose.)

Modifiers

Modifiers, on the other hand, use the ComposeModifierView to perform actions, including changing the context passed to the modified view. Here is the .opacity modifier:

extension View {
    public func opacity(_ opacity: Double) -> some View {
        #if SKIP
        return ComposeModifierView(targetView: self) { context in
            context.modifier = context.modifier.alpha(Float(opacity))
        }
        #else
        return self
        #endif
    }
}

Some modifiers have their own composition logic. These modifiers use a different ComposeModifierView constructor whose block defines the composition. Here, for example, .frame composes the view within a Compose Box with the proper dimensions and alignment:

extension View {
    public func frame(width: CGFloat? = nil, height: CGFloat? = nil, alignment: Alignment = .center) -> some View {
        #if SKIP
        return ComposeModifierView(contentView: self) { view, context in
            var modifier = context.modifier
            if let width {
                modifier = modifier.width(width.dp)
            }
            if let height {
                modifier = modifier.height(height.dp)
            }
            let contentContext = context.content()
            ComposeContainer(modifier: modifier, fixedWidth: width != nil, fixedHeight: height != nil) { modifier in
                Box(modifier: modifier, contentAlignment: alignment.asComposeAlignment()) {
                    view.Compose(context: contentContext)
                }
            }
        }
        #else
        return self
        #endif
    }
}

Like other SwiftUI components, modifiers use #if SKIP ... #else ... to stub the Swift implementation and keep SkipUI buildable in Xcode.

ComposeView

ComposeView is an Android-only SwiftUI view that you can use to embed Compose code directly into your SwiftUI view tree. In the following example, we use a SwiftUI Text to write “Hello from SwiftUI”, followed by calling the androidx.compose.material3.Text() Compose function to write “Hello from Compose” below it:

VStack {
    Text("Hello from SwiftUI")
    #if SKIP
    ComposeView { _ in
        androidx.compose.material3.Text("Hello from Compose")
        return .ok
    }
    #endif
}

Skip also enhances all SwiftUI views with a Compose() method, allowing you to use SwiftUI views from within Compose. The following example again uses a SwiftUI Text to write “Hello from SwiftUI”, but this time from within a ComposeView:

#if SKIP
ComposeView { context in 
    androidx.compose.foundation.layout.Column(modifier: context.modifier) {
        Text("Hello from SwiftUI").Compose(context: context.content())
        androidx.compose.material3.Text("Hello from Compose")
    }
    return .ok
}
#endif

Or:

#if SKIP
ComposeView { context in 
    VStack {
        Text("Hello from SwiftUI").Compose(context: context.content())
        androidx.compose.material3.Text("Hello from Compose")
    }.Compose(context: context) // Returns .ok
}
#endif

With ComposeView and the Compose() function, you can move fluidly between SwiftUI and Compose code. These techniques work not only with standard SwiftUI and Compose components, but with your own custom SwiftUI views and Compose functions as well.

Note that ComposeView and the Compose() function are only available in Android, so you must guard all uses with the #if SKIP or #if os(Android) compiler directives.

Tests

SkipUI utilizes a combination of unit tests, UI tests, and basic snapshot tests in which the snapshots are converted into ASCII art for easy processing.

Perhaps the most common way to test SkipUI’s support for a SwiftUI component, however, is through the Skip playground app. Whenever you add or update support for a visible element of SwiftUI, make sure there is a playground that exercises the element. This not only gives us a mechanism to test appearance and behavior, but the playground app becomes a showcase of supported SwiftUI components on Android over time.

Supported SwiftUI

The following table summarizes SkipUI’s SwiftUI support on Android. Note that in your iOS-only code - i.e. code within #if !SKIP blocks - you can use any SwiftUI you want.

• ✅ – Full
• 🟢 – High
• 🟡 – Medium
• 🔴 – Low

Environment Keys

SwiftUI has many built-in environment keys. These keys are defined in EnvironmentValues and typically accessed with the @Environment property wrapper. In additional to supporting your custom environment keys, SkipUI exposes the following built-in environment keys:

• autocorrectionDisabled
• backgroundStyle
• dismiss
• font
• isEnabled
• isSearching
• lineLimit

Gestures

SkipUI currently supports tap, long press, and drag gestures. You can use either the general .gesture modifier or the specialized modifiers like .onTapGesture to add gesture support to your views. The following limitations apply:

• @GestureState is not yet supported. Use the Gesture.onEnded modifier to reset your state.
• Tap counts > 2 are not supported.
• Gesture velocity and predicted end location are always reported as zero and the current location, respectively.
• Only the onChanged and onEnded gesture modifiers are supported.
• Customization of minimum touch duration, distance, etc. is not supported.

Images

SkipUI supports loading images from URLs using SwiftUI’s AsyncImage. Our implementation uses the Coil library to download images on Android.

To display a standard SwiftUI Image, SkipUI currently only supports the Image(systemName:) constructor. The table below details the mapping between iOS and Android system images. Other system names are not supported, though you can display any emoji using Text. These restrictions also apply to other components that load images, such as Label.

Skip cannot yet read iOS asset catalogs, so Image(name:) is not yet available on Android. You can, however, use AsyncImage to display local image resources. This works on both iOS and through Skip on Android. So if you have an image Sources/MyModule/Resources/sample.jpg and your .target in Package.swift properly marks the Resources folder for SPM resource processing:

.target(name: "MyModule", dependencies: ..., resources: [.process("Resources")], plugins: skipstone)

Then the following SwiftUI will display the image on both platforms:

AsyncImage(url: Bundle.module.url(forResource: "sample", withExtension: "jpg"))

If these image display options do not meet your needs, consider embedding Compose code directly until resource loading is implemented.

In Android-only code, you can also supply any androidx.compose.material.icons.Icons image name as the systemName. For example:

#if SKIP
Image(systemName: "Icons.Filled.Settings")
#endif

Lists

SwiftUI Lists are powerful and flexible components. SkipUI currently supports the following patterns for specifying List content.

Static content. Embed a child view for each row directly within the List:

List {
    Text("Row 1")
    Text("Row 2")
    Text("Row 3")
}

Indexed content. Specify an Int range and a closure to create a row for each index:

List(1...100) { index in 
    Text("Row \(index)")
}

Collection content. Supply any RandomAccessCollection - typically an Array - and a closure to create a row for each element. If the elements do not implement the Identifiable protocol, specify the key path to a property that can be used to uniquely identify each element:

List([person1, person2, person3], id: \.fullName) { person in
    HStack {
        Text(person.fullName)
        Spacer()
        Text(person.age)
    } 
}

ForEach content. Use ForEach to specify indexed or collection content. This allows you to mix content types.

List {
    Text("People").bold()
    ForEach([person1, person2, person3], id: \.fullName) { person in
        HStack {
            Text(person.fullName)
            Spacer()
            Text(person.age)
        }
    }
}

When using collection content or a ForEach with collection content, you can enable swipe-to-delete and drag-to-reorder by supplying a binding to the collection and the appropriate set of edit actions.

List($people, id: \.fullName, editActions: .all) { $person in
    Text(person.fullName)
        .deleteDisabled(!person.isDeletable)
    }
}

You can also enable editing by using a ForEach with the .onDelete and .onMove modifiers. Make sure your ForEach also supplies an id for each item.

List Limitations

• Compose requires that every id value in a List is unique. This applies even if your list consists of multiple Sections or uses multiple ForEach components to define its content.
• Additionally, id values must follow our Restrictions on Identifiers.
• Section and ForEach views must be defined inline within their owning List. In other words, if your List contains MyView, MyView will be rendered as a single list row even if it contains Section or ForEach content.
• SkipUI does not support placing modifiers on Section or ForEach views within lists, other than ForEach.onDelete and ForEach.onMove.

Navigation

SwiftUI has three primary forms of navigation: TabView, NavigationStack, and modal presentations. SkipUI has implemented all three, albeit with the restrictions explained below.

SkipUI’s TabView does yet not support SwiftUI’s overflow tab behavior. Adding too many tabs will just result in too many tabs rather than SwiftUI’s automatic “More” tab.

Otherwise, TabView acts as you would expect. NavigationStack, however, has several restrictions you must be aware of.

In SwiftUI, you push vies onto a NavigationStack with NavigationLink. NavigationLink has two ways to specify its destination view: embedding the view directly, or specifying a value that is mapped to a view through the .navigationDestination modifier, as in the following code sample:

NavigationStack {
    ListView()
        .navigationTitle(Self.title)
}

struct ListView : View {
    var body: some View {
        List(City.allCases) { city in
            NavigationLink(value: city) {
                rowView(city: city)
            }
        }
        .navigationDestination(for: City.self) { city in
            CityView(city: city)
        }
    }
}

SkipUI does not support embedding a destination view directly in a NavigationLink, and future support may prove difficult. Compose forces you to define fixed navigation routes, making any dynamic navigation a challenge.

SkipUI does support navigation with .navigationDestination as in the example above, because we can map each modifier to a fixed Compose navigation route. Even this support, however, required some abuse of Compose’s system in order to allow newly-pushed views to defined additional .navigationDestinations. In fact, it is currently the case that if a pushed view defines a new .navigationDestination for key type T, it will overwrite any previous stack view’s T destination mapping. Take care not to unintentionally re-map the same key type in the same navigation stack.

Compose imposes an additional restriction as well: we must be able to stringify .navigationDestination key types. See Restrictions on Identifiers below.

Finally, SkipUI does not yet support binding to an array of destination values to specify the navigation stack.

For modal presentations, SkipUI supports the .sheet(isPresented:onDismiss:content:) modifier only. We will add support for other forms of modal presentations in the future.

Restrictions on Identifiers

Compose requires all state values to be serializable. This restriction is typically transparent to your code, because when you use property wrappers like @State, SkipUI automatically tracks your state objects and gives Compose serializable identifiers in their place. Some SwiftUI values, however, must be stored directly in Compose, including navigationDestination values and List item identifiers. When this is the case, SkipUI creates a String from the value you supply using the following algorithm:

• If the value is Identifiable, use String(describing: value.id)
• If the value is RawRepresentable, use String(describing: value.rawValue)
• Else use String(describing: value)

Please ensure that when using these API, the above algorithm will create unique, stable strings for unique values.