objc.io

00:06 Today we'll start building a token view. We can find such an input view in the Mac Mail app, which lets us type in a search term, and it turns the term into a blue bubble. The original Mac version is implemented using NSTextView, which takes care of a lot of the functionality, but we want to see how far we can get in pure SwiftUI.

00:56 Currently, one of the problems with the state of SwiftUI is that we can't really use an existing component, like NSTextView, and customize it for our use case. So, because we can't hook into a text field, we have to basically start from scratch.

Token Views

01:22 Our TokenField view will be generic over a Token type, which is Identifiable, and over a TokenView:

struct TokenField<Token: Identifiable, TokenView: View>: View {
    // ...
}

01:43 We add properties to accept an array of tokens and a view builder. We call the view builder to generate a view for each token, placing those views in an HStack:

struct TokenField<Token: Identifiable, TokenView: View>: View {
    var tokens: [Token]
    @ViewBuilder var tokenView: (Token) -> TokenView
    
    var body: some View {
        HStack {
            ForEach(tokens) { token in
                tokenView(token)
            }
        }
    }
}

02:22 We write a sample MyToken struct, and we create some sample values to work with:

struct MyToken: Identifiable {
    var id = UUID()
    var text: String
}

let sample = [
    "mail@objc.io",
    "mail@apple.com",
    "test@test.example"
].map { MyToken(text: $0) }

02:52 In our ContentView, we add a TokenField, and we pass in the sample tokens:

struct ContentView: View {
    var body: some View {
        TokenField(tokens: sample) { token in
            // ...
        }
        .padding()
    }
}

03:13 In the view builder closure of the TokenView, we output each token's text in a Text view with some small padding and a light blue background with rounded corners:

struct ContentView: View {
    var body: some View {
        TokenField(tokens: sample) { token in
            Text(token.text)
                .padding(.horizontal, 8)
                .padding(.vertical, 4)
                .background(Color.blue.opacity(0.2), in: .rect(cornerRadius: 4))
        }
        .padding()
    }
}

TODO screenshot at 04:35

Insertion Point

04:37 Next, we want to be able to navigate the token field with a cursor, or an insertion point. Since we're not in an actual text field, we'll fake the cursor by creating our own view for it. The position of this cursor will be defined by a state property, selection. For now, we'll use an integer to represent this position, where 0 means the cursor is in front of the first token view:

struct TokenField<Token: Identifiable, TokenView: View>: View {
    var tokens: [Token]
    @ViewBuilder var tokenView: (Token) -> TokenView
    
    @State private var selection = 0

    // ...
}

06:04 We create an InsertionPoint view consisting of a rectangle that's two points wide, and we add it in an overlay over the HStack of tokens:

struct TokenField<Token: Identifiable, TokenView: View>: View {
    var tokens: [Token]
    @ViewBuilder var tokenView: (Token) -> TokenView
    
    @State private var selection = 0

    var body: some View {
        HStack {
            ForEach(tokens) { token in
                tokenView(token)
            }
        }
        .overlay {
            InsertionPoint()
        }
    }
}

struct InsertionPoint: View {
    var body: some View {
        Color.blue
            .frame(width: 2)
    }
}

06:47 The insertion point shows up in the middle, but we want to align it to the selection. Let's do this by applying a matched geometry effect between the tokens and the insertion point. We add the effect to each token view using the token's ID as the identifier for the effect:

struct TokenField<Token: Identifiable, TokenView: View>: View {
    var tokens: [Token]
    @ViewBuilder var tokenView: (Token) -> TokenView
    
    @State private var selection = 0
    @Namespace private var namespace

    var body: some View {
        HStack {
            ForEach(tokens) { token in
                tokenView(token)
                    .matchedGeometryEffect(id: token.id, in: namespace)
            }
        }
        .overlay {
            InsertionPoint()
        }
    }
}

07:31 One of these instances of the matched geometry effect will be used as the source for the insertion point's geometry, depending on the selected token. We add a computed property that returns the ID of the token at the position marked by selection, so we can use it for the insertion point's matched geometry effect. We also define this effect to not be the source, so that it will take on the selected token view's geometry:

struct TokenField<Token: Identifiable, TokenView: View>: View {
    var tokens: [Token]
    @ViewBuilder var tokenView: (Token) -> TokenView
    
    @State private var selection = 0
    @Namespace private var namespace

    var selectedID: Token.ID {
        return tokens[selection].id
    }

    var spacing: CGFloat = 6

    var body: some View {
        HStack {
            ForEach(tokens) { token in
                tokenView(token)
                    .matchedGeometryEffect(id: token.id, in: namespace)
            }
        }
        .padding(.trailing, spacing)
        .matchedGeometryEffect(id: "endPosition", in: namespace)
        .overlay {
            InsertionPoint()
                .matchedGeometryEffect(id: selectedID, in: namespace, isSource: false)
        }
    }
}

08:57 This moves the insertion point, but it's still not at the leading edge of the first token view. We can try specifying a leading anchor point for the matched geometry effect:

// ...
InsertionPoint()
    .matchedGeometryEffect(id: selectedID, in: namespace, anchor: .leading, isSource: false)
// ...

09:29 Still not quite right. We instead add a flexible frame around the two-points-wide insertion point, and we give the frame a .leading alignment:

// ...
InsertionPoint()
    .frame(maxWidth: .infinity, alignment: .leading)
    .matchedGeometryEffect(id: selectedID, in: namespace, isSource: false)
// ...

09:57 Now it sits at the beginning of the token view. If we look closely at what Mail does, we see that the insertion point should be centered to the leading edge of the token view. We achieve this by giving it an offset of one point:

// ...
InsertionPoint()
    .offset(x: -1)
    .frame(maxWidth: .infinity, alignment: .leading)
    .matchedGeometryEffect(id: selectedID, in: namespace, isSource: false)
// ...

10:42 To make the rectangle look more like an insertion point, we want to make it blink. In onAppear, we set a didAppear state property to true, and we use this property to change the opacity of the rectangle. By wrapping the mutation of didAppear in a withAnimation block, and by using an repeating animation, the cursor starts to blink:

struct InsertionPoint: View {
    @State private var didAppear = false
    
    var body: some View {
        Color.blue
            .frame(width: 2)
            .opacity(didAppear ? 0 : 1)
            .onAppear {
                withAnimation(.linear(duration: 8/60).delay(0.4).repeatForever()) {
                    didAppear = true
                }
            }
    }
}

Moving the Selection

12:01 Now we need a way to move the cursor between the token views. Using the onKeyPress modifier, we can define an action to be called when the user presses a key on their keyboard. If the pressed key is the right arrow key, we increment the selection integer by one — and we'll check that we don't go out of bounds later on:

struct TokenField<Token: Identifiable, TokenView: View>: View {
    var tokens: [Token]
    @ViewBuilder var tokenView: (Token) -> TokenView
    
    @State private var selection = 0
    @Namespace private var namespace

    var selectedID: Token.ID {
        return tokens[selection].id
    }

    var body: some View {
        HStack {
            // ...
        }
        .overlay {
            // ...
        }
        .onKeyPress { press in
            switch press.key {
            case .rightArrow:
                selection += 1
                return .handled
            default:
                return .ignored
            }
        }
    }
}

12:40 This isn't working yet, because we need to make our view focusable first. We also disable the focus effect so that we don't see a blue ring around the TokenField view:

struct TokenField<Token: Identifiable, TokenView: View>: View {
    var tokens: [Token]
    @ViewBuilder var tokenView: (Token) -> TokenView
    
    @State private var selection = 0
    @Namespace private var namespace

    var selectedID: Token.ID {
        return tokens[selection].id
    }

    var body: some View {
        HStack {
            // ...
        }
        .overlay {
            // ...
        }
        .focusable()
        .focusEffectDisabled()
        .onKeyPress { press in
            switch press.key {
            case .rightArrow:
                selection += 1
                return .handled
            default:
                return .ignored
            }
        }
    }
}

Checking Bounds

13:03 To prevent crashing when we press the arrow key too many times, we check if the selection is smaller than the number of tokens. This still allows a crash if the button is pressed when the cursor is at the last token, but we'll want to handle this as a special case:

.onKeyPress { press in
    switch press.key {
    case .rightArrow:
        if selection < tokens.count {
            selection += 1
        }
        return .handled
    default:
        return .ignored
    }
}

13:41 We want to allow the insertion point to be placed after the last token, which means we have to make the selectedID optional. If it's nil, we know that we have to draw the insertion point at the end of the token field, ready for new input:

var selectedID: Token.ID? {
    guard selection < tokens.count else { return nil }
    return tokens[selection].id
}

14:38 When selectedID returns nil, we could use a matched geometry effect on an extra view at the end of the HStack:

struct TokenField<Token: Identifiable, TokenView: View>: View {
    var tokens: [Token]
    @ViewBuilder var tokenView: (Token) -> TokenView
    
    @State private var selection = 0
    @Namespace private var namespace

    var selectedID: Token.ID? {
        guard selection < tokens.count else { return nil }
        return tokens[selection].id
    }

    var body: some View {
        HStack {
            ForEach(tokens) { token in
                tokenView(token)
                    .matchedGeometryEffect(id: AnyHashable(token.id), in: namespace)
            }
            Color.clear
                .frame(width: 0)
                .matchedGeometryEffect(id: "endPosition", in: namespace)
        }
        .overlay {
            // ...
        }
        .focusable()
        .focusEffectDisabled()
        .onKeyPress { press in
            // ...
        }
    }
}

16:12 In the overlay, we adjust the insertion point's matched geometry effect to use the "endPosition" ID in case selectID is nil. But to make this possible, we have to wrap both IDs in AnyHashable, because the nil-coalescing operator requires the same type of value to be returned from both sides of the operator:

.overlay {
    InsertionPoint()
        .offset(x: -1)
        .frame(maxWidth: .infinity, alignment: selectedID == nil ? .trailing : .leading)
        .matchedGeometryEffect(id: selectedID.map { AnyHashable($0) } ?? AnyHashable("endPosition") , in: namespace, isSource: false)
}

17:38 Our cursor is now as tall as the window, because it adopts the geometry of the Color.clear shape, which accepts all the available space proposed to it. So perhaps it's not a good idea to use this view as a source for the geometry. Instead, we can place the matched geometry effect on the HStack, so that the cursor becomes only as tall as the maximum height of the token views:

struct TokenField<Token: Identifiable, TokenView: View>: View {
    var tokens: [Token]
    @ViewBuilder var tokenView: (Token) -> TokenView
    
    @State private var selection = 0
    @Namespace private var namespace

    var selectedID: Token.ID? {
        guard selection < tokens.count else { return nil }
        return tokens[selection].id
    }

    var body: some View {
        HStack {
            ForEach(tokens) { token in
                tokenView(token)
                    .matchedGeometryEffect(id: AnyHashable(token.id), in: namespace)
            }
        }
        .matchedGeometryEffect(id: "endPosition", in: namespace)
        .overlay {
            // ...
        }
        .focusable()
        .focusEffectDisabled()
        .onKeyPress { press in
            // ...
        }
    }
}

18:31 Then we change the alignment of the flexible frame around the insertion point to .trailing if selectID is nil, which places the cursor at the end of the stack view:

struct TokenField<Token: Identifiable, TokenView: View>: View {
    var tokens: [Token]
    @ViewBuilder var tokenView: (Token) -> TokenView
    
    @State private var selection = 0
    @Namespace private var namespace

    var selectedID: Token.ID? {
        guard selection < tokens.count else { return nil }
        return tokens[selection].id
    }

    var body: some View {
        HStack {
            ForEach(tokens) { token in
                tokenView(token)
                    .matchedGeometryEffect(id: AnyHashable(token.id), in: namespace)
            }
        }
        .padding(.trailing, spacing)
        .matchedGeometryEffect(id: "endPosition", in: namespace)
        .overlay {
            InsertionPoint()
                .offset(x: -1)
                .frame(maxWidth: .infinity, alignment: selectedID == nil ? .trailing : .leading)
                .matchedGeometryEffect(id: selectedID.map { AnyHashable($0) } ?? AnyHashable("endPosition") , in: namespace, isSource: false)
        }
        .focusable()
        .focusEffectDisabled()
        .onKeyPress { press in
            // ...
        }
    }
}

18:47 That works, but something's off when we try to move the cursor to one of the token views. Perhaps we can display some debug information to see what's going on. Using the safeAreaInset modifier, we can easily attach a text view to the bottom of the TokenField. We can use this text view to display the current value of selection:

struct TokenField<Token: Identifiable, TokenView: View>: View {
    var tokens: [Token]
    @ViewBuilder var tokenView: (Token) -> TokenView
    
    @State private var selection = 0
    @Namespace private var namespace

    var selectedID: Token.ID? {
        guard selection < tokens.count else { return nil }
        return tokens[selection].id
    }

    var body: some View {
        HStack {
            // ...
        }
        // ...
        .safeAreaInset(edge: .bottom) {
            debugInfo
        }
    }

    var debugInfo: some View {
        Text("\(selection)")
    }
}

20:25 The selection changes correctly when we press the right arrow key, so something must be off with our matched geometry effects. And indeed, we forgot to also wrap the IDs of the token views in AnyHashable, so there was a mismatch between the effect ID of the insertion point and the IDs of the source effects:

// ...
HStack {
    ForEach(tokens) { token in
        tokenView(token)
            .matchedGeometryEffect(id: AnyHashable(token.id), in: namespace)
        }
    }
}
// ...

20:46 That fixes the navigation, so now we only have to add some extra padding at the end of the stack view to create some space between the last token view and the cursor:

struct TokenField<Token: Identifiable, TokenView: View>: View {
    var tokens: [Token]
    @ViewBuilder var tokenView: (Token) -> TokenView
    
    @State private var selection = 0
    @Namespace private var namespace

    var selectedID: Token.ID? {
        guard selection < tokens.count else { return nil }
        return tokens[selection].id
    }

    var spacing: CGFloat = 6

    var body: some View {
        HStack(spacing: spacing) {
            ForEach(tokens) { token in
                tokenView(token)
                    .matchedGeometryEffect(id: AnyHashable(token.id), in: namespace)
            }
        }
        .padding(.trailing, spacing)
        .matchedGeometryEffect(id: "endPosition", in: namespace)
        .overlay {
            // ...
        }
        // ...
    }

    // ...
}

TODO screenshot with cursor visible at end, at 21:20

More Keypresses

21:21 Let's also listen for presses of the left arrow key to move the cursor back:

// ...
.onKeyPress { press in
    switch press.key {
    case .leftArrow:
        if selection > 0 {
            selection -= 1
        }
        return .handled
    case .rightArrow:
        if selection < tokens.count {
            selection += 1
        }
        return .handled
    default:
        return .ignored
    }
}
// ...

21:38 We need to return .handled or .ignored from the modifier's closure to tell the system if the key press event should bubble up further. By returning .handled, we consume the event, and we prevent other actions from being triggered by the same key press.

21:51 Holding down the command key should make the cursor jump to the beginning or the end of the token field. This can be easily supported by checking the modifiers property of the key press:

// ...
.onKeyPress { press in
    switch press.key {
    case .leftArrow:
        if press.modifiers.contains(.command) {
            selection = 0
        } else if selection > 0 {
            selection -= 1
        }
        return .handled
    case .rightArrow:
        if press.modifiers.contains(.command) {
            selection = tokens.count
        } else  if selection < tokens.count {
            selection += 1
        }
        return .handled
    default:
        return .ignored
    }
}
// ...

22:32 We now have basic navigation in place. A next step could be to abstract away the selection logic and add a range to the selection so that we can highlight the tokens. By pulling the logic out, we'll make it possible to test the token field's behavior in response to key presses.