Swift Talk # 291

Advanced Alignment (Part 1)

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We prepare a tree diagram view in SwiftUI to experiment with advanced alignment techniques.

00:06 Today, we'll once again draw some tree diagrams. In our last series on this topic, we manually computed a tree's node positions using the Knuth algorithm. That was interesting, but it was also a lot of work. This time, we want to use built-in components like HStack and VStack in combination with alignment guides to lay out a tree diagram.

01:00 This will be a case study of what we can create using stack views and alignment guides. But before we get to the more complex stuff, we have to prepare a few things, as we'll be building everything up from scratch.

Getting Started

01:26 The first thing we need is a simple Tree structure. We make this struct generic over the type of value it holds. To easily create trees, we add an initializer that skips the value label and provides a default empty array for the children argument:

struct Tree<A> {
    init(_ value: A, children: [Tree<A>] = []) {
        self.value = value
        self.children = children
    }
    
    var value: A
    var children: [Tree<A>] = []
}

01:56 We construct a sample tree:

let sample = Tree("Root", children: [
    Tree("First Child"),
    Tree("Second"),
])

02:14 The next step is drawing the tree. We write a Diagram view, which is generic over the tree's value type. The view takes a view builder that can turn a single value into a node view. For the body view, we run the view builder with — for now — just the tree's root value:

struct Diagram<A, Node: View>: View {
    var tree: Tree<A>
    @ViewBuilder var node: (A) -> Node
    
    var body: some View {
        node(tree.value)
    }
}

03:11 We add a Diagram view with the sample tree to ContentView. For the diagram's node view builder, we return a Text with the node's string value. We use the fixedSize modifier so that the text view renders its string at its ideal width. We also give the text view some padding and a background color:

struct ContentView: View {
    var body: some View {
        Diagram(tree: sample) { value in
            Text(value)
                .fixedSize()
                .padding()
                .background(.tertiary)
        }
        .padding()
        .frame(maxWidth: .infinity, maxHeight: .infinity)
    }
}

Rendering Child Nodes

03:45 To draw the tree's child nodes below the root node, we wrap the root in a VStack, together with an HStack containing the children. We render the children recursively by adding a Diagram to the HStack for each child, passing on the node view builder:

struct Diagram<A, Node: View>: View {
    var tree: Tree<A>
    @ViewBuilder var node: (A) -> Node
    
    var body: some View {
        VStack {
            node(tree.value)
            HStack {
                ForEach(tree.children) { child in
                    Diagram(tree: child, node: node)
                }
            }
        }
    }
}

04:42 To make ForEach happy, we conform Tree to Identifiable:

struct Tree<A>: Identifiable {
    init(_ value: A, children: [Tree<A>] = []) {
        self.value = value
        self.children = children
    }
    
    var value: A
    var children: [Tree<A>] = []
    let id = UUID()
}

04:59 We give the HStack and the VStack some uniform spacing:

struct Diagram<A, Node: View>: View {
    var tree: Tree<A>
    @ViewBuilder var node: (A) -> Node
    
    var body: some View {
        VStack(spacing: 20) {
            node(tree.value)
            HStack(spacing: 20) {
                ForEach(tree.children) { child in
                    Diagram(tree: child, node: node)
                }
            }
        }
    }
}

Drawing Lines

05:25 The root node is centered above the combined frame of its children. When we draw lines from the root to the children, we'll see that these lines aren't symmetrical, which looks a little odd.

06:07 First, we write a Line shape:

struct Line: Shape {
    var from: CGPoint
    var to: CGPoint
    
    func path(in rect: CGRect) -> Path {
        Path { p in
            p.move(to: from)
            p.addLine(to: to)
        }
    }
}

06:50 To draw lines between the root node and each of the child nodes, we need to know the frames of all nodes in a common coordinate space. We can create this common coordinate space by calling coordinateSpace on the outer stack view:

struct Diagram<A, Node: View>: View {
    var tree: Tree<A>
    @ViewBuilder var node: (A) -> Node
    
    let coordinateSpace = "diagram"
    
    var body: some View {
        VStack(spacing: 20) {
            node(tree.value)
            HStack(spacing: 20) {
                ForEach(tree.children) { child in
                    Diagram(tree: child, node: node)
                }
            }
        }
        .coordinateSpace(name: coordinateSpace)
    }
}

08:03 Next, we'll write a helper that measures a view's frame and propagates it up the view tree along with the identifier of the view's node. We pass in the node identifiers so that we can later find a specific node's frame in a collection of measured frames:

struct Diagram<A, Node: View>: View {
    var tree: Tree<A>
    @ViewBuilder var node: (A) -> Node
    
    let coordinateSpace = "diagram"
    
    var body: some View {
        VStack(spacing: 20) {
            node(tree.value)
                .measureFrame(in: .named(coordinateSpace), id: tree.id)
            HStack(spacing: 20) {
                ForEach(tree.children) { child in
                    Diagram(tree: child, node: node)
                        .measureFrame(in: .named(coordinateSpace), id: child.id)
                }
            }
        }
        .coordinateSpace(name: coordinateSpace)
    }
}

08:39 We write a preference key to store a dictionary of UUID and CGRect. Its default value is an empty dictionary, and its combine function merges two dictionaries:

struct FrameKey: PreferenceKey {
    static var defaultValue: [UUID: CGRect] { [:] }
    static func reduce(value: inout [UUID : CGRect], nextValue: () -> [UUID : CGRect]) {
        value.merge(nextValue(), uniquingKeysWith: { $1 })
    }
}

09:19 The measureFrame helper method takes a coordinate space and a UUID. It then uses a geometry reader to measure the view's frame, and it stores this frame in a preference:

extension View {
    func measureFrame(in coordinateSpace: CoordinateSpace, id: UUID) -> some View {
        background(GeometryReader { proxy in
            Color.clear.preference(key: FrameKey.self, value: [id: proxy.frame(in: coordinateSpace)])
        })
    }
}

10:35 All these preferences are propagated up the view tree and combined into a single dictionary. We usually call onPreferenceChanged to read the dictionary and store it in a state property, but there's another API we can use. It takes a preference value and produces a background view from it, which fits our use case perfectly:

.backgroundPreferenceValue(FrameKey.self) { frames in
    // build some `View` using `frames`
}

11:44 The dictionary contains the frames of the root node and the child nodes. We want to draw a line from the root node to each of the children, so we first look up the root node's frame. Then, we filter the root node out of the frames dictionary to get an array of the child frames.

13:00 We have to help the compiler out by explicitly stating which overload of filter we want to use — i.e. the one that returns an array of key-value tuples instead of a new dictionary:

struct Diagram<A, Node: View>: View {
    var tree: Tree<A>
    // ...
    
    var body: some View {
        VStack(spacing: 20) {
            // ...
        }
        .backgroundPreferenceValue(FrameKey.self) { frames in
            let rootFrame = frames[tree.id]!
            let childFrames: [(UUID, CGRect)] = frames.filter { $0.key != tree.id }


        }
        .coordinateSpace(name: coordinateSpace)
    }
}

14:17 Now, we loop over the child frames to draw a line to each child. To start, we just use the frame origins, but we don't get the result we'd expect:

struct Diagram<A, Node: View>: View {
    var tree: Tree<A>
    // ...
    
    var body: some View {
        VStack(spacing: 20) {
            // ...
        }
        .backgroundPreferenceValue(FrameKey.self) { frames in
            let rootFrame = frames[tree.id]!
            let childFrames: [(UUID, CGRect)] = frames.filter { $0.key != tree.id }
            ForEach(childFrames, id: \.0) { (_, childFrame) in
                Line(from: rootFrame.origin, to: childFrame.origin)
                    .stroke(lineWidth: 1)
            }
        }
        .coordinateSpace(name: coordinateSpace)
    }
}

15:00 Within the Diagram view, we measure the root node's frame, and for each of the children, we recursively create a Diagram view and measure its frame. Inside each child's Diagram view, there's also a root node measuring its frame and propagating up. By clearing out the FrameKey preference value outside the Diagram view, we stop the propagation of measured frames from child trees:

struct Diagram<A, Node: View>: View {
    var tree: Tree<A>
    @ViewBuilder var node: (A) -> Node
    
    let coordinateSpace = "diagram"
    
    var body: some View {
        VStack(spacing: 20) {
            node(tree.value)
                .measureFrame(in: .named(coordinateSpace), id: tree.id)
            HStack(spacing: 20) {
                ForEach(tree.children) { child in
                    Diagram(tree: child, node: node)
                        .measureFrame(in: .named(coordinateSpace), id: child.id)
                }
            }
        }
        .backgroundPreferenceValue(FrameKey.self) { frames in
            let rootFrame = frames[tree.id]!
            let childFrames: [(UUID, CGRect)] = frames.filter { $0.key != tree.id }
            ForEach(childFrames, id: \.0) { (_, childFrame) in
                Line(from: rootFrame.origin, to: childFrame.origin)
                    .stroke(lineWidth: 1)
            }
        }
        .coordinateSpace(name: coordinateSpace)
        .preference(key: FrameKey.self, value: [:])
    }
}

Now the correct lines are drawn:

16:53 Rather than drawing lines between the frame origins, we want to draw a line from the root node's bottom to the top of each child node. In an extension on CGRect, we write a subscript to get the desired point from a frame using a UnitPoint, which defines handy static properties such as bottom and top:

struct Diagram<A, Node: View>: View {
    // ...
    var body: some View {
        VStack(spacing: 20) {
            // ...
        }
        .backgroundPreferenceValue(FrameKey.self) { frames in
            let rootFrame = frames[tree.id]!
            let childFrames: [(UUID, CGRect)] = frames.filter { $0.key != tree.id }
            ForEach(childFrames, id: \.0) { (_, childFrame) in
                Line(from: rootFrame[.bottom], to: childFrame[.top])
                    .stroke(lineWidth: 1)
            }
        }
        .coordinateSpace(name: coordinateSpace)
        .preference(key: FrameKey.self, value: [:])
    }
}

extension CGRect {
    subscript(point: UnitPoint) -> CGPoint {
        CGPoint(x: minX + point.x * width, y: minY + point.y * height)
    }
}

18:07 That doesn't look too bad. But when we give the child nodes strings of different lengths, the tree starts to look a little strange:

18:20 We want the root node to be moved to the right so that the lines to its children become symmetrical. In the next episode, we'll see we can use custom alignment guides to achieve this.

Resources

  • Sample Code

    Written in Swift 5.5

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