SwiftUI 中的 PencilKit

Question

我正在尝试在 SwiftUI 中使用 PencilKit。

1. 如何在updateUIView-函数中检测到Binding-variable 已更新？例如，我不想在更改颜色时清除画布。
2. 还有比切换布尔值更好的方法来清除画布吗？切换布尔值会强制执行 updateUIView-函数。

import SwiftUI
import UIKit
import PencilKit

struct ContentView: View {
    @State var color = UIColor.black
    @State var clear = false

    var body: some View {
        VStack{
            PKCanvas(color: $color, clear:$clear)
            VStack(){
                Button("Change to BLUE"){ self.color = UIColor.blue }
                Button("Change to GREEN"){ self.color = UIColor.green }
                Button("Clear Canvas"){ self.clear.toggle() }
            }
        }
    }
}

struct PKCanvas: UIViewRepresentable {
    class Coordinator: NSObject, PKCanvasViewDelegate {
        var pkCanvas: PKCanvas

        init(_ pkCanvas: PKCanvas) {
            self.pkCanvas = pkCanvas
        }
    }

    @Binding var color:UIColor
    @Binding var clear:Bool

    func makeCoordinator() -> Coordinator {
        Coordinator(self)
    }

    func makeUIView(context: Context) -> PKCanvasView {
        let canvas = PKCanvasView()
        canvas.tool = PKInkingTool(.pen, color: color, width: 10)

        canvas.delegate = context.coordinator
        return canvas
    }

    func updateUIView(_ canvasView: PKCanvasView, context: Context) {
        // clears the canvas
        canvasView.drawing = PKDrawing()

        // sets a new color
        canvasView.tool = PKInkingTool(.pen, color: color, width: 10)
    }
}

Answer 1

这样就可以了：

    func updateUIView(_ canvasView: PKCanvasView, context: Context) {
        if clear != context.coordinator.pkCanvas.clear{
            canvasView.drawing = PKDrawing()
        }
        canvasView.tool = PKInkingTool(.pen, color: color, width: 10)
    }

Answer 2

我多次观察到类似的模式，在这里我们得到了SwiftUI-value-declarative-state-managedworld 和xKit-OOP-imperative-action-managedworld 之间的相互通信......并且有人尝试将所有内容从一个到另一个，反之亦然......

我有一个想法，可能最好为每个人保留每个性质并在两者之间有一些调解人或演员......所以对于你的用例，我认为我会采取不同的方式，比如说（粗糙，未测试，供考虑）：

struct ContentView: View {
    //@State var color = UIColor.black // < both these have nothing to ContentView
    //@State var clear = false

    let pkActor = PKCanvasActor() // < mediator, reference type

    var body: some View {
        VStack{
            PKCanvas(actor: pkActor)
            VStack(){
                Button("Change to BLUE"){ self.pkActor.use(color: UIColor.blue) }
                Button("Change to GREEN"){ self.pkActor.use(color: UIColor.green) }
                Button("Clear Canvas"){ self.pkActor.clear() }
            }
        }
    }
}

这里的某个地方

func makeUIView(context: Context) -> PKCanvasView {
    let canvas = PKCanvasView()
    self.actor.canvas = canvas // but think to avoid cycle reference

和纯 OOP 部分

class PKCanvasActor {
  var canvas: PKCanvasView

   func use(color: Color) {
      // do anything with canvas color
   }

   func clear() {
      canvas.drawing = PKDrawing()
   }

   // ... any more actions 
}

当然，我在SwiftUI Button interact with Map 中为类似场景提出了简单的方法......但上面的方法对我来说看起来更可取。

备注：有人可能会说 Coordinator 就是为了这个目的，但它的生命周期由 SwiftUI-internals 管理，并且它参与那些内部工作流，所以我会避免侵入这些关系...