iOS SwiftUI-3 Callback使用方式

1.簡易Callback使用方法
2.搭配ObservableObject使用callback

1. 簡易Callback使用方法

    //當SubView按下按鈕時，SubView會回call，並打印 index 0
    struct MainView: View {
        var body: some View {
            SubView(callback: .constant({ index in
                print("index \(index)")
            }))
        }
    }
    struct SubView: View {
        @Binding var callback: (Int) -> ()
        var body: some View {
            Button(action: {
                callback(0)
            }, label: {
                Text("click")
            })
        }
    }
   

2. 搭配ObservableObject使用callback
   ```
   class MainDelegate: ObservableObject {

    var data = false {
        didSet {
            dataCallback(data)
        }
    }
    var dataCallback: (Bool) -> () = {_ in}
    func setDataCallback(callback: @escaping (Bool) -> ()) {
        dataCallback = callback
    }
   

   }

struct MainView: View {
    @State var opacity: Double = 1
    @State var animation = false
    @ObservedObject var delegate: MainDelegate = MainDelegate()
    var body: some View {
        delegate.setDataCallback(callback: { flag in
            print("123")
        })
        return Button(action: {
            delegate.data.toggle()
        }, label: {
            Text("onClick")
        })
    }
}

```

更新於2021/01/12，後續有新的會再更新