为什么这个 swiftui 视图没有更新？

Question

Consider the following code block: import SwiftUI

struct MeasurementReading: View, Equatable {

    @ObservedObject var ble: BluetoothConnectionmanager
    @GestureState var isDetectTap = false
    @State var MyText:String = "Wait"

    static func == (lhs: MeasurementReading, rhs: MeasurementReading)->Bool{
        return lhs.MyText == rhs.MyText
    }

    var body: some View {
        let timer = Timer.publish(every: 1, on: .main, in: .common).autoconnect()

        return HStack {
            Spacer()
            VStack{
                Button(action:{
                    self.MyText = "\(self.ble.getValue()!) mV"
                    print("Text is \(self.MyText as NSString)")
                }, label: {
                    Text(MyText)
                        .font(.system(size: 40))
                        .bold()
                        .foregroundColor(Color.black)
                        .padding(.trailing, 15)
                        .frame(height: 100)
                })
                Button(action: {
                    self.MyText = "\(self.ble.getValue()!) mV"
                    print("Text is \(self.MyText as NSString)")

                }, label: {
                    Text(MyText)
                        .font(.system(size: 25))
                        .padding(.top, -20)
                        .padding(.bottom, 20)
                        .foregroundColor(Color.black)
                })
            }
        }.onReceive(timer)
        { _ in // TIMER FUNCTIONALITY HERE
            self.MyText = "\(self.ble.getValue()!) mV"
            print("Text is \(self.MyText)")
        }
    }
}


struct MeasurementReading_Previews: PreviewProvider {
    static var previews: some View {
        MeasurementReading(ble: BluetoothConnectionmanager())
    }
}

Every 1 second the correct value read from the BLE system is assigned to MyText and then MyText is printed to the debug output properly with the updated value.

The problem here is that view MeasurementReading does not update. Also, using a closure on any item also has the same behavior (variable is updated, it is output properly but no view update) ex .onTap{....} will have the same behavior or any other .onXXXX closure. The only way I could get the view to update at all with new values for the MyText state is to put the behavior in a Button.

My question is this: Why does the view not update even when the state variable changes via Timer or .onXXXX closure?

Answer 1

You need to be setting the ble value to the updated timer value:

Without testing this properly. I also think your BluetoothConnectionmanager needs to be a @State property for this to work.

@State var ble: BluetoothConnectionmanager

.onReceive(timer) { value in // value is the updated value
    self.ble.value = value
    self.MyText = "\(self.ble.getValue()!) mV"
    print("Text is \(self.MyText)")
}

Take a look at this example to see how a timer works with a Date() object.