SwiftUI DatePicker Binding optional Date, valid nil

问题

I'm experimenting code from https://alanquatermain.me/programming/swiftui/2019-11-15-CoreData-and-bindings/

my goal is to have DatePicker bind to Binding< Date? > which allow for nil value instead of initiate to Date(); this is useful, if you have Date attribute in your core data model entity which accept nil as valid value.

Here is my swift playground code:

extension Binding {
    init<T>(isNotNil source: Binding<T?>, defaultValue: T) where Value == Bool {
        self.init(get: { source.wrappedValue != nil },
                  set: { source.wrappedValue = $0 ? defaultValue : nil})
    }
}

struct LiveView: View {
    @State private var testDate: Date? = nil
    var body: some View {
        VStack {
            Text("abc")

            Toggle("Has Due Date",
                   isOn: Binding(isNotNil: $testDate, defaultValue: Date()))

            if testDate != nil {
                DatePicker(
                    "Due Date",
                    selection: Binding($testDate)!,
                    displayedComponents: .date
                )
            }
        }
    }
}

let liveView = LiveView()
PlaygroundPage.current.liveView = UIHostingController(rootView: liveView)

I can't find solution to fix this code. It works when the toggle first toggled to on, but crash when the toggle turned back off.

The code seems to behave properly when I removed the DatePicker, and change the code to following:

extension Binding {
    init<T>(isNotNil source: Binding<T?>, defaultValue: T) where Value == Bool {
        self.init(get: { source.wrappedValue != nil },
                  set: { source.wrappedValue = $0 ? defaultValue : nil})
    }
}

struct LiveView: View {
    @State private var testDate: Date? = nil
    var body: some View {
        VStack {
            Text("abc")

            Toggle("Has Due Date",
                   isOn: Binding(isNotNil: $testDate, defaultValue: Date()))

            if testDate != nil {
                Text("\(testDate!)")
            }
        }
    }
}

let liveView = LiveView()
PlaygroundPage.current.liveView = UIHostingController(rootView: liveView)

I suspect it's something to do with this part of the code

DatePicker("Due Date", selection: Binding($testDate)!, displayedComponents: .date )

problem when the source.wrappedValue set back to nil (refer to Binding extension)

回答1:

The problem is that DatePicker grabs binding and is not so fast to release it even when you remove it from view, due to Toggle action, so it crashes on force unwrap optional, which becomes nil ...

The solution for this crash is

DatePicker(
    "Due Date",
    selection: Binding<Date>(get: {self.testDate ?? Date()}, set: {self.testDate = $0}),
    displayedComponents: .date
)

来源：https://stackoverflow.com/questions/59272801/swiftui-datepicker-binding-optional-date-valid-nil

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ios

swift

core-data

swiftui

swift-playground