We often use onChange () modifier in SwiftUI to react to environment or state changes, but it can be used with any properties that have
Equatable value, even a computed property.
If we need to perform an action when either of multiple properties changes, then it’s simpler to make a computed property, rather than add multiple
onChange() modifiers for each of them.
Let’s say we have a view to set an alarm with a
DatePicker and an activation
Toggle. When user activates the toggle we set an alarm for selected date. When user changes the date, we cancel the previous alarm if it has not been canceled yet and set a new one. When user deactivates the toggle, we cancel the alarm.
In our case we depend on changes in the
DatePicker and the
Toggle. We could set up two
onChange() modifiers to react to changes in each of them. But we could also define an intermediary computed property that will only return a date if alarm is activated and otherwise return
nil. Then we can react to changes in that computed date.
struct AlarmView: View @State private var date = Date() @State private var isOn = false var alarmDate: Date? isOn ? date : nil var body: some View VStack DatePicker( "Alarm Time", selection: $date, displayedComponents: [.hourAndMinute] ) Toggle("Activate", isOn: $isOn) .onChange(of: alarmDate) [alarmDate] newDate in if alarmDate != nil AlartScheduler.cancelAlarm() if let date = newDate AlartScheduler.setNewAlarm(for: date) struct AlartScheduler static func cancelAlarm() print("Cancelling old alarm...") static func setNewAlarm(for date: Date) print("Setting alarm for: (date)")
Note that we are capturing the old value of
onChange closure, so that we only cancel the alarm if the old value isn’t
nil. If the old value is
nilit means that the alarm was already canceled.
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