使用onChange modifier 而不是onAppear() 来观察textString 绑定。
struct TypewriterTextView: View {
@Binding var textString:String
@State private var typingInterval = 0.3
@State private var typedString = ""
var body: some View {
Text(typedString).onChange(of: textString) {
typedString = ""
Timer.scheduledTimer(withTimeInterval: self.typingInterval, repeats: true, block: { timer in
if self.typedString.length < self.textString.length {
self.typedString = self.typedString + self.textString[self.typedString.length]
}
else { timer.invalidate() }
})
}
}
}
兼容性
onChange 修饰符是在 WWDC 2020 上推出的，仅在
- macOS 11+
- iOS 14+
- tvOS 14+
- watchOS 7+
如果您想在旧系统上使用此功能，您可以使用以下填充程序。它基本上是使用旧 SwiftUI 重新实现的 onChange 方法：
import Combine
import SwiftUI
/// See `View.onChange(of: value, perform: action)` for more information
struct ChangeObserver<Base: View, Value: Equatable>: View {
let base: Base
let value: Value
let action: (Value)->Void
let model = Model()
var body: some View {
if model.update(value: value) {
DispatchQueue.main.async { self.action(self.value) }
}
return base
}
class Model {
private var savedValue: Value?
func update(value: Value) -> Bool {
guard value != savedValue else { return false }
savedValue = value
return true
}
}
}
extension View {
/// Adds a modifier for this view that fires an action when a specific value changes.
///
/// You can use `onChange` to trigger a side effect as the result of a value changing, such as an Environment key or a Binding.
///
/// `onChange` is called on the main thread. Avoid performing long-running tasks on the main thread. If you need to perform a long-running task in response to value changing, you should dispatch to a background queue.
///
/// The new value is passed into the closure. The previous value may be captured by the closure to compare it to the new value. For example, in the following code example, PlayerView passes both the old and new values to the model.
///
/// ```
/// struct PlayerView : View {
/// var episode: Episode
/// @State private var playState: PlayState
///
/// var body: some View {
/// VStack {
/// Text(episode.title)
/// Text(episode.showTitle)
/// PlayButton(playState: $playState)
/// }
/// }
/// .onChange(of: playState) { [playState] newState in
/// model.playStateDidChange(from: playState, to: newState)
/// }
/// }
/// ```
///
/// - Parameters:
/// - value: The value to check against when determining whether to run the closure.
/// - action: A closure to run when the value changes.
/// - newValue: The new value that failed the comparison check.
/// - Returns: A modified version of this view
func onChange<Value: Equatable>(of value: Value, perform action: @escaping (_ newValue: Value)->Void) -> ChangeObserver<Self, Value> {
ChangeObserver(base: self, value: value, action: action)
}
}