swift iOS应用程序中的延迟搜索

Question

我有用于输入搜索字符串的 UITextField 和带有结果的 UITableView。 我想要的是当用户输入超过 3 个字母并且自最后一个符号添加到 UITextView 以来至少经过 0.5 秒时运行搜索功能。

我找到了 (Detect when user stopped / paused typing in Swift) 函数，并将它添加到我的 ViewController 中，它具有类 SearchVC 和方法 server_search

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    NSObject.cancelPreviousPerformRequests(
        withTarget: self,
        selector: #selector(SearchVC.server_search),
        object: textField)
    self.perform(
        #selector(SearchVC.server_search),
        with: textField,
        afterDelay: 0.5)
    return true
}

但是什么也没发生。

Answer 1

您可以通过Timer 来实现这一点……

var timer: Timer?

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {

    timer?.invalidate()  // Cancel any previous timer

    // If the textField contains at least 3 characters…
    let currentText = textField.text ?? ""
    if (currentText as NSString).replacingCharacters(in: range, with: string).characters.count >= 3 {

         // …schedule a timer for 0.5 seconds
         timer = Timer.scheduledTimer(timeInterval: 0.5, target: self, selector: #selector(performSearch()), userInfo: nil, repeats: false)
    }

    return true
}

func performSearch() {

} 

并且不要忘记将视图控制器设置为您的UITextField 的delegate

Answer 2

虽然使用 Timer 仍然是一个有效的答案，但从 Swift 3 开始，您可以使用 DispatchWorkItem...

var workItem: DispatchWorkItem?

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {

    // Cancel any outstanding search
    self.workItem?.cancel() 

    guard let text = textField.text, let textRange = Range(range, in: text) else {
        return true
    }

    let updatedText = text.replacingCharacters(in: textRange, with: string)

    guard updatedText.count >= 3 else { 
        return true
    }

    // Set up a DispatchWorkItem to perform the search
    let workItem = DispatchWorkItem { [weak self] in
        self?.performSearch(updatedText)
    }

    // Run this block after 0.5 seconds
    DispatchQueue.main.asyncAfter(deadline: .now() + 0.5, execute: workItem)

    // Keep a reference to it so it can be cancelled
    self.workItem = workItem 

    return true   
}

func performSearch(_ text: String) {

} 

Answer 3

使用定时器有一些好处。通过您的实施，您将取消 您的对象的所有执行，这可能会失控。

相反，计时器可以让您精细控制它。请参阅以下实现：

var searchTimer: Timer?
func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    // Invalidate remove the previous timer, since it's optional if the previous timer is already cancelled or nil, it won't affect execution
    searchTimer?.invalidate()
    searchTimer = Timer.scheduledTimer(withTimeInterval: 0.5, repeats: false, block: { (timer) in
        //do Something crazy
        self.server_search()
    })
    return true
}