为什么我的 if 函数不能阻止转至下一个视图控制器？

Question

我有一系列 if 函数来检查两个文本字段是否为空（或其中一个），如果是，则在标签中显示错误消息。如果两个文本字段都不为空，那么它应该执行一个计算，然后转到下面的视图控制器，并在标签中显示等式的值。然而，即使使用 if 函数来检查文本字段是否为 nil，segue 仍然会执行，即使它们为 nil，错误标签也是如此。为什么 if 函数不能阻止这种情况？

@IBAction func CalcFlowRateEnd(_ sender: UIButton) {
  
    let volume = Double(VolTextField.text!)
    let time = Double(SecTextField.text!)
    let units = volumeUnitsPicker4.selectedRow(inComponent: 0)
    var selectedUnit = data[units]

        
        //milliliters error labels
    if selectedUnit == "milliliters" && volume == nil && time == nil {
            volErrorLabel.text = "Enter Volume"
            secErrorLabel.text = "Enter Seconds"}
        
    if selectedUnit == "milliliters" && volume != nil && time == nil {
            secErrorLabel.text = "Enter Seconds"}
        
    if selectedUnit == "milliliters" && volume == nil && time != nil {
            volErrorLabel.text = "Enter Volume"}
        
        
        //milliliters calc
    if selectedUnit == "milliliters" && volume != nil && time != nil{
    
    let volumeConvert = Double(volume!/1000)
    let timeConvert = Double(time! / 60)
    let flowRateMetric = Double(volumeConvert / timeConvert)
    let volumeImperial = Double(volume!/4546)
    let flowRateImperial = Double(volumeImperial / timeConvert)
    let roundedMetric = round(flowRateMetric * 100) / 100.0
    let roundedImperial = round(flowRateImperial * 100) / 100.0
            
    calcFlowRate.text = "\(roundedMetric) liters per min or \(roundedImperial) gal per min"
        
    func prepare(for segue: UIStoryboardSegue, sender: Any?) {
                 
    let flowRate = calcFlowRate.text ?? ""
    let destinationVC = segue.destination as! FlowRateCalcEndViewController
    destinationVC.flowRate = flowRate
            
    }

Answer 1

prepare(for segue) 函数不应嵌套在 @IBAction 函数中。

func prepare(for segue) 应该被覆盖。它在执行 segue 之前调用，因此您有机会“做某事”

var isFlowRateCalculated = false

@IBAction func CalcFlowRateEnd(_ sender: UIButton) {
    if selectedUnit == "milliliters" && volume != nil && time != nil {
    // logic

    isFlowRateCalculated = true
   }
}

override func shouldPerformSegue(withIdentifier identifier: String, sender: Any?) -> Bool {
    return isFlowRateCalculated
}

override func prepare(for segue) {
    // hand off data for view being presented
}

这并不能解决您的全部问题，但它是朝着正确方向迈出的一步。很高兴继续和你谈论这个:)

我还建议您看看 Medium

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为什么我的 if 函数不能阻止转至下一个视图控制器？

Why are my if functions not preventing the segue to the next view controller?

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if-statement

segue