Question

我正在iTunes U上进行斯坦福iOS课程，并且在调用getter方法时我会遇到奇怪的行为

import UIKit

class ViewController: UIViewController {
    @IBOutlet weak var display: UILabel!
    var newNumber = true
    var operandStack = Array<Double>()
    var displayValue: Double{
        get{
            return NSNumberFormatter().numberFromString(display.text!)!.doubleValue
        }
        set{
            display.text = "\(newNumber)"
            newNumber = true
        }
    }

    @IBAction func appendDigit(sender: UIButton) {
        let digit = sender.currentTitle!
        if(newNumber){
            display.text = digit
            newNumber = false
        }else{
            display.text = display.text! + digit
        }
    }

    @IBAction func enter() {
        if(!newNumber){
            newNumber = true
        }
        operandStack.append(displayValue)
        println("\(operandStack)")
    }

}

当调用enter()方法时，我遇到一个崩溃，断点指向displayValue get方法中的return语句。如果我忽略了get方法并调用

operandStack.append(NSNumberFormatter().numberFromString(display.text!)!.doubleValue)

一切正常，但我不明白为什么get方法崩溃，是不是和上面的代码完全一样呢？

这是完整的回溯

* thread #1: tid = 0x133463, 0x000e4738 Kalkylator`Kalkylator.ViewController.displayValue.getter : Swift.Double(self=0x7986d560) + 24 at ViewController.swift:17, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
  * frame #0: 0x000e4738 Kalkylator`Kalkylator.ViewController.displayValue.getter : Swift.Double(self=0x7986d560) + 24 at ViewController.swift:17
    frame #1: 0x000e57c2 Kalkylator`Kalkylator.ViewController.enter (self=0x7986d560)() -> () + 130 at ViewController.swift:39
    frame #2: 0x000e5992 Kalkylator`@objc Kalkylator.ViewController.enter (Kalkylator.ViewController)() -> () + 34 at ViewController.swift:0
    frame #3: 0x01d0c7cd libobjc.A.dylib`-[NSObject performSelector:withObject:withObject:] + 84
    frame #4: 0x00b1aa90 UIKit`-[UIApplication sendAction:to:from:forEvent:] + 99
    frame #5: 0x00b1aa22 UIKit`-[UIApplication sendAction:toTarget:fromSender:forEvent:] + 64
    frame #6: 0x00c5b18a UIKit`-[UIControl sendAction:to:forEvent:] + 69
    frame #7: 0x00c5b5a7 UIKit`-[UIControl _sendActionsForEvents:withEvent:] + 598
    frame #8: 0x00c5a811 UIKit`-[UIControl touchesEnded:withEvent:] + 660
    frame #9: 0x00b72cfa UIKit`-[UIWindow _sendTouchesForEvent:] + 874
    frame #10: 0x00b737d6 UIKit`-[UIWindow sendEvent:] + 792
    frame #11: 0x00b316d1 UIKit`-[UIApplication sendEvent:] + 242
    frame #12: 0x00b41b08 UIKit`_UIApplicationHandleEventFromQueueEvent + 21484
    frame #13: 0x00b15337 UIKit`_UIApplicationHandleEventQueue + 2300
    frame #14: 0x0021706f CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 15
    frame #15: 0x0020cb7d CoreFoundation`__CFRunLoopDoSources0 + 253
    frame #16: 0x0020c0d8 CoreFoundation`__CFRunLoopRun + 952
    frame #17: 0x0020ba5b CoreFoundation`CFRunLoopRunSpecific + 443
    frame #18: 0x0020b88b CoreFoundation`CFRunLoopRunInMode + 123
    frame #19: 0x03fa32c9 GraphicsServices`GSEventRunModal + 192
    frame #20: 0x03fa3106 GraphicsServices`GSEventRun + 104
    frame #21: 0x00b19106 UIKit`UIApplicationMain + 1526
    frame #22: 0x000eec64 Kalkylator`main + 180 at AppDelegate.swift:12
    frame #23: 0x02407ac9 libdyld.dylib`start + 1

Answer 1

var newNumber = true

这是一个布尔值。

display.text = "\(newNumber)"

您正在将标签的文本设置为该布尔值

NSNumberFormatter().numberFromString(display.text

你正在从布尔值创建一个数字 - 而不是NSNumber - 这里。

你可能的意思是“newValue”。

Answer 2

这是修改后的工作代码：

您遇到崩溃的行已被替换。

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var display: UILabel!

    var newNumber = true
    var operandStack = Array<Double>()
    var displayValue: Double {
        get{
            return (display.text! as NSString).doubleValue // Changed Line
        }
        set{
            display.text = "\(newNumber)"
            newNumber = true
        }
    }

    @IBAction func appendDigit(sender: UIButton) {
        let digit = sender.currentTitle!
        if(newNumber){
            display.text = digit
            newNumber = false
        }else{
            display.text = display.text! + digit
        }
    }

    @IBAction func enter() {
        if(!newNumber){
            newNumber = true
        }
        operandStack.append(displayValue)
        println("\(operandStack)")
    }

}

Answer 3

原来我只是意外地放置了一个断点而不知道它是什么。删除它当然解决了问题

在Swift中调用getter方法时崩溃

3 个答案: