异常处理 – 异常处理的Swift异常

选择1：返回可选类型

如果您的函数可能失败,并且这种情况经常发生,那么请考虑返回一个可选的类型变量.例如,在您的情况下,您的方法add可以返回NSDecimalNumber？而不是普通的NSDecimalNumber.在这种情况下,您的方法将检查可能出错的所有内容,并在这些情况下返回nil.溢出和下溢将返回nil,并且所有其他情况将返回NSDecimalNumber.调用者必须检查和解包可选的NSDecimalNumber,如下所示：

let brain = MathWhiz()
if let sum = brain.add("1",op2: "1e127") {
    println("the result was (sum)")
} else
    println("something went wrong with MathWhiz add")
}

选择2：返回枚举类型

如果要返回有关出错的更多信息,可以创建一个枚举类型,其中包含每个错误的值和一个嵌入答案的成功值.例如,您可以这样做：

enum MathWhizResult {
    case Overflow
    case Underflow
    case Success(NSDecimalNumber)
}

然后将定义add以返回MathWhizResult：

func add(op1: String,op2: String) -> MathWhizResult

如果出现错误,add将返回.Overflow或.Underflow.如果成功,add将返回Success(结果).调用者必须检查枚举并解压缩结果.可以使用一个开关：

switch (brain.add("1",op2: "1e128")) {
case .Overflow
    println("darn,it overflowed")
case .Underflow
    println("underflow condition happened")
case .Success(let answer)
    println("the result was (answer)"
}

选择3：选择不明确处理错误

在前两个选项中解压缩结果对于非常罕见的错误可能是过多的开销.您可以选择仅返回结果,并让调用者处理下溢或溢出情况的可能性.在这种情况下,他们必须在调用add之前自己检查这些条件.好处是,他们知道他们的程序永远不会导致下溢或溢出(因为他们正在处理单个数字的数字),他们没有负担解压缩结果.

我创建了一个小应用程序来演示如何使用NSDecimalNumbers执行此操作.我在Xcode中创建了一个单视图应用程序.在StoryBoard的ViewController中,我添加了3个TextFields(操作数1,操作数2和结果各一个)和我标记的Button.

ViewController.swift

import UIKit

class ViewController: UIViewController {
    @IBOutlet var operand1 : UITextField!
    @IBOutlet var operand2 : UITextField!
    @IBOutlet var result   : UITextField!

    var brain = MathWhiz()

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view,typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    @IBAction func addButton(sender : UIButton) {
        var op1 = operand1.text
        var op2 = operand2.text

        // Perform the add with the contents of the operand fields.
        // Print the answer,or "No Result" if add returns nil.
        if let answer = brain.add(op1,op2: op2)?.description {
            result.text = answer
        } else {
            result.text = "No Result"
        }
    }
}

MathWhiz.swift

import UIKit

// Declare that we implement NSDecimalNumberBehaviors so that we can handle
// exceptions without them being raised.
class MathWhiz: NSDecimalNumberBehaviors {
    var badException = false

    // Required function of NSDecimalNumberBehaviors protocol
    func roundingMode() -> NSRoundingMode {
        return .RoundPlain
    }

    // Required function of NSDecimalNumberBehaviors protocol
    func scale() -> CShort {
        return CShort(NSDecimalNoScale)
    }

    // Required function of NSDecimalNumberBehaviors protocol
    // Here we process the exceptions
    func exceptionDuringOperation(operation: Selector,error: NSCalculationError,leftOperand: NSDecimalNumber,rightOperand: NSDecimalNumber) -> NSDecimalNumber? {
        var errorstr = ""

        switch(error) {
        case .NoError:
            errorstr = "NoError"
        case .LossOfPrecision:
            errorstr = "LossOfPrecision"
        case .Underflow:
            errorstr = "Underflow"
            badException = true
        case .Overflow:
            errorstr = "Overflow"
            badException = true
        case .DivideByZero:
            errorstr = "DivideByZero"
            badException = true
        }
        println("Exception called for operation (operation) -> (errorstr)")

        return nil
    }

    // Add two numbers represented by the strings op1 and op2.  Return nil
    // if a bad exception occurs.
    func add(op1: String,op2: String) -> NSDecimalNumber? {
        let dn1 = NSDecimalNumber(string: op1)
        let dn2 = NSDecimalNumber(string: op2)

        // Init badException to false.  It will be set to true if an
        // overflow,underflow,or divide by zero exception occur.
        badException = false

        // Add the NSDecimalNumbers,passing ourselves as the implementor
        // of the NSDecimalNumbersBehaviors protocol.
        let dn3 = dn1.decimalNumberByAdding(dn2,withBehavior: self)

        // Return nil if a bad exception happened,otherwise return the result
        // of the add.
        return badException ? nil : dn3
    }
}