swift - 展开可选值时意外发现 nil(AppleScript 结果)

Question

我正在尝试在 Swift 2 中制作一个程序，该程序运行并获取 AppleScript 脚本的结果。

这是我的代码:

import Foundation

func runAppleScript(script:String) -> String
{
    let errorInfo = AutoreleasingUnsafeMutablePointer<NSDictionary?>()
    let startAtLoginScript: NSAppleScript = NSAppleScript(source: script)!
    let theDiscriptor:NSAppleEventDescriptor = startAtLoginScript.executeAndReturnError(errorInfo)
    let theResult:String = theDiscriptor.stringValue! //This is whats causing the error

    return theResult
}

let scriptResult = runAppleScript("tell app \"Spotify\" to playpause")

NSLog("\(scriptResult)")

问题是程序崩溃并输出:

fatal error: unexpectedly found nil while unwrapping an Optional value

在控制台中。我也尝试过 if let else，但这也不起作用。我该如何解决这个问题？

这是通过使用 swift 语言的 OS X 命令行模板进行测试的。

Answer 1

实际上错误可能来自 NSAppleScript(source: script)! 所以 正确的 解决方案是返回一个可选字符串并且根本不使用强制解包 :

func runAppleScript(script:String) -> String? {
    let errorInfo: AutoreleasingUnsafeMutablePointer<NSDictionary?> = nil
    let startAtLoginScript = NSAppleScript(source: script)
    let theDescriptor = startAtLoginScript?.executeAndReturnError(errorInfo)
    return theDescriptor?.stringValue
}

if let scriptResult = runAppleScript("tell app \"Spotify\" to playpause") {
    NSLog("\(scriptResult)")
} else {
    print("the script execution failed")
}

如果您希望在失败时使用默认值而不是 nil，则无需返回 Optional:

func runAppleScript(script:String) -> String {
    let errorInfo: AutoreleasingUnsafeMutablePointer<NSDictionary?> = nil
    let startAtLoginScript = NSAppleScript(source: script)
    let theDescriptor = startAtLoginScript?.executeAndReturnError(errorInfo)
    return theDescriptor?.stringValue ?? ""  // if nil, returns the default ""
}

let scriptResult = runAppleScript("tell app \"Spotify\" to playpause")
NSLog("\(scriptResult)")

至于使用新的 Swift 2 错误处理系统，您在 runAppleScript 中使用的所有方法都不会抛出错误，因此它只有在您使用自定义错误类型并自己抛出错误。示例:

enum MyAppleScriptError: ErrorType {
    case ExecutingScriptFailed
    case GettingStringValueFailed
}

func runAppleScript(script:String) throws -> String {
    let errorInfo: AutoreleasingUnsafeMutablePointer<NSDictionary?> = nil
    let startAtLoginScript = NSAppleScript(source: script)
    guard let theDescriptor = startAtLoginScript?.executeAndReturnError(errorInfo) else {
        throw MyAppleScriptError.ExecutingScriptFailed
    }
    guard let value = theDescriptor.stringValue else {
        throw MyAppleScriptError.GettingStringValueFailed
    }
    return value
}

do {
    let scriptResult = try runAppleScript("tell app \"Spotify\" to playpause")
    NSLog("\(scriptResult)")
} catch {
    print(error)
}

swift 3

相同的想法，但一些实现细节不同。

func runAppleScript(_ script:String) -> String? {
    let errorInfo: AutoreleasingUnsafeMutablePointer<NSDictionary?>? = nil
    if let startAtLoginScript = NSAppleScript(source: script) {
        let theDescriptor = startAtLoginScript.executeAndReturnError(errorInfo)
        return theDescriptor.stringValue
    }
    return nil
}

if let scriptResult = runAppleScript("tell app \"Spotify\" to playpause") {
    NSLog("\(scriptResult)")
} else {
    print("no return value")
}

错误处理:

enum MyAppleScriptError: ErrorProtocol {
    case ExecutingScriptFailed
    case GettingStringValueFailed
}

func runAppleScript(_ script:String) throws -> String {
    let errorInfo: AutoreleasingUnsafeMutablePointer<NSDictionary?>? = nil
    let startAtLoginScript = NSAppleScript(source: script)
    guard let theDescriptor = startAtLoginScript?.executeAndReturnError(errorInfo) else {
        throw MyAppleScriptError.ExecutingScriptFailed
    }
    guard let value = theDescriptor.stringValue else {
        throw MyAppleScriptError.GettingStringValueFailed
    }
    return value
}

do {
    let scriptResult = try runAppleScript("tell app \"Spotify\" to playpause")
    NSLog("\(scriptResult)")
} catch {
    print(error)
}