使用NSExpression时正确的方式来捕获NSInvalidArgumentException

问题描述

我想验证用户创建的表达式（如2 + 2，5 + 7或更复杂）。我使用NSExpression类来解析和计算这个表达式。这是我的Playground代码：

I want to validate user created expressions (like "2+2", "5+7" or more complex). I use NSExpression class to parse and calculate this expressions. This is my Playground code:

import UIKit

let string = "2+2"

var ex:NSExpression?
do {
    ex = NSExpression(format: string)
}
catch {
    print("String is not valid expression")
}

if let result = ex?.expressionValue(with: nil, context: nil) as! NSNumber? {
    print("result is \(result)")
}

当我使用有效的表达式（2 + 2） - 我得到结果。但有时用户可以提供错误的字符串（例如2+）。有了这个字符串，我的应用程序会崩溃：

When I use valid expression ("2+2") - I get the result. But sometime user can provide wrong string ("2+" for example). With this string my app crashes with this:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Unable to parse the format string "2+ == 1"'

我不明白我怎么可以捕获这个异常，为什么上面的代码不这样做。现在我使用Objective C类（具有相同的逻辑），从我的swift代码调用这个方法，在该类中我真的可以捕获这样的异常：

I don't understand how I can catch this exception and why code above don't do this. Now I use Objective C class (with same logic), calling this method from my swift code, and in that class I really can catch such exception:

+(NSNumber *)solveExpression:(NSString *)string
{
    id value;
    @try {
        NSExpression *ex = [NSExpression expressionWithFormat:string];
        value = [ex expressionValueWithObject:nil context:nil];
    }
    @catch (NSException *e) { }
    return value;
}

这个工作，我可以得到正确的解析状态（nil表示表达式的问题）和结果（NSNumber），但我真的想了解如何在Swift中正确和完整地完成所有这些事情。

This works and I can get correct parse state (nil means problems with expression) and result (NSNumber), but I really want to understand how to do all this things correct and entirely in Swift.

推荐答案

是什么书籍使用Swift与Cocoa和Objective-C ：

This is what the book Using Swift with Cocoa and Objective-C has to say:

[我的粗体]

刚刚删除了 NSExpression的引用我看不出有关这个问题的简单方法。上面的引用建议写一些Objective-C代码来做。最简单的方法可能是创建一个C函数：

Having just skimmed the reference for NSExpression I can't see a simple way around the issue. The quote above recommends writing a a bit of Objective-C code to do it. The simplest way is probably to create a C function:

声明：

extern NSExpression* _Nullable makeExpression(NSString* format _Nonnull);

定义

NSExpression* _Nullable makeExpression(NSString* format _Nonnull)
{
    NSExpression* ret = nil;
    @try
    {
        // create and assign the expression to ret
    }
    @catch(NSException* e)
    {
        // ignore
    }
    return ret;
}

该函数对于错误的表达式返回nil。

The function returns nil for expressions that are in error.

您可能会添加一个 NSError ** 参数，以便在发生故障时使用。您也可以在 NSExpression 中的类别中使用此方法，然后返回无效的错误/填充NSError模式可能会导入Swift作为Swift投掷方法。

You could probably add an NSError** parameter to be used when there is a failure. You could also probably make this a method in a category on NSExpression and then the return nil for error/fill in NSError pattern would probably be imported to Swift as a Swift throwing method.

顺便说一下，Objective-C异常并没有真正保证让程序保持一致状态。在这种情况下，手指越过它就可以。

I should say, by the way, that an Objective-C exception is not really guaranteed to leave your program in a consistent state. Fingers crossed it is OK in this case.

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