ios 如何在我的 iPhone 应用程序中使用 NSError？

How can I use NSError in my iPhone App?

提问by Nic Hubbard

I am working on catching errors in my app, and I am looking into using NSError. I am slightly confused about how to use it, and how to populate it.

我正在努力捕捉我的应用程序中的错误，我正在考虑使用NSError. 我对如何使用它以及如何填充它感到有些困惑。

Could someone provide an example on how I populate then use NSError?

有人可以提供一个关于我如何填充然后使用的例子NSError吗？

回答by Alex

Well, what I usually do is have my methods that could error-out at runtime take a reference to a NSErrorpointer. If something does indeed go wrong in that method, I can populate the NSErrorreference with error data and return nil from the method.

好吧，我通常做的是让我的方法在运行时可能出错，引用一个NSError指针。如果该方法确实出现问题，我可以NSError使用错误数据填充引用并从该方法返回 nil。

Example:

例子：

- (id) endWorldHunger:(id)largeAmountsOfMonies error:(NSError**)error {
    // begin feeding the world's children...
    // it's all going well until....
    if (ohNoImOutOfMonies) {
        // sad, we can't solve world hunger, but we can let people know what went wrong!
        // init dictionary to be used to populate error object
        NSMutableDictionary* details = [NSMutableDictionary dictionary];
        [details setValue:@"ran out of money" forKey:NSLocalizedDescriptionKey];
        // populate the error object with the details
        *error = [NSError errorWithDomain:@"world" code:200 userInfo:details];
        // we couldn't feed the world's children...return nil..sniffle...sniffle
        return nil;
    }
    // wohoo! We fed the world's children. The world is now in lots of debt. But who cares? 
    return YES;
}

We can then use the method like this. Don't even bother to inspect the error object unless the method returns nil:

然后我们可以使用这样的方法。除非该方法返回 nil，否则甚至不必检查错误对象：

// initialize NSError object
NSError* error = nil;
// try to feed the world
id yayOrNay = [self endWorldHunger:smallAmountsOfMonies error:&error];
if (!yayOrNay) {
   // inspect error
   NSLog(@"%@", [error localizedDescription]);
}
// otherwise the world has been fed. Wow, your code must rock.

We were able to access the error's localizedDescriptionbecause we set a value for NSLocalizedDescriptionKey.

我们能够访问错误，localizedDescription因为我们为 设置了一个值NSLocalizedDescriptionKey。

The best place for more information is Apple's documentation. It really is good.

获取更多信息的最佳位置是Apple 的文档。这真的很好。

There is also a nice, simple tutorial on Cocoa Is My Girlfriend.

Cocoa Is My Girlfriend上还有一个不错的简单教程。

回答by Wolfgang Schreurs

I would like to add some more suggestions based on my most recent implementation. I've looked at some code from Apple and I think my code behaves in much the same way.

我想根据我最近的实施添加更多建议。我看过一些来自 Apple 的代码，我认为我的代码的行为方式大致相同。

The posts above already explain how to create NSError objects and return them, so I won't bother with that part. I'll just try to suggest a good way to integrate errors (codes, messages) in your own app.

上面的帖子已经解释了如何创建 NSError 对象并返回它们，所以我不会打扰那部分。我只是尝试提出一种在您自己的应用程序中集成错误（代码、消息）的好方法。

---

---

I recommend creating 1 header that will be an overview of all the errors of your domain (i.e. app, library, etc..). My current header looks like this:

我建议创建 1 个标头，用于概述您的域（即应用程序、库等）的所有错误。我当前的标题如下所示：

FSError.h

错误文件

FOUNDATION_EXPORT NSString *const FSMyAppErrorDomain;

enum {
    FSUserNotLoggedInError = 1000,
    FSUserLogoutFailedError,
    FSProfileParsingFailedError,
    FSProfileBadLoginError,
    FSFNIDParsingFailedError,
};

FSError.m

FSError.m

#import "FSError.h" 

NSString *const FSMyAppErrorDomain = @"com.felis.myapp";

Now when using the above values for errors, Apple will create some basic standard error message for your app. An error could be created like the following:

现在，当使用上述错误值时，Apple 将为您的应用程序创建一些基本的标准错误消息。可能会产生如下错误：

+ (FSProfileInfo *)profileInfoWithData:(NSData *)data error:(NSError **)error
{
    FSProfileInfo *profileInfo = [[FSProfileInfo alloc] init];
    if (profileInfo)
    {
        /* ... lots of parsing code here ... */

        if (profileInfo.username == nil)
        {
            *error = [NSError errorWithDomain:FSMyAppErrorDomain code:FSProfileParsingFailedError userInfo:nil];            
            return nil;
        }
    }
    return profileInfo;
}

The standard Apple-generated error message (error.localizedDescription) for the above code will look like the following:

苹果error.localizedDescription为上述代码生成的标准错误消息 ( ) 将如下所示：

Error Domain=com.felis.myapp Code=1002 "The operation couldn't be completed. (com.felis.myapp error 1002.)"

Error Domain=com.felis.myapp Code=1002 "The operation couldn't be completed. (com.felis.myapp error 1002.)"

The above is already quite helpful for a developer, since the message displays the domain where the error occured and the corresponding error code. End users will have no clue what error code 1002means though, so now we need to implement some nice messages for each code.

以上对开发人员来说已经非常有帮助了，因为消息显示了发生错误的域和相应的错误代码。最终用户将不知道错误代码的1002含义，所以现在我们需要为每个代码实现一些很好的消息。

For the error messages we have to keep localisation in mind (even if we don't implement localized messages right away). I've used the following approach in my current project:

对于错误消息，我们必须牢记本地化（即使我们没有立即实现本地化消息）。我在当前项目中使用了以下方法：

---

---

1) create a stringsfile that will contain the errors. Strings files are easily localizable. The file could look like the following:

1) 创建一个strings包含错误的文件。字符串文件很容易本地化。该文件可能如下所示：

FSError.strings

FSError.strings

"1000" = "User not logged in.";
"1001" = "Logout failed.";
"1002" = "Parser failed.";
"1003" = "Incorrect username or password.";
"1004" = "Failed to parse FNID."

---

---

2) Add macros to convert integer codes to localized error messages. I've used 2 macros in my Constants+Macros.h file. I always include this file in the prefix header (MyApp-Prefix.pch) for convenience.

2) 添加宏以将整数代码转换为本地化的错误消息。我在我的 Constants+Macros.h 文件中使用了 2 个宏。MyApp-Prefix.pch为方便起见，我总是将此文件包含在前缀标题 ( ) 中。

Constants+Macros.h

常量+宏.h

// error handling ...

#define FS_ERROR_KEY(code)                    [NSString stringWithFormat:@"%d", code]
#define FS_ERROR_LOCALIZED_DESCRIPTION(code)  NSLocalizedStringFromTable(FS_ERROR_KEY(code), @"FSError", nil)

---

---

3) Now it's easy to show a user friendly error message based on an error code. An example:

3) 现在很容易根据错误代码显示用户友好的错误消息。一个例子：

UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Error" 
            message:FS_ERROR_LOCALIZED_DESCRIPTION(error.code) 
            delegate:nil 
            cancelButtonTitle:@"OK" 
            otherButtonTitles:nil];
[alert show];

回答by jlmendezbonini

Great answer Alex. One potential issue is the NULL dereference. Apple's reference on Creating and Returning NSError objects

很好的答案亚历克斯。一个潜在的问题是 NULL 取消引用。Apple 关于创建和返回 NSError 对象的参考

...
[details setValue:@"ran out of money" forKey:NSLocalizedDescriptionKey];

if (error != NULL) {
    // populate the error object with the details
    *error = [NSError errorWithDomain:@"world" code:200 userInfo:details];
}
// we couldn't feed the world's children...return nil..sniffle...sniffle
return nil;
...

回答by AlBeebe

Objective-C

目标-C

NSError *err = [NSError errorWithDomain:@"some_domain"
                                   code:100
                               userInfo:@{
                                           NSLocalizedDescriptionKey:@"Something went wrong"
                               }];

Swift 3

斯威夫特 3

let error = NSError(domain: "some_domain",
                      code: 100,
                  userInfo: [NSLocalizedDescriptionKey: "Something went wrong"])

回答by Tirth

Please refer following tutorial

请参考以下教程

i hope it will helpful for you but prior you have to read documentation of NSError

我希望它对你有帮助，但在你必须阅读NSError 的文档之前

This is very interesting link i found recently ErrorHandling

这是我最近发现的非常有趣的链接ErrorHandling

回答by Gabriele Petronella

I'll try summarize the great answer by Alex and the jlmendezbonini's point, adding a modification that will make everything ARC compatible (so far it's not since ARC will complain since you should return id, which means "any object", but BOOLis not an object type).

我将尝试总结 Alex 和 jlmendezbonini 的观点，添加一个修改，使所有内容与 ARC 兼容（到目前为止，不是因为 ARC 会抱怨，因为您应该返回id，这意味着“任何对象”，但BOOL不是对象类型）。

- (BOOL) endWorldHunger:(id)largeAmountsOfMonies error:(NSError**)error {
    // begin feeding the world's children...
    // it's all going well until....
    if (ohNoImOutOfMonies) {
        // sad, we can't solve world hunger, but we can let people know what went wrong!
        // init dictionary to be used to populate error object
        NSMutableDictionary* details = [NSMutableDictionary dictionary];
        [details setValue:@"ran out of money" forKey:NSLocalizedDescriptionKey];
        // populate the error object with the details
        if (error != NULL) {
             // populate the error object with the details
             *error = [NSError errorWithDomain:@"world" code:200 userInfo:details];
        }
        // we couldn't feed the world's children...return nil..sniffle...sniffle
        return NO;
    }
    // wohoo! We fed the world's children. The world is now in lots of debt. But who cares? 
    return YES;
}

Now instead of checking for the return value of our method call, we check whether erroris still nil. If it's not we have a problem.

现在，我们不再检查方法调用的返回值，而是检查是否error仍然是nil。如果不是，我们就有问题。

// initialize NSError object
NSError* error = nil;
// try to feed the world
BOOL success = [self endWorldHunger:smallAmountsOfMonies error:&error];
if (!success) {
   // inspect error
   NSLog(@"%@", [error localizedDescription]);
}
// otherwise the world has been fed. Wow, your code must rock.

回答by Senseful

Another design pattern that I have seen involves using blocks, which is especially useful when a method is being run asynchronously.

我见过的另一种设计模式涉及使用块，这在异步运行方法时特别有用。

Say we have the following error codes defined:

假设我们定义了以下错误代码：

typedef NS_ENUM(NSInteger, MyErrorCodes) {
    MyErrorCodesEmptyString = 500,
    MyErrorCodesInvalidURL,
    MyErrorCodesUnableToReachHost,
};

You would define your method that can raise an error like so:

您将定义可以引发错误的方法，如下所示：

- (void)getContentsOfURL:(NSString *)path success:(void(^)(NSString *html))success failure:(void(^)(NSError *error))failure {
    if (path.length == 0) {
        if (failure) {
            failure([NSError errorWithDomain:@"com.example" code:MyErrorCodesEmptyString userInfo:nil]);
        }
        return;
    }

    NSString *htmlContents = @"";

    // Exercise for the reader: get the contents at that URL or raise another error.

    if (success) {
        success(htmlContents);
    }
}

And then when you call it, you don't need to worry about declaring the NSError object (code completion will do it for you), or checking the returning value. You can just supply two blocks: one that will get called when there is an exception, and one that gets called when it succeeds:

然后当你调用它时，你不需要担心声明 NSError 对象（代码完成会为你做），或者检查返回值。你可以只提供两个块：一个在出现异常时被调用，一个在成功时被调用：

[self getContentsOfURL:@"http://google.com" success:^(NSString *html) {
    NSLog(@"Contents: %@", html);
} failure:^(NSError *error) {
    NSLog(@"Failed to get contents: %@", error);
    if (error.code == MyErrorCodesEmptyString) { // make sure to check the domain too
        NSLog(@"You must provide a non-empty string");
    }
}];

回答by Mike.R

Well it's a little bit out of question scope but in case you don't have an option for NSError you can always display the Low level error:

好吧，这有点超出问题范围，但如果您没有 NSError 选项，您始终可以显示低级错误：

 NSLog(@"Error = %@ ",[NSString stringWithUTF8String:strerror(errno)]);

回答by Hemang

extension NSError {
    static func defaultError() -> NSError {
        return NSError(domain: "com.app.error.domain", code: 0, userInfo: [NSLocalizedDescriptionKey: "Something went wrong."])
    }
}

which I can use NSError.defaultError()whenever I don't have valid error object.

NSError.defaultError()每当我没有有效的错误对象时，我就可以使用它。

let error = NSError.defaultError()
print(error.localizedDescription) //Something went wrong.