AFNetworking 2.6.3 源码解析

作者: FlyElephant | 来源:发表于2017-06-26 09:31 被阅读78次

到了现在大部分项目使用的AFNetworking应该是3.0以上版本，源码分析基本上都是都是3.0的文章介绍，关于2.0的介绍只有bang神的博客.本文分析以AFNetworking 2.6.3为基础进行分析，如果大家已经看过了类似文章，可以忽略我.

AFNetworking主要由NSURLConnction，NSURLSession，Security，Reachability和Serialization五个文件夹组成，AFNetworking对常见的UIKit组件进行扩展，这一部分就不介绍了，项目中实际使用的不多.

NSURLConnection

AFURLConnectionOperation是NSOperation的子类，实现的协议如下:
<pre><code>@interface AFURLConnectionOperation : NSOperation <NSURLConnectionDelegate, NSURLConnectionDataDelegate, NSSecureCoding, NSCopying></code></pre>

相对于日常开发中常用的NSCoding协议NSSecureCoding更加安全，可以防止信息被篡改，如果解档的类和要求的类类型不一致的时候会抛出异常，详情见参考链接.

单例请求的队列queue和队列组group，queue的命名堪称典范:
<pre><code>`static dispatch_group_t url_request_operation_completion_group() {
static dispatch_group_t af_url_request_operation_completion_group;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
af_url_request_operation_completion_group = dispatch_group_create();
});

return af_url_request_operation_completion_group;

}

static dispatch_queue_t url_request_operation_completion_queue() {
static dispatch_queue_t af_url_request_operation_completion_queue;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
af_url_request_operation_completion_queue = dispatch_queue_create("com.alamofire.networking.operation.queue", DISPATCH_QUEUE_CONCURRENT );
});

return af_url_request_operation_completion_queue;

}`</code></pre>

静态内联函数，static inline可以理解为static函数加入了inline属性，编译的时候会将其展开编译而不是为函数生成独立的汇编码.
<pre><code>`static inline NSString * AFKeyPathFromOperationState(AFOperationState state) {
switch (state) {
case AFOperationReadyState:
return @"isReady";
case AFOperationExecutingState:
return @"isExecuting";
case AFOperationFinishedState:
return @"isFinished";
case AFOperationPausedState:
return @"isPaused";
default: {

pragma clang diagnostic push

pragma clang diagnostic ignored "-Wunreachable-code"

        return @"state";

pragma clang diagnostic pop

}
}

}`</code></pre>

创建线程默认是主线的Runloop下，Runloop模式默认为NSDefaultRunLoopMode模式，如果用户操作UI滑动屏幕，会切换到UITrackingRunLoopMode模式下，导致请求失效.AFNetworking创建一条常驻线程:
<pre><code>`+ (void)networkRequestThreadEntryPoint:(id)__unused object {
@autoreleasepool {
[[NSThread currentThread] setName:@"AFNetworking"];

    NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
    [runLoop addPort:[NSMachPort port] forMode:NSDefaultRunLoopMode];
    [runLoop run];
}

}

(NSThread *)networkRequestThread {
static NSThread *_networkRequestThread = nil;
static dispatch_once_t oncePredicate;
dispatch_once(&oncePredicate, ^{
_networkRequestThread = [[NSThread alloc] initWithTarget:self selector:@selector(networkRequestThreadEntryPoint:) object:nil];
[_networkRequestThread start];
});

return _networkRequestThread;
}`</code></pre>

批量请求的代码，线程的异步转同步，线程之间的依赖写的很👍:
<pre><code>`+ (NSArray *)batchOfRequestOperations:(NSArray *)operations
progressBlock:(void (^)(NSUInteger numberOfFinishedOperations, NSUInteger totalNumberOfOperations))progressBlock
completionBlock:(void (^)(NSArray *operations))completionBlock
{
if (!operations || [operations count] == 0) {
return @[[NSBlockOperation blockOperationWithBlock:^{
dispatch_async(dispatch_get_main_queue(), ^{
if (completionBlock) {
completionBlock(@[]);
}
});
}]];
}

__block dispatch_group_t group = dispatch_group_create();
NSBlockOperation *batchedOperation = [NSBlockOperation blockOperationWithBlock:^{
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        if (completionBlock) {
            completionBlock(operations);
        }
    });
}];

for (AFURLConnectionOperation *operation in operations) {
    operation.completionGroup = group;
    void (^originalCompletionBlock)(void) = [operation.completionBlock copy];
    __weak __typeof(operation)weakOperation = operation;
    operation.completionBlock = ^{
        __strong __typeof(weakOperation)strongOperation = weakOperation;

pragma clang diagnostic push

pragma clang diagnostic ignored "-Wgnu"

        dispatch_queue_t queue = strongOperation.completionQueue ?: dispatch_get_main_queue();

pragma clang diagnostic pop

        dispatch_group_async(group, queue, ^{
            if (originalCompletionBlock) {
                originalCompletionBlock();
            }

            NSUInteger numberOfFinishedOperations = [[operations indexesOfObjectsPassingTest:^BOOL(id op, NSUInteger __unused idx,  BOOL __unused *stop) {
                return [op isFinished];
            }] count];

            if (progressBlock) {
                progressBlock(numberOfFinishedOperations, [operations count]);
            }

            dispatch_group_leave(group);
        });
    };

    dispatch_group_enter(group);
    [batchedOperation addDependency:operation];
}

return [operations arrayByAddingObject:batchedOperation];

}`</code></pre>

AFHTTPRequestOperation继承自AFURLConnectionOperation，其中的暂停的操作对文件流的操作代码如下:
<pre><code>`- (void)pause {
[super pause];

u_int64_t offset = 0;
if ([self.outputStream propertyForKey:NSStreamFileCurrentOffsetKey]) {
    offset = [(NSNumber *)[self.outputStream propertyForKey:NSStreamFileCurrentOffsetKey] unsignedLongLongValue];
} else {
    offset = [(NSData *)[self.outputStream propertyForKey:NSStreamDataWrittenToMemoryStreamKey] length];
}

NSMutableURLRequest *mutableURLRequest = [self.request mutableCopy];
if ([self.response respondsToSelector:@selector(allHeaderFields)] && [[self.response allHeaderFields] valueForKey:@"ETag"]) {
    [mutableURLRequest setValue:[[self.response allHeaderFields] valueForKey:@"ETag"] forHTTPHeaderField:@"If-Range"];
}
[mutableURLRequest setValue:[NSString stringWithFormat:@"bytes=%llu-", offset] forHTTPHeaderField:@"Range"];
self.request = mutableURLRequest;

}
`</code></pre>

AFHTTPRequestOperationManager对所有的Operation进行管理，通过初始化就覆盖了所有的核心类文件，序列化，安全协议和网络请求管理文件.
<pre><code>`- (instancetype)initWithBaseURL:(NSURL *)url {
self = [super init];
if (!self) {
return nil;
}

// Ensure terminal slash for baseURL path, so that NSURL +URLWithString:relativeToURL: works as expected
if ([[url path] length] > 0 && ![[url absoluteString] hasSuffix:@"/"]) {
    url = [url URLByAppendingPathComponent:@""];
}

self.baseURL = url;

self.requestSerializer = [AFHTTPRequestSerializer serializer];
self.responseSerializer = [AFJSONResponseSerializer serializer];

self.securityPolicy = [AFSecurityPolicy defaultPolicy];

self.reachabilityManager = [AFNetworkReachabilityManager sharedManager];

self.operationQueue = [[NSOperationQueue alloc] init];

self.shouldUseCredentialStorage = YES;

return self;

}
`</code></pre>

提供http请求的六种方式GET，HEAD，POST，PUT，PATCH，DELETE.
<pre><code>`- (nullable AFHTTPRequestOperation *)GET:(NSString *)URLString
parameters:(nullable id)parameters
success:(nullable void (^)(AFHTTPRequestOperation *operation, id responseObject))success
failure:(nullable void (^)(AFHTTPRequestOperation * __nullable operation, NSError *error))failure;

/**
Creates and runs an AFHTTPRequestOperation with a HEAD request.

@param URLString The URL string used to create the request URL.
@param parameters The parameters to be encoded according to the client request serializer.
@param success A block object to be executed when the request operation finishes successfully. This block has no return value and takes a single arguments: the request operation.
@param failure A block object to be executed when the request operation finishes unsuccessfully, or that finishes successfully, but encountered an error while parsing the response data. This block has no return value and takes a two arguments: the request operation and the error describing the network or parsing error that occurred.

@see -HTTPRequestOperationWithRequest:success:failure:
*/

(nullable AFHTTPRequestOperation *)HEAD:(NSString *)URLString
parameters:(nullable id)parameters
success:(nullable void (^)(AFHTTPRequestOperation *operation))success
failure:(nullable void (^)(AFHTTPRequestOperation * __nullable operation, NSError *error))failure;

/**
Creates and runs an AFHTTPRequestOperation with a POST request.

@param URLString The URL string used to create the request URL.
@param parameters The parameters to be encoded according to the client request serializer.
@param success A block object to be executed when the request operation finishes successfully. This block has no return value and takes two arguments: the request operation, and the response object created by the client response serializer.
@param failure A block object to be executed when the request operation finishes unsuccessfully, or that finishes successfully, but encountered an error while parsing the response data. This block has no return value and takes a two arguments: the request operation and the error describing the network or parsing error that occurred.

@see -HTTPRequestOperationWithRequest:success:failure:
*/

(nullable AFHTTPRequestOperation *)POST:(NSString *)URLString
parameters:(nullable id)parameters
success:(nullable void (^)(AFHTTPRequestOperation *operation, id responseObject))success
failure:(nullable void (^)(AFHTTPRequestOperation * __nullable operation, NSError *error))failure;

/**
Creates and runs an AFHTTPRequestOperation with a multipart POST request.

@param URLString The URL string used to create the request URL.
@param parameters The parameters to be encoded according to the client request serializer.
@param block A block that takes a single argument and appends data to the HTTP body. The block argument is an object adopting the AFMultipartFormData protocol.
@param success A block object to be executed when the request operation finishes successfully. This block has no return value and takes two arguments: the request operation, and the response object created by the client response serializer.
@param failure A block object to be executed when the request operation finishes unsuccessfully, or that finishes successfully, but encountered an error while parsing the response data. This block has no return value and takes a two arguments: the request operation and the error describing the network or parsing error that occurred.

@see -HTTPRequestOperationWithRequest:success:failure:
*/

(nullable AFHTTPRequestOperation *)POST:(NSString *)URLString
parameters:(nullable id)parameters
constructingBodyWithBlock:(nullable void (^)(id <AFMultipartFormData> formData))block
success:(nullable void (^)(AFHTTPRequestOperation *operation, id responseObject))success
failure:(nullable void (^)(AFHTTPRequestOperation * __nullable operation, NSError *error))failure;

/**
Creates and runs an AFHTTPRequestOperation with a PUT request.

@param URLString The URL string used to create the request URL.
@param parameters The parameters to be encoded according to the client request serializer.
@param success A block object to be executed when the request operation finishes successfully. This block has no return value and takes two arguments: the request operation, and the response object created by the client response serializer.
@param failure A block object to be executed when the request operation finishes unsuccessfully, or that finishes successfully, but encountered an error while parsing the response data. This block has no return value and takes a two arguments: the request operation and the error describing the network or parsing error that occurred.

@see -HTTPRequestOperationWithRequest:success:failure:
*/

(nullable AFHTTPRequestOperation *)PUT:(NSString *)URLString
parameters:(nullable id)parameters
success:(nullable void (^)(AFHTTPRequestOperation *operation, id responseObject))success
failure:(nullable void (^)(AFHTTPRequestOperation * __nullable operation, NSError *error))failure;

/**
Creates and runs an AFHTTPRequestOperation with a PATCH request.

@param URLString The URL string used to create the request URL.
@param parameters The parameters to be encoded according to the client request serializer.
@param success A block object to be executed when the request operation finishes successfully. This block has no return value and takes two arguments: the request operation, and the response object created by the client response serializer.
@param failure A block object to be executed when the request operation finishes unsuccessfully, or that finishes successfully, but encountered an error while parsing the response data. This block has no return value and takes a two arguments: the request operation and the error describing the network or parsing error that occurred.

@see -HTTPRequestOperationWithRequest:success:failure:
*/

(nullable AFHTTPRequestOperation *)PATCH:(NSString *)URLString
parameters:(nullable id)parameters
success:(nullable void (^)(AFHTTPRequestOperation *operation, id responseObject))success
failure:(nullable void (^)(AFHTTPRequestOperation * __nullable operation, NSError *error))failure;

/**
Creates and runs an AFHTTPRequestOperation with a DELETE request.

@param URLString The URL string used to create the request URL.
@param parameters The parameters to be encoded according to the client request serializer.
@param success A block object to be executed when the request operation finishes successfully. This block has no return value and takes two arguments: the request operation, and the response object created by the client response serializer.
@param failure A block object to be executed when the request operation finishes unsuccessfully, or that finishes successfully, but encountered an error while parsing the response data. This block has no return value and takes a two arguments: the request operation and the error describing the network or parsing error that occurred.

@see -HTTPRequestOperationWithRequest:success:failure:
*/

(nullable AFHTTPRequestOperation *)DELETE:(NSString *)URLString
parameters:(nullable id)parameters
success:(nullable void (^)(AFHTTPRequestOperation *operation, id responseObject))success
failure:(nullable void (^)(AFHTTPRequestOperation * __nullable operation, NSError *error))failure;`</code></pre>

整个网络请求回调顺序:

AFNetworking.png

NSURLSession

NSURLSession文件夹下有两个文件AFURLSessionManager和AFHTTPSessionManager.
<pre><code>@interface AFURLSessionManager : NSObject <NSURLSessionDelegate, NSURLSessionTaskDelegate, NSURLSessionDataDelegate, NSURLSessionDownloadDelegate, NSSecureCoding, NSCopying></code></pre>

AFURLSessionManager有一个私有类_AFURLSessionTaskSwizzling，交换NSURLSessionTask中的暂停和继续方法.

交换函数:

<pre><code>`static inline void af_swizzleSelector(Class theClass, SEL originalSelector, SEL swizzledSelector) {
Method originalMethod = class_getInstanceMethod(theClass, originalSelector);
Method swizzledMethod = class_getInstanceMethod(theClass, swizzledSelector);
method_exchangeImplementations(originalMethod, swizzledMethod);
}

static inline BOOL af_addMethod(Class theClass, SEL selector, Method method) {
return class_addMethod(theClass, selector, method_getImplementation(method), method_getTypeEncoding(method));
}
`</code></pre>

交换过程:

<pre><code>`

(void)load {
/**
WARNING: Trouble Ahead
https://github.com/AFNetworking/AFNetworking/pull/2702
*/

if (NSClassFromString(@"NSURLSessionTask")) {
/**
iOS 7 and iOS 8 differ in NSURLSessionTask implementation, which makes the next bit of code a bit tricky.
Many Unit Tests have been built to validate as much of this behavior has possible.
Here is what we know:
- NSURLSessionTasks are implemented with class clusters, meaning the class you request from the API isn't actually the type of class you will get back.
- Simply referencing [NSURLSessionTask class] will not work. You need to ask an NSURLSession to actually create an object, and grab the class from there.
- On iOS 7, localDataTask is a __NSCFLocalDataTask, which inherits from __NSCFLocalSessionTask, which inherits from __NSCFURLSessionTask.
- On iOS 8, localDataTask is a __NSCFLocalDataTask, which inherits from __NSCFLocalSessionTask, which inherits from NSURLSessionTask.
- On iOS 7, __NSCFLocalSessionTask and __NSCFURLSessionTask are the only two classes that have their own implementations of resume and suspend, and __NSCFLocalSessionTask DOES NOT CALL SUPER. This means both classes need to be swizzled.
- On iOS 8, NSURLSessionTask is the only class that implements resume and suspend. This means this is the only class that needs to be swizzled.
- Because NSURLSessionTask is not involved in the class hierarchy for every version of iOS, its easier to add the swizzled methods to a dummy class and manage them there.
```
   Some Assumptions:
      - No implementations of `resume` or `suspend` call super. If this were to change in a future version of iOS, we'd need to handle it.
      - No background task classes override `resume` or `suspend`
   
   The current solution:
      1) Grab an instance of `__NSCFLocalDataTask` by asking an instance of `NSURLSession` for a data task.
      2) Grab a pointer to the original implementation of `af_resume`
      3) Check to see if the current class has an implementation of resume. If so, continue to step 4.
      4) Grab the super class of the current class.
      5) Grab a pointer for the current class to the current implementation of `resume`.
      6) Grab a pointer for the super class to the current implementation of `resume`.
      7) If the current class implementation of `resume` is not equal to the super class implementation of `resume` AND the current implementation of `resume` is not equal to the original implementation of `af_resume`, THEN swizzle the methods
      8) Set the current class to the super class, and repeat steps 3-8
   */
  NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration ephemeralSessionConfiguration];
  NSURLSession * session = [NSURLSession sessionWithConfiguration:configuration];
```

pragma GCC diagnostic push

pragma GCC diagnostic ignored "-Wnonnull"

    NSURLSessionDataTask *localDataTask = [session dataTaskWithURL:nil];

pragma clang diagnostic pop

    IMP originalAFResumeIMP = method_getImplementation(class_getInstanceMethod([self class], @selector(af_resume)));
    Class currentClass = [localDataTask class];
    
    while (class_getInstanceMethod(currentClass, @selector(resume))) {
        Class superClass = [currentClass superclass];
        IMP classResumeIMP = method_getImplementation(class_getInstanceMethod(currentClass, @selector(resume)));
        IMP superclassResumeIMP = method_getImplementation(class_getInstanceMethod(superClass, @selector(resume)));
        if (classResumeIMP != superclassResumeIMP &&
            originalAFResumeIMP != classResumeIMP) {
            [self swizzleResumeAndSuspendMethodForClass:currentClass];
        }
        currentClass = [currentClass superclass];
    }
    
    [localDataTask cancel];
    [session finishTasksAndInvalidate];
}

}

(void)swizzleResumeAndSuspendMethodForClass:(Class)theClass {
Method afResumeMethod = class_getInstanceMethod(self, @selector(af_resume));
Method afSuspendMethod = class_getInstanceMethod(self, @selector(af_suspend));

if (af_addMethod(theClass, @selector(af_resume), afResumeMethod)) {
af_swizzleSelector(theClass, @selector(resume), @selector(af_resume));
}

if (af_addMethod(theClass, @selector(af_suspend), afSuspendMethod)) {
af_swizzleSelector(theClass, @selector(suspend), @selector(af_suspend));
}
}`</code></pre>

初始化过程与AFURLConnectionOperation的常驻线程处理方式不一样，设置最大并发数量为1，想当于是串行队列.
<pre><code>`- (instancetype)initWithSessionConfiguration:(NSURLSessionConfiguration *)configuration {
self = [super init];
if (!self) {
return nil;
}

if (!configuration) {
    configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
}

self.sessionConfiguration = configuration;

self.operationQueue = [[NSOperationQueue alloc] init];
self.operationQueue.maxConcurrentOperationCount = 1;

self.session = [NSURLSession sessionWithConfiguration:self.sessionConfiguration delegate:self delegateQueue:self.operationQueue];

self.responseSerializer = [AFJSONResponseSerializer serializer];

self.securityPolicy = [AFSecurityPolicy defaultPolicy];

if !TARGET_OS_WATCH

self.reachabilityManager = [AFNetworkReachabilityManager sharedManager];

endif

self.mutableTaskDelegatesKeyedByTaskIdentifier = [[NSMutableDictionary alloc] init];

self.lock = [[NSLock alloc] init];
self.lock.name = AFURLSessionManagerLockName;

[self.session getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {
    for (NSURLSessionDataTask *task in dataTasks) {
        [self addDelegateForDataTask:task completionHandler:nil];
    }

    for (NSURLSessionUploadTask *uploadTask in uploadTasks) {
        [self addDelegateForUploadTask:uploadTask progress:nil completionHandler:nil];
    }

    for (NSURLSessionDownloadTask *downloadTask in downloadTasks) {
        [self addDelegateForDownloadTask:downloadTask progress:nil destination:nil completionHandler:nil];
    }
}];

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(taskDidResume:) name:AFNSURLSessionTaskDidResumeNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(taskDidSuspend:) name:AFNSURLSessionTaskDidSuspendNotification object:nil];

return self;

}`</code></pre>

任务的暂停和继续通过通知来执行:
<pre><code>`- (void)taskDidResume:(NSNotification *)notification {
NSURLSessionTask *task = notification.object;
if ([task respondsToSelector:@selector(taskDescription)]) {
if ([task.taskDescription isEqualToString:self.taskDescriptionForSessionTasks]) {
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:AFNetworkingTaskDidResumeNotification object:task];
});
}
}
}

(void)taskDidSuspend:(NSNotification *)notification {
NSURLSessionTask *task = notification.object;
if ([task respondsToSelector:@selector(taskDescription)]) {
if ([task.taskDescription isEqualToString:self.taskDescriptionForSessionTasks]) {
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:AFNetworkingTaskDidSuspendNotification object:task];
});
}
}
}`</code></pre>

进度管理通过AFURLSessionManagerTaskDelegate来实现：
<pre><code>@interface AFURLSessionManagerTaskDelegate : NSObject <NSURLSessionTaskDelegate, NSURLSessionDataDelegate, NSURLSessionDownloadDelegate> @property (nonatomic, weak) AFURLSessionManager *manager; @property (nonatomic, strong) NSMutableData *mutableData; @property (nonatomic, strong) NSProgress *progress; @property (nonatomic, copy) NSURL *downloadFileURL; @property (nonatomic, copy) AFURLSessionDownloadTaskDidFinishDownloadingBlock downloadTaskDidFinishDownloading; @property (nonatomic, copy) AFURLSessionTaskCompletionHandler completionHandler; @end</code></pre>

异步转同步的另外一种实现方式:
<pre><code>`- (NSArray *)tasksForKeyPath:(NSString *)keyPath {
__block NSArray *tasks = nil;
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
[self.session getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {
if ([keyPath isEqualToString:NSStringFromSelector(@selector(dataTasks))]) {
tasks = dataTasks;
} else if ([keyPath isEqualToString:NSStringFromSelector(@selector(uploadTasks))]) {
tasks = uploadTasks;
} else if ([keyPath isEqualToString:NSStringFromSelector(@selector(downloadTasks))]) {
tasks = downloadTasks;
} else if ([keyPath isEqualToString:NSStringFromSelector(@selector(tasks))]) {
tasks = [@[dataTasks, uploadTasks, downloadTasks] valueForKeyPath:@"@unionOfArrays.self"];
}

    dispatch_semaphore_signal(semaphore);
}];

dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

return tasks;

}

(NSArray *)tasks {
return [self tasksForKeyPath:NSStringFromSelector(_cmd)];
}
`</code></pre>

AFHTTPSessionManger是AFURLSessionManager的子类，提供了GET，HEAD，POST，PUT，PATCH和DELETE六种访问方式.
<pre><code>`- (nullable NSURLSessionDataTask *)GET:(NSString *)URLString
parameters:(nullable id)parameters
success:(nullable void (^)(NSURLSessionDataTask *task, id responseObject))success
failure:(nullable void (^)(NSURLSessionDataTask * __nullable task, NSError *error))failure;

/**
Creates and runs an NSURLSessionDataTask with a HEAD request.

@param URLString The URL string used to create the request URL.
@param parameters The parameters to be encoded according to the client request serializer.
@param success A block object to be executed when the task finishes successfully. This block has no return value and takes a single arguments: the data task.
@param failure A block object to be executed when the task finishes unsuccessfully, or that finishes successfully, but encountered an error while parsing the response data. This block has no return value and takes a two arguments: the data task and the error describing the network or parsing error that occurred.