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面试题：Objective-C的Alamofire网络请求库在大型项目中的优化与架构设计

在一个大型的Objective-C项目中，使用Alamofire作为网络请求库。阐述如何对Alamofire进行封装以提高代码的可维护性和复用性，并且从性能优化角度，说明如何处理大量并发请求、缓存策略以及错误处理机制的设计，结合实际项目经验举例说明。
23.2万 热度难度
编程语言Objective-C

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面试题答案

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1. Alamofire 封装提高可维护性和复用性

  1. 创建网络请求基类
    • 定义一个继承自 NSObject 的基类，例如 NetworkManager。在这个类中封装通用的 Alamofire 请求配置，如设置公共的请求头、请求超时时间等。
    @interface NetworkManager : NSObject
+ (instancetype)sharedManager;
- (NSURLSessionDataTask *)requestWithURL:(NSString *)url
                                 method:(Alamofire.HTTPMethod)method
                            parameters:(NSDictionary *)parameters
                               headers:(NSDictionary *)headers
                          completion:(void (^)(id responseObject, NSError *error))completion;
@end

@implementation NetworkManager
+ (instancetype)sharedManager {
    static NetworkManager *manager = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        manager = [[NetworkManager alloc] init];
    });
    return manager;
}

- (NSURLSessionDataTask *)requestWithURL:(NSString *)url
                                 method:(Alamofire.HTTPMethod)method
                            parameters:(NSDictionary *)parameters
                               headers:(NSDictionary *)headers
                          completion:(void (^)(id responseObject, NSError *error))completion {
    NSMutableURLRequest *request = [Alamofire.request(url, method: method, parameters: parameters, headers: headers).request mutableCopy];
    request.timeoutInterval = 30; // 设置超时时间
    // 添加公共请求头
    [request setValue:@"application/json" forHTTPHeaderField:@"Content-Type"];
    return [Alamofire request:request].responseJSON(completionHandler:^(NSURLResponse * _Nonnull response, id  _Nullable responseObject, NSError * _Nullable error) {
        if (completion) {
            completion(responseObject, error);
        }
    });
}
@end
  2. 业务请求分类封装
    • 根据业务模块，创建不同的类继承自 NetworkManager，例如用户模块 UserNetworkManager，商品模块 ProductNetworkManager 等。在这些类中定义具体业务的请求方法。
    @interface UserNetworkManager : NetworkManager
- (NSURLSessionDataTask *)loginWithUsername:(NSString *)username password:(NSString *)password completion:(void (^)(id responseObject, NSError *error))completion;
@end

@implementation UserNetworkManager
- (NSURLSessionDataTask *)loginWithUsername:(NSString *)username password:(NSString *)password completion:(void (^)(id responseObject, NSError *error))completion {
    NSString *url = @"https://example.com/api/login";
    NSDictionary *parameters = @{@"username": username, @"password": password};
    return [self requestWithURL:url method:.post parameters:parameters headers:nil completion:completion];
}
@end
  3. 使用链式调用风格（可选）
    • 可以进一步优化封装，使用链式调用风格，使代码更易读。例如：
    @interface NetworkRequestBuilder : NSObject
- (NetworkRequestBuilder *)url:(NSString *)url;
- (NetworkRequestBuilder *)method:(Alamofire.HTTPMethod)method;
- (NetworkRequestBuilder *)parameters:(NSDictionary *)parameters;
- (NetworkRequestBuilder *)headers:(NSDictionary *)headers;
- (NSURLSessionDataTask *)execute:(void (^)(id responseObject, NSError *error))completion;
@end

@implementation NetworkRequestBuilder {
    NSString *_url;
    Alamofire.HTTPMethod _method;
    NSDictionary *_parameters;
    NSDictionary *_headers;
}
- (NetworkRequestBuilder *)url:(NSString *)url {
    _url = url;
    return self;
}
- (NetworkRequestBuilder *)method:(Alamofire.HTTPMethod)method {
    _method = method;
    return self;
}
- (NetworkRequestBuilder *)parameters:(NSDictionary *)parameters {
    _parameters = parameters;
    return self;
}
- (NetworkRequestBuilder *)headers:(NSDictionary *)headers {
    _headers = headers;
    return self;
}
- (NSURLSessionDataTask *)execute:(void (^)(id responseObject, NSError *error))completion {
    return [[NetworkManager sharedManager] requestWithURL:_url method:_method parameters:_parameters headers:_headers completion:completion];
}
@end
    • 使用方式：
    [[[NetworkRequestBuilder alloc] init]
 url:@"https://example.com/api/data"
 method:.get
 parameters:nil
 headers:nil
 execute:^(id responseObject, NSError *error) {
     // 处理响应
 }];

2. 性能优化 - 处理大量并发请求

  1. 使用队列控制并发数量
    • 创建一个 NSOperationQueue，并设置其最大并发数。例如，设置最大并发数为 5，以避免过多请求占用过多资源。
    NSOperationQueue *requestQueue = [[NSOperationQueue alloc] init];
requestQueue.maxConcurrentOperationCount = 5;
for (int i = 0; i < 20; i++) {
    NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
        [[NetworkManager sharedManager] requestWithURL:@"https://example.com/api/data" method:.get parameters:nil headers:nil completion:^(id responseObject, NSError *error) {
            // 处理响应
        }];
    }];
    [requestQueue addOperation:operation];
}
  2. 请求优先级设置
    • 根据业务需求，为不同的请求设置优先级。例如，用户登录请求优先级高于获取广告数据请求。在 Alamofire 中，可以通过 NSURLSessionDataTask 的 priority 属性设置优先级。
    NSURLSessionDataTask *loginTask = [[NetworkManager sharedManager] requestWithURL:@"https://example.com/api/login" method:.post parameters:loginParameters headers:nil completion:^(id responseObject, NSError *error) {
    // 处理登录响应
}];
loginTask.priority = NSURLSessionTaskPriorityHigh;

NSURLSessionDataTask *adTask = [[NetworkManager sharedManager] requestWithURL:@"https://example.com/api/ad" method:.get parameters:nil headers:nil completion:^(id responseObject, NSError *error) {
    // 处理广告响应
}];
adTask.priority = NSURLSessionTaskPriorityLow;

3. 性能优化 - 缓存策略

  1. 使用系统缓存
    • Alamofire 支持系统默认的缓存策略。可以通过设置 NSURLRequest 的 cachePolicy 属性来实现。例如，使用 NSURLRequestReturnCacheDataElseLoad 策略，优先返回缓存数据，如果缓存数据不存在则发起网络请求。
    NSMutableURLRequest *request = [Alamofire.request(url, method: method, parameters: parameters, headers: headers).request mutableCopy];
request.cachePolicy = NSURLRequestReturnCacheDataElseLoad;
return [Alamofire request:request].responseJSON(completionHandler:^(NSURLResponse * _Nonnull response, id  _Nullable responseObject, NSError * _Nullable error) {
    // 处理响应
});
  2. 自定义缓存
    • 创建一个本地缓存数据库（如 SQLite）或使用内存缓存（如 NSCache）。在请求成功后，将响应数据存储到缓存中，并在下次请求相同 URL 时先检查缓存。
    • 以 NSCache 为例：
    @interface NetworkCache : NSObject
+ (instancetype)sharedCache;
- (void)setCacheObject:(id)object forURL:(NSString *)url;
- (id)cacheObjectForURL:(NSString *)url;
@end

@implementation NetworkCache
+ (instancetype)sharedCache {
    static NetworkCache *cache = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        cache = [[NetworkCache alloc] init];
    });
    return cache;
}

- (void)setCacheObject:(id)object forURL:(NSString *)url {
    NSCache *cache = [[NSCache alloc] init];
    [cache setObject:object forKey:url];
}

- (id)cacheObjectForURL:(NSString *)url {
    NSCache *cache = [[NSCache alloc] init];
    return [cache objectForKey:url];
}
@end

- (NSURLSessionDataTask *)requestWithURL:(NSString *)url
                                 method:(Alamofire.HTTPMethod)method
                            parameters:(NSDictionary *)parameters
                               headers:(NSDictionary *)headers
                          completion:(void (^)(id responseObject, NSError *error))completion {
    id cachedObject = [[NetworkCache sharedCache] cacheObjectForURL:url];
    if (cachedObject) {
        if (completion) {
            completion(cachedObject, nil);
        }
        return nil;
    }
    NSMutableURLRequest *request = [Alamofire.request(url, method: method, parameters: parameters, headers: headers).request mutableCopy];
    return [Alamofire request:request].responseJSON(completionHandler:^(NSURLResponse * _Nonnull response, id  _Nullable responseObject, NSError * _Nullable error) {
        if (responseObject &&!error) {
            [[NetworkCache sharedCache] setCacheObject:responseObject forURL:url];
        }
        if (completion) {
            completion(responseObject, error);
        }
    });
}

4. 错误处理机制设计

  1. 统一错误处理
    • 在 NetworkManager 的请求完成回调中，对错误进行统一处理。例如，根据 NSError 的 code 判断错误类型，并给出相应的提示。
    - (NSURLSessionDataTask *)requestWithURL:(NSString *)url
                                 method:(Alamofire.HTTPMethod)method
                            parameters:(NSDictionary *)parameters
                               headers:(NSDictionary *)headers
                          completion:(void (^)(id responseObject, NSError *error))completion {
    //...
    return [Alamofire request:request].responseJSON(completionHandler:^(NSURLResponse * _Nonnull response, id  _Nullable responseObject, NSError * _Nullable error) {
        if (error) {
            NSString *errorMessage = @"网络请求失败";
            if (error.code == NSURLErrorTimedOut) {
                errorMessage = @"请求超时，请稍后重试";
            } else if (error.code == NSURLErrorNotConnectedToInternet) {
                errorMessage = @"请检查网络连接";
            }
            UIAlertController *alertController = [UIAlertController alertControllerWithTitle:@"错误" message:errorMessage preferredStyle:.alert];
            UIAlertAction *okAction = [UIAlertAction actionWithTitle:@"确定" style:.default handler:nil];
            [alertController addAction:okAction];
            [[UIApplication sharedApplication].keyWindow.rootViewController presentViewController:alertController animated:YES completion:nil];
        }
        if (completion) {
            completion(responseObject, error);
        }
    });
}
  2. 业务层错误处理
    • 在业务请求类（如 UserNetworkManager）中，可以根据业务逻辑对响应数据进行进一步的错误判断。例如，登录接口返回的状态码不为 200 时，提示用户名或密码错误。
    - (NSURLSessionDataTask *)loginWithUsername:(NSString *)username password:(NSString *)password completion:(void (^)(id responseObject, NSError *error))completion {
    //...
    return [self requestWithURL:url method:.post parameters:parameters headers:nil completion:^(id responseObject, NSError *error) {
        if (!error) {
            NSInteger statusCode = [(NSHTTPURLResponse *)response statusCode];
            if (statusCode!= 200) {
                error = [NSError errorWithDomain:@"UserLoginErrorDomain" code:statusCode userInfo:@{NSLocalizedDescriptionKey: @"用户名或密码错误"}];
            }
        }
        if (completion) {
            completion(responseObject, error);
        }
    }];
}

实际项目经验举例

在一个电商项目中，使用上述封装方法。通过 NetworkManager 统一配置请求头和超时时间，不同业务模块（如商品展示、用户订单等）有各自的网络请求类。在处理商品图片加载时，面临大量并发请求，通过设置 NSOperationQueue 的最大并发数为 3，避免了因过多请求导致的卡顿。同时，对商品列表数据采用了 NSURLRequestReturnCacheDataElseLoad 缓存策略，减少了不必要的网络请求，提高了响应速度。在错误处理方面，统一处理网络连接错误和超时错误，业务层如订单提交失败时，根据服务器返回的错误码给出具体的错误提示，如库存不足等，提升了用户体验。