Objective-C iOS Development for Legacy & Enterprise Apps · 课时

使用 NSURLSession 调用 API

使用 NSURLSession 执行网络请求、处理响应，并通过 RESTful API 管理数据传输。

使用 NSURLSession 调用 API 是 CoddyKit 上的免费 Objective-C iOS Development for Legacy & Enterprise Apps 课时。这是第 1 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Objective-C iOS Development for Legacy & Enterprise Apps 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Objective-C iOS Development for Legacy & Enterprise Apps 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Connecting Your App to the World

Mobile apps often need to talk to the internet to fetch or send data. This is called networking.

Think of checking the weather, loading your social media feed, or sending a message. All these involve your app making requests to a server.

Servers expose APIs (Application Programming Interfaces), which are sets of rules for how your app can interact with them. Many modern APIs follow the REST architectural style.

Meet NSURLSession

In Objective-C, the primary way to perform network requests is using NSURLSession. It's a powerful and flexible API for downloading content over HTTP/HTTPS.

It replaced older methods like NSURLConnection.
It handles data tasks, download tasks, and upload tasks.
It supports background transfers and authentication.

NSURLSession makes network communication more efficient and easier to manage.

NSURLSession's Building Blocks

NSURLSession works with a few key components:

NSURLSession: The main object that coordinates network tasks.
NSURLSessionConfiguration: Defines the behavior of a session (e.g., caching, timeouts, cellular access).
NSURLSessionTask: An abstract class for tasks. Concrete subclasses include NSURLSessionDataTask (for data), NSURLSessionDownloadTask (for files), and NSURLSessionUploadTask (for uploading files).

Making a Simple GET Request

Let's make a basic GET request to fetch data from a server. A GET request is used to retrieve information.

We'll use NSURLSessionDataTask. This task takes a URL and a completion handler (a block of code) that runs when the request finishes.

Remember to call [task resume]; to start the task!

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // 1. Create a URL
        NSURL *url = [NSURL URLWithString:@"https://jsonplaceholder.typicode.com/todos/1"];

        // 2. Create a session (using the default shared session)
        NSURLSession *session = [NSURLSession sharedSession];

        // 3. Create a data task
        NSURLSessionDataTask *dataTask = [session dataTaskWithURL:url completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
            if (error) {
                NSLog(@"Error: %@", error.localizedDescription);
            } else {
                // Check for HTTP status code (e.g., 200 OK)
                NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
                if (httpResponse.statusCode == 200) {
                    NSString *responseString = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
                    NSLog(@"Response Data: %@", responseString);
                } else {
                    NSLog(@"Server returned status code: %ld", (long)httpResponse.statusCode);
                }
            }
            CFRunLoopStop(CFRunLoopGetCurrent()); // Stop the run loop once task finishes
        }];

        // 4. Start the task
        [dataTask resume];

        // Keep the main thread alive until the async task completes
        CFRunLoopRun();
    }
    return 0;
}

Understanding the Response

The completion handler block for a data task provides three parameters:

data: The raw data received from the server (e.g., JSON, image bytes).
response: An NSURLResponse object, which can be cast to NSHTTPURLResponse for HTTP-specific details like status codes (200 OK, 404 Not Found, etc.) and headers.
error: An NSError object if something went wrong during the request (e.g., no internet connection, server unreachable).

Always check for error first, then the HTTP status code.

Decoding JSON Responses

Most REST APIs return data in JSON (JavaScript Object Notation) format. This is a lightweight, human-readable data interchange format.

Objective-C provides NSJSONSerialization to convert raw NSData into Objective-C objects (NSDictionary or NSArray) and vice-versa.

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSURL *url = [NSURL URLWithString:@"https://jsonplaceholder.typicode.com/todos/1"];
        NSURLSession *session = [NSURLSession sharedSession];
        NSURLSessionDataTask *dataTask = [session dataTaskWithURL:url completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
            if (error) {
                NSLog(@"Error: %@", error.localizedDescription);
            } else {
                NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
                if (httpResponse.statusCode == 200) {
                    NSError *jsonError = nil;
                    // Attempt to parse JSON data
                    id jsonObject = [NSJSONSerialization JSONObjectWithData:data
                                                                    options:0 // No special options
                                                                      error:&jsonError];
                    if (jsonError) {
                        NSLog(@"JSON Parsing Error: %@", jsonError.localizedDescription);
                    } else if ([jsonObject isKindOfClass:[NSDictionary class]]) {
                        NSDictionary *todoItem = (NSDictionary *)jsonObject;
                        NSLog(@"Todo Title: %@", todoItem[@"title"]);
                        NSLog(@"Completed: %@", todoItem[@"completed"] ? @"YES" : @"NO");
                    } else {
                        NSLog(@"Received unexpected JSON format.");
                    }
                } else {
                    NSLog(@"Server returned status code: %ld", (long)httpResponse.statusCode);
                }
            }
            CFRunLoopStop(CFRunLoopGetCurrent());
        }];
        [dataTask resume];
        CFRunLoopRun();
    }
    return 0;
}

Tailoring NSURLSession with Configuration

For more control, you can create a custom NSURLSession using an NSURLSessionConfiguration object.

Common configurations include:

defaultSessionConfiguration: Uses a global shared cookie storage, credential storage, and disk-based cache.
ephemeralSessionConfiguration: Similar to default, but stores no data to disk (e.g., no caches, cookies). Ideal for private browsing.
backgroundSessionConfigurationWithIdentifier:: Allows transfers to continue when your app is suspended or terminated.

You can set properties like timeoutIntervalForRequest, HTTPAdditionalHeaders, and more.

Sending Data with POST

While GET retrieves data, POST requests are used to send new data to the server (e.g., submitting a form, creating a new resource).

For POST, you typically create an NSURLRequest object, set its HTTP method to "POST", and attach the data you want to send as HTTPBody.

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSURL *url = [NSURL URLWithString:@"https://jsonplaceholder.typicode.com/posts"];
        NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
        request.HTTPMethod = @"POST"; // Set HTTP method to POST

        // Prepare data to send (e.g., a new post title and body)
        NSDictionary *postData = @{
            @"title": @"foo",
            @"body": @"bar",
            @"userId": @1
        };
        NSError *jsonError = nil;
        NSData *jsonData = [NSJSONSerialization dataWithJSONObject:postData options:0 error:&jsonError];

        if (jsonError) {
            NSLog(@"JSON serialization error: %@", jsonError.localizedDescription);
            CFRunLoopStop(CFRunLoopGetCurrent());
            return 1;
        }

        request.HTTPBody = jsonData;
        [request setValue:@"application/json" forHTTPHeaderField:@"Content-Type"];
        [request setValue:@"application/json" forHTTPHeaderField:@"Accept"];

        NSURLSession *session = [NSURLSession sharedSession];
        NSURLSessionDataTask *dataTask = [session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
            if (error) {
                NSLog(@"Error: %@", error.localizedDescription);
            } else {
                NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
                if (httpResponse.statusCode >= 200 && httpResponse.statusCode < 300) {
                    NSString *responseString = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
                    NSLog(@"POST Success! Response: %@", responseString);
                } else {
                    NSLog(@"Server returned status code: %ld", (long)httpResponse.statusCode);
                }
            }
            CFRunLoopStop(CFRunLoopGetCurrent());
        }];
        [dataTask resume];
        CFRunLoopRun();
    }
    return 0;
}

Handling Network Errors Gracefully

Network requests can fail for many reasons. Robust error handling is crucial for a good user experience.

Check the error object first: This indicates fundamental issues like no internet, DNS lookup failure, or server connection problems.
Check NSHTTPURLResponse status code: After confirming no error, check the HTTP status code (e.g., 400 Bad Request, 401 Unauthorized, 500 Internal Server Error).
Provide user feedback: Inform the user about the issue, e.g., "No internet connection" or "Could not load data."

Always assume network requests can fail and plan for it!

Quick Check: NSURLSession

Consider the following Objective-C code snippet for a network request:

NSURL *url = [NSURL URLWithString:@"https://example.com/data"];
NSURLSession *session = [NSURLSession sharedSession];
NSURLSessionDataTask *task = [session dataTaskWithURL:url completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
    if (error) {
        NSLog(@"Failed with error: %@", error.localizedDescription);
    } else {
        NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
        if (httpResponse.statusCode == 200) {
            NSLog(@"Data received!");
        } else {
            NSLog(@"HTTP Status: %ld", (long)httpResponse.statusCode);
        }
    }
}];
// Missing line here

Recap: Mastering NSURLSession

In this lesson, we explored how to perform network requests using NSURLSession in Objective-C.

We learned about NSURLSession, NSURLSessionConfiguration, and NSURLSessionTask.
You can make GET requests to fetch data and POST requests to send data.
We covered how to handle the response, including checking for errors and parsing JSON data using NSJSONSerialization.
Always remember to call [task resume]; to start a task and implement robust error handling.

Next, you'll dive into concurrency with Grand Central Dispatch to keep your UI responsive during network operations!

常见问题解答

「使用 NSURLSession 调用 API」课时是免费的吗？

是的 — 「使用 NSURLSession 调用 API」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Objective-C iOS Development for Legacy & Enterprise Apps 课程的其余内容，请升级到 CoddyKit PRO。 Objective-C iOS Development for Legacy & Enterprise Apps 课程共包含 4 节课。

「使用 NSURLSession 调用 API」这节课中我会学到什么？

使用 NSURLSession 执行网络请求、处理响应，并通过 RESTful API 管理数据传输。你通过在浏览器中直接运行的动手代码来练习 Objective-C iOS Development for Legacy & Enterprise Apps，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Objective-C iOS Development for Legacy & Enterprise Apps 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Objective-C iOS Development for Legacy & Enterprise Apps 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 1 节课，共 4 节。

「使用 NSURLSession 调用 API」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Objective-C iOS Development for Legacy & Enterprise Apps 课中编写并运行代码吗？

能。每节 Objective-C iOS Development for Legacy & Enterprise Apps 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

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