面试题：Objective-C Protocol的代理模式应用

Question

假设你正在开发一个视图控制器（ViewController），它需要和一个网络请求类（NetworkRequest）进行交互。请使用Protocol和代理模式实现以下功能：网络请求完成后，将数据传递给视图控制器进行展示。请详细说明设计思路，并给出关键代码片段。

Answer 1

设计思路

1. 定义协议：创建一个协议，用于声明网络请求完成后传递数据给视图控制器的方法。
2. 网络请求类：在网络请求类中定义一个代理属性，类型为上述定义的协议。当网络请求完成时，调用代理方法传递数据。
3. 视图控制器：视图控制器遵循该协议，实现协议方法以处理接收到的数据并进行展示。同时，将自身设置为网络请求类的代理。

关键代码片段（以Swift为例）

1. 定义协议

protocol NetworkRequestDelegate {
    func networkRequestDidFinish(_ request: NetworkRequest, withData data: Data?)
}

2. 网络请求类

class NetworkRequest {
    var delegate: NetworkRequestDelegate?
    
    func startRequest() {
        // 模拟网络请求
        DispatchQueue.global().asyncAfter(deadline: .now() + 2) { [weak self] in
            let mockData = "Mock response data".data(using: .utf8)
            self?.delegate?.networkRequestDidFinish(self!, withData: mockData)
        }
    }
}

3. 视图控制器

class ViewController: UIViewController, NetworkRequestDelegate {
    let networkRequest = NetworkRequest()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        networkRequest.delegate = self
        networkRequest.startRequest()
    }
    
    func networkRequestDidFinish(_ request: NetworkRequest, withData data: Data?) {
        if let data = data, let text = String(data: data, encoding: .utf8) {
            print("Received data: \(text)")
            // 在这里进行数据展示相关操作，比如更新UI
        }
    }
}

关键代码片段（以Objective - C为例）

1. 定义协议

@protocol NetworkRequestDelegate <NSObject>
- (void)networkRequestDidFinish:(NetworkRequest *)request withData:(NSData *)data;
@end

2. 网络请求类

@interface NetworkRequest : NSObject
@property (nonatomic, weak) id<NetworkRequestDelegate> delegate;
- (void)startRequest;
@end

@implementation NetworkRequest
- (void)startRequest {
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSString *mockData = @"Mock response data";
        NSData *data = [mockData dataUsingEncoding:NSUTF8StringEncoding];
        [self.delegate networkRequestDidFinish:self withData:data];
    });
}
@end

3. 视图控制器

@interface ViewController : UIViewController <NetworkRequestDelegate>
@property (nonatomic, strong) NetworkRequest *networkRequest;
@end

@implementation ViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    self.networkRequest = [[NetworkRequest alloc] init];
    self.networkRequest.delegate = self;
    [self.networkRequest startRequest];
}

- (void)networkRequestDidFinish:(NetworkRequest *)request withData:(NSData *)data {
    NSString *text = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
    NSLog(@"Received data: %@", text);
    // 在这里进行数据展示相关操作，比如更新UI
}
@end