Objective-C 代理（委託）設計模式完整實作指南

代理模式核心概念

代理模式讓一個物件（委託者）將特定任務交由另一個物件（代理者）執行，實現「兩個物件協同解決問題」的鬆耦合通訊。其優勢在於：

• 簡化物件行為，降低類別間耦合度。
• 無需子類化即可擴充功能。
• 實作彈性高，易於維護與擴展。

租房子案例：真實角色與代理角色

房東（RealSubject）委託仲介（Proxy）處理租屋事宜，租客（Client）只與仲介互動。

1. 定義協議

#import <Foundation/Foundation.h>

@protocol FindApartment <NSObject>
- (void)findApartment;
@end

2. 代理角色（仲介）

// Agent.h
#import <Foundation/Foundation.h>
#import "FindApartment.h"

@interface Agent : NSObject <FindApartment>
@end

// Agent.m
#import "Agent.h"

@implementation Agent
- (void)findApartment {
    NSLog(@"findApartment");
}
@end

3. 真實角色（房東/租客）

// Person.h
#import <Foundation/Foundation.h>
#import "FindApartment.h"

@interface Person : NSObject {
    NSString *_name;
    id<FindApartment> _delegate;
}
@property (nonatomic, copy) NSString *name;
@property (nonatomic, assign) id<FindApartment> delegate;
- (id)initWithName:(NSString *)name withDelegate:(id<FindApartment>)delegate;
- (void)wantToFindApartment;
@end

// Person.m
#import "Person.h"

@interface Person()
- (void)startFindApartment:(NSTimer *)timer;
@end

@implementation Person
@synthesize name = _name;
@synthesize delegate = _delegate;

- (id)initWithName:(NSString *)name withDelegate:(id<FindApartment>)delegate {
    self = [super init];
    if (self) {
        self.name = name;
        self.delegate = delegate;
    }
    return self;
}

- (void)wantToFindApartment {
    [NSTimer scheduledTimerWithTimeInterval:2 target:self selector:@selector(startFindApartment:) userInfo:@"Hello" repeats:YES];
}

- (void)startFindApartment:(NSTimer *)timer {
    [self.delegate findApartment];
}
@end

4. 主程式測試

#import <Foundation/Foundation.h>
#import "Person.h"
#import "Agent.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSLog(@"Hello, World!");
        Agent *agent = [[Agent alloc] init];
        Person *jack = [[Person alloc] initWithName:@"jack" withDelegate:agent];
        [jack wantToFindApartment];
        [[NSRunLoop currentRunLoop] run];
        [jack autorelease];
    }
    return 0;
}

此模式廣泛應用於 iOS 開發（如 UITableViewDelegate、UIScrollViewDelegate），是物件間通訊的基石。