Objective-C如何自己实现一个基于数组下标的属性访问模式

在iOS6.0以及OS X10.8之后，Apple引入了一套非正式协议（informal protocol）与Objective-C语法直接绑定。当你实现了这其中的方法之后即可使用数组下标来访问属性元素。

在Foundation库中，NSArray类实现了- (id)objectAtIndexedSubscript:(NSUInteger)idx方法。因此，我们可以这么来访问数组元素：

NSArray *arr = @[@100, @200, @300];
NSNumber *num = arr[0];

上述arr[0]就相当于[arr objectAtIndex:0]。

而NSMutableArray在基于NSArray的基础上又实现了- (void)setObject:(id)anObject atIndexedSubscript:(NSUInteger)index方法。这样我们可以通过数组下标来读写相应元素，比如：

NSMutableArray *arr = [NSMutableArray arrayWithArray:@[@100, @200, @300]];
arr[2] = arr[0];

而NSDictionary类实现了- (id)objectForKeyedSubscript:(id)key方法。这样我们能以数组下标的形式来访问相应键的值。比如：

NSDictionary *dict = @{@"key" : @"value"};
NSString *value = dict[@"key"];

而NSMutableDictionary在NSDictionary类的基础上又实现了- (void)setObject:(id)object forKeyedSubscript:(id < NSCopying >)aKey方法。这样，我们能以数组下标的方式来读写相应键的值。比如：

NSMutableDictionary *dict = [NSMutableDictionary dictionaryWithDictionary:@{@"key":"@Hello"}];
dict[dict[@"key"]] = @"world";

下面我们通过实现这四个方法，自己实现一个能同时使用这四种下标方式访问模式的类。

//
//  main.m
//  objCTest
//
//  Created by Zenny Chen on 12-2-7.
//  Copyright (c) 2014年 Neon Media Studio. All rights reserved.
//

#import <Foundation/Foundation.h>


@interface MyContainer : NSObject
{
@private
    
    NSMutableDictionary *mDict;
    NSMutableArray *mArray;
}

- (void)setObject:(id)object forKeyedSubscript:(id < NSCopying >)aKey;
- (id)objectForKeyedSubscript:(id)key;
- (void)setObject:(id)anObject atIndexedSubscript:(NSUInteger)index;
- (id)objectAtIndexedSubscript:(NSUInteger)idx;

@end

@implementation MyContainer

- (instancetype)init
{
    self = [super init];
    
    mDict = [[NSMutableDictionary alloc] initWithDictionary:@{@"key1":@"value1", @"key2":@"value2"}];
    
    mArray = [[NSMutableArray alloc] initWithArray:@[@100, @200, @300, @400]];
    
    return self;
}

- (void)dealloc
{
    if(mDict != nil)
    {
        [mDict removeAllObjects];
        [mDict release];
        mDict = nil;
    }
    
    if(mArray != nil)
    {
        [mArray removeAllObjects];
        [mArray release];
        mArray = nil;
    }
    
    [super dealloc];
}

- (void)setObject:(id)object forKeyedSubscript:(id < NSCopying >)aKey
{
    [mDict setObject:object forKey:aKey];
}

- (id)objectForKeyedSubscript:(id)key
{
    return [mDict objectForKey:key];
}

- (void)setObject:(id)anObject atIndexedSubscript:(NSUInteger)index
{
    const NSUInteger length = [mArray count];
    if(index > length)
        return;
    
    if(index == length)
        [mArray addObject:anObject];
    else
        [mArray replaceObjectAtIndex:index withObject:anObject];
}

- (id)objectAtIndexedSubscript:(NSUInteger)idx
{
    if(idx >= [mArray count])
        return nil;
    
    return [mArray objectAtIndex:idx];
}

@end


int main (int argc, const char * argv[])
{
    @autoreleasepool
    {
        // insert code here...
        
        MyContainer *cont = [[MyContainer alloc] init];
        
        cont[@"mykey"] = @"myvalye";
        
        NSLog(@"key1 is: %@", cont[@"key1"]);
        NSLog(@"key2 is: %@", cont[@"key2"]);
        NSLog(@"mykey is: %@", cont[@"mykey"]);
        
        cont[4] = @500;
        cont[2] = @-300;
        
        NSLog(@"The value[4] = %@", cont[4]);
        NSLog(@"The value[3] = %@", cont[3]);
        NSLog(@"The value[2] = %@", cont[2]);
    }
    
    return 0;
}