objective-c - 基础 objective-c : Dictionary with array; dict with dict

Question

假设我有一个 NSDictionary，它包含一个 NSArray 和一个 NSDictionary 的两个子集合:

NSMutableDictionary *mkDict(void){
    NSMutableDictionary *dict=[NSMutableDictionary dictionary];
    NSMutableDictionary *sub=[NSMutableDictionary dictionary];
    NSMutableArray *array= [NSMutableArray array];
    [dict setObject:array forKey:@"array_key"];
    [dict setObject:sub forKey:@"dict_key"];
    return dict;
}

访问子集合的单个元素的方法有很多种，我选择对其中的三种进行计时。

第一种方式是通过访问父元素的键来间接访问子元素:

void KVC1(NSMutableDictionary *dict, int count){

    for(int i=0; i<count; i++){
        char buf1[40], buf2[sizeof buf1];
        snprintf(buf1,sizeof(buf1),"element %i", i);
        snprintf(buf2, sizeof buf2, "key %i", i);

        [[dict objectForKey:@"array_key"] 
          addObject:
             [NSString stringWithUTF8String:buf1]];
        [[dict objectForKey:@"dict_key"] 
          setObject:[NSString stringWithUTF8String:buf1] 
          forKey:[NSString stringWithUTF8String:buf2]];
    }
}

第二种是使用KeyPath访问:

void KVC2(NSMutableDictionary *dict, int count){

    for(int i=0; i<count; i++){
        char buf1[40], buf2[sizeof buf1], buf3[sizeof buf1];
        snprintf(buf1,sizeof(buf1),"element %i", i);
        snprintf(buf2, sizeof buf2, "key %i", i);
        snprintf(buf3, sizeof buf3, "dict_key.key %i",i);

        [dict insertValue:
             [NSString stringWithUTF8String:buf1] 
            atIndex:i inPropertyWithKey:@"array_key"];
        [dict setValue:
             [NSString stringWithUTF8String:buf1] 
            forKeyPath:
             [NSString stringWithUTF8String:buf3]];
    }
}

第三个与第一个类似，是访问指向子元素的指针，然后使用该指针:

void KVC3(NSMutableDictionary *dict, int count){

    NSMutableArray *subArray = [dict objectForKey:@"array_key"];
    NSMutableDictionary *subDict = [dict objectForKey:@"dict_key"];

    for(int i=0; i<count; i++){
        char buf1[40], buf2[sizeof buf1];
        snprintf(buf1,sizeof(buf1),"element %i", i);
        snprintf(buf2, sizeof buf2, "key %i", i);

        [subArray addObject:[NSString stringWithUTF8String:buf1]];
        [subDict 
           setObject:
            [NSString stringWithUTF8String:buf1] 
           forKey:
            [NSString stringWithUTF8String:buf2]];
    }
}

这是时间代码:

#import <Foundation/Foundation.h>
#import <mach/mach_time.h>

// KVC1, KVC2 and KVC3 from above...

#define TIME_THIS(func,times) \
({\
mach_timebase_info_data_t info; \
mach_timebase_info(&info); \
uint64_t start = mach_absolute_time(); \
for(int i=0; i<(int)times; i++) \
func ; \
uint64_t duration = mach_absolute_time() - start; \
duration *= info.numer; \
duration /= info.denom; \
duration /= 1000000; \
NSLog(@"%i executions of line %i took %lld milliseconds", times, __LINE__, duration); \
});

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
    NSMutableDictionary *dict=mkDict();
    NSMutableDictionary *dict2=mkDict();
    NSMutableDictionary *dict3=mkDict();

    TIME_THIS(KVC1(dict,1000),10);
    TIME_THIS(KVC2(dict2,1000),10);
    TIME_THIS(KVC3(dict3,1000),10);

    if([dict isEqualToDictionary:dict2])
        NSLog(@"And they are the same...");
    [pool drain];
    return 0;
}

结果如下:

10 executions of line 256 took 57 milliseconds
10 executions of line 257 took 7930 milliseconds
10 executions of line 258 took 46 milliseconds
And they are the same...

问题:为什么 OS X Snow Leopard/Lion 建议的使用 KeyPaths 的方法如此缓慢？如果将 count 的大小增加到 10,000 或更多，KVC2 会变得无限慢，而其他两种方法呈线性增加。

我做错了什么吗？有没有更好的习惯用法来访问字典中子集合的单个元素？

Answer 1

在KVC2() , 你发送

[dict insertValue:[NSString stringWithUTF8String:buf1] 
          atIndex:i
inPropertyWithKey:@"array_key"]; 

该方法的文档说明如下:

The method insertIn<Key>:atIndex: is invoked if it exists. If no corresponding scripting-KVC-compliant method (insertIn<Key>:atIndex:) is found, this method invokes mutableArrayValueForKey: and mutates the result.

由于消息被发送到 dict , NSDictionary 的实例, 没有 -insertIn<Key>:atIndex:方法，因此 -mutableArrayValueForKey:已发送。此方法的文档说明如下:

Return Value A mutable array proxy that provides read-write access to the ordered to-many relationship specified by key.

Discussion Objects added to the mutable array become related to the receiver, and objects removed from the mutable array become unrelated. The default implementation recognizes the same simple accessor methods and array accessor methods as valueForKey:, and follows the same direct instance variable access policies, but always returns a mutable collection proxy object instead of the immutable collection that valueForKey: would return.

所以发生的事情是在每次迭代中:

1. 代理可变数组被创建为原始数组的可变副本；
2. 一个对象被添加到代理数组中；
3. 代理数组将相同的对象添加到原始数组。

如果您使用 Instruments 分析您的程序，您会注意到大约 50% 的处理时间花在了 -[NSKeyValueSlowMutableArray insertObject:atIndex:] 上。 — 我认为可以安全地假设NSKeyValueSlowMutableArray是代理数组，它的名字应该是它性能的线索。