高效的把NSMutableArray變成NSArray

標籤：nsmutablearray   nsarray   object   objective-c   

高效的把NSMutableArray變成NSArray

我們在方法內部可能會用NSMutableArray,但是返回的時候希望其他人不去改我們的Array

   NSMutableArray *array1 = [@[] mutableCopy];    for (int i = 0; i < 10000000; i++)    {        [array1 addObject:@(i)];    }    return array1;

這個時候我們通常的做法是拷貝array1或者是產生一個新的array1

return [array1 copy];return [NSArray arrayWithArray:array1];

這裡呢,我們列印了下時間

    NSMutableArray *array1 = [@[] mutableCopy];    NSMutableArray *array2 = [@[] mutableCopy];    PERF_ENTER_(init)    for (int i = 0; i < 10000000; i++)    {        [array1 addObject:@(i)];        [array2 addObject:@(i)];    }    PERF_LEAVE_(init)    PERF_ENTER_(copy)    NSArray *array3 = [array1 copy];    PERF_LEAVE_(copy)    NSLog(@"%@", [array3 class]);    PERF_ENTER_(arrayWithArray)    NSArray *array4 = [NSArray arrayWithArray:array1];    PERF_LEAVE_(arrayWithArray)    NSLog(@"%@", [array4 class]);/*    2015-01-07 00:57:36.869 JoinShow[1134:68543] +[XYPerformance recordName:andTime:] 'init' = 5.492105(s)2015-01-07 00:57:37.125 JoinShow[1134:68543] +[XYPerformance recordName:andTime:] 'copy' = 0.255650(s)2015-01-07 00:57:37.125 JoinShow[1134:68543] __NSArrayI2015-01-07 00:57:37.373 JoinShow[1134:68543] +[XYPerformance recordName:andTime:] 'arrayWithArray' = 0.247696(s)2015-01-07 00:57:37.373 JoinShow[1134:68543] __NSArrayI*/

發現正常拷貝一個大array需要0.2s,這已經是不能忍的時間了.需要對方法進行最佳化.

問題的本質是什麼,是我們需要返回一個不可變的array,因此我們可以直接把 NSMutableArray 對象的isa 修改成 NSArray.

- (NSArray *)immutable{    object_setClass(self, [NSArray class]);    return self;}    PERF_ENTER_(immutable)    NSArray *array5 = [array2 immutable];    PERF_LEAVE_(immutable)    NSLog(@"%@", [array5 class]);/*2015-01-07 00:57:37.373 JoinShow[1134:68543] +[XYPerformance recordName:andTime:] 'immutable' = 0.000030(s)2015-01-07 00:57:37.373 JoinShow[1134:68543] NSArray*/

看下時間,果然快很多.

高效的把NSMutableArray變成NSArray