[iOS]NSHashTable和NSMapTable用法

2019年5月26日 150次阅读来源: 肖浩呗

一个项目中的需求

在iOS项目开发过程中，我们经常会使用到NSSet、NSArray、NSDictionary三个类，它们为我们设计较友好的数据结构时提供了很方便的方法

先准备本文中将要使用的对象:

#import <Foundation/Foundation.h>

@interface HHHuman : NSObject

@property (nonatomic ,strong) NSString *name;

+ (instancetype) humanWithName:(NSString *)n;

@end

@implementation HHHuman

+ (instancetype) humanWithName:(NSString *)n
{
    HHHuman *human = [[HHHuman alloc] init];
    human.name = n;
    
    return [human autorelease];
}
- (NSString *)description
{
    return [NSString stringWithFormat:@"%@'s retainCount is %lu",self.name,[self retainCount]];
}
- (void)dealloc
{
    self.name = nil;
    [super dealloc];
}
@end

在程序开发过程中,经常会用到诸如此类的Model对象.
用法呢也大致会有如下几种方式:
1.通过有序的数列进行存储,数组NSArray;

    HHHuman *human_1 = [HHHuman humanWithName:@"lilei"];
    HHHuman *human_2 = [HHHuman humanWithName:@"hanmeimei"];
    HHHuman *human_3 = [HHHuman humanWithName:@"lewis"];
    HHHuman *human_4 = [HHHuman humanWithName:@"xiaohao"];
    HHHuman *human_5 = [HHHuman humanWithName:@"beijing"];

    id list = @[human_1,human_2,human_3,human_4,human_5];
    NSLog(@"%@",list);

输出的结果如下:

(
    "lilei's retainCount is 2",
    "hanmeimei's retainCount is 2",
    "lewis's retainCount is 2",
    "xiaohao's retainCount is 2",
    "beijing's retainCount is 2"
)

2.通过统一的关键字进行存储,字典NSDictionary;

    HHHuman *human_1 = [HHHuman humanWithName:@"lilei"];
    HHHuman *human_2 = [HHHuman humanWithName:@"hanmeimei"];
    HHHuman *human_3 = [HHHuman humanWithName:@"lewis"];
    HHHuman *human_4 = [HHHuman humanWithName:@"xiaohao"];
    HHHuman *human_5 = [HHHuman humanWithName:@"beijing"];
    id dic = @{@"excellent":human_1};
    //同样在控制台输出上文字典，用来查看每个对象的保留值
    NSLog(@"%@",list);

输出的结果如下:

(
    "lilei's retainCount is 3",
    "hanmeimei's retainCount is 3",
    "lewis's retainCount is 2",
    "xiaohao's retainCount is 2",
    "beijing's retainCount is 2"
)

通过上述两个例子我们能够发现一个问题，即将对象添加到容器时，会对该对象的引用技术+1
这样就会有可能发生循环持有的问题，例如如下代码:

@interface HHHuman : NSObject

@property (nonatomic ,strong) NSString *name;
@property (nonatomic ,strong) NSMutableArray *family;

+ (instancetype) humanWithName:(NSString *)n;

@end

@implementation HHHuman

+ (instancetype) humanWithName:(NSString *)n
{
    HHHuman *human = [[HHHuman alloc] init];
    human.name = n;
    human.family = [[NSMutableArray alloc] init];
    [human.family addObject:human];

    return [human autorelease];
}
- (NSString *)description
{
    return [NSString stringWithFormat:@"%@'s retainCount is %lu",self.name,[self retainCount]];
}
- (void)dealloc
{
    self.name = nil;
    self.family = nil;
    [super dealloc];
}

@end

在以上代码中,一个human的实例对象中包含一个strong修饰的family属性，但是在family属性中，又添加了human自身对象，这样会造成循环持有的问题，而导致内存泄漏。
但是项目需求又要求我们在该Model对象中完成如此代码，我们不得已会多创建一个类HHHumanRelationShip,如下所示:

#import <Foundation/Foundation.h>



@interface HHHuman : NSObject

@property (nonatomic ,strong) NSString *name;

+ (instancetype) humanWithName:(NSString *)n;

@end

@implementation HHHuman

+ (instancetype) humanWithName:(NSString *)n
{
    HHHuman *human = [[HHHuman alloc] init];
    human.name = n;

    return [human autorelease];
}
- (NSString *)description
{
    return [NSString stringWithFormat:@"%@'s retainCount is %lu",self.name,[self retainCount]];
}
- (void)dealloc
{
    self.name = nil;
    [super dealloc];
}

@end

@interface HHHumanRelationShip : NSObject

@property (nonatomic ,strong) HHHuman *human;
@property (nonatomic ,strong) NSArray *family;

+ (instancetype)relationShipWithHuman:(HHHuman *)human family:(NSArray *)members;

@end

@implementation HHHumanRelationShip

+ (instancetype)relationShipWithHuman:(HHHuman *)human family:(NSArray *)members
{
    HHHumanRelationShip *rs = [[HHHumanRelationShip alloc] init];
    rs.human = human;
    rs.family = members;
    
    return [rs autorelease];
}

- (NSString *)description
{
    return [NSString stringWithFormat:@"%@'s family's member is %@",self.human,self.family];
}
- (void)dealloc
{
    self.human = nil;
    self.family = nil;
    [super dealloc];
}

@end

int main(int argc, const char * argv[])
{
    
    HHHuman *human_0 = [HHHuman humanWithName:@"parent"];
    HHHuman *human_1 = [HHHuman humanWithName:@"lilei"];
    HHHuman *human_2 = [HHHuman humanWithName:@"hanmeimei"];
    HHHuman *human_3 = [HHHuman humanWithName:@"lewis"];
    HHHuman *human_4 = [HHHuman humanWithName:@"xiaohao"];
    HHHuman *human_5 = [HHHuman humanWithName:@"beijing"];
    

    id list = @[human_1,human_2,human_3,human_4,human_5];
    

    HHHumanRelationShip *relationShip = [HHHumanRelationShip relationShipWithHuman:human_0 family:list];
    NSLog(@"%@",relationShip);
    
    return 0;
}

NSHashTable

很明显，大家能够看到这样造成了程序代码的臃肿
根据上述需求和功能，在iOS6之后，Objective-C Foundation框架中添加了两个类分别是NSHashTable和NSMapTable

NSHashTable
- 构造函数
  - - (instancetype)initWithOptions:(NSPointerFunctionsOptions)options capacity:(NSUInteger)initialCapacity
  - - (instancetype)initWithPointerFunctions:(NSPointerFunctions *)functions capacity:(NSUInteger)initialCapacity
  - + (NSHashTable *)hashTableWithOptions:(NSPointerFunctionsOptions)options;
  - + (id)hashTableWithWeakObjects;
  - + (NSHashTable *)weakObjectsHashTable;

在创建NSHashTable对象时，会传NSPointerFunctionsOptions参数，列举如下:

NSHashTableStrongMemory
- 将HashTable容器内的对象引用计数+1一次
NSHashTableZeroingWeakMemory
- 在OSX 10.8之后已经废弃
NSHashTableCopyIn
- 将添加到容器的对象通过NSCopying中的方法，复制一个新的对象存入HashTable容器
NSHashTableObjectPointerPersonality
- 使用移位指针(shifted pointer)来做hash检测及确定两个对象是否相等；
NSHashTableWeakMemory
- 不会修改HashTable容器内对象元素的引用计数，并且对象释放后，会被自动移除

对于我们来说，NSHashTable吸引力比较大的即NSHashTableWeakMemory特性.
使用一段代码来展示功能:

#import <Foundation/Foundation.h>

@interface HHHuman : NSObject

@property (nonatomic ,strong) NSString      *name;
@property (nonatomic ,strong) NSHashTable   *family;

+ (instancetype) humanWithName:(NSString *)n;

@end

@implementation HHHuman

+ (instancetype) humanWithName:(NSString *)n
{
    HHHuman *human = [[HHHuman alloc] init];
    human.name = n;
    human.family = [NSHashTable hashTableWithOptions:NSHashTableWeakMemory];
    [human.family addObject:human];

    return [human autorelease];
}
- (NSString *)description
{
    return [NSString stringWithFormat:@"%@'s retainCount is %lu",self.name,[self retainCount]];
}
- (void)dealloc
{
    self.name = nil;
    self.family = nil;
    [super dealloc];
}

@end

int main(int argc, const char * argv[])
{
    //创建一个NSHashTableWeakMemory特性的HashTable对象
    NSHashTable *hash_tab = [NSHashTable hashTableWithOptions:NSHashTableWeakMemory];

    //创建自动释放池对象
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    
    //通过便利构造器获取一个name属性是lewis的human对象
    HHHuman *human = [HHHuman humanWithName:@"lewis"];

    //将该对象添加到HashTable容器中
    [hash_tab addObject:human];
    
    //释放之前打印human
    NSLog(@"before pool:%@",human);
    
    //将自动释放池释放掉
    [pool drain];
    
    //释放之后打印hash_tab
    NSLog(@"after pool:%@",hash_tab);
    return 0;
}

在控制台输出的结果如下

before pool:lewis's retainCount is 1
after pool:NSHashTable {
}

我们可以看到，当pool对象释放时，human的引用计数会执行一次-1，human对象在内存中就会自动释放，并且相应的hash_tab对象中的对象也会被自动移除.
而我们在创建hash_tab时使用的是NSHashTableStrongMemory特性话,那么控制台输出的结果如下:

before pool:lewis's retainCount is 2
after pool:NSHashTable {
[13] lewis's retainCount is 1
}

有了NSHashTable就可以完成我们文章一开始的需求了.

#import <Foundation/Foundation.h>

@interface HHHuman : NSObject

@property (nonatomic ,strong) NSString      *name;
@property (nonatomic ,strong) NSHashTable   *family;

+ (instancetype) humanWithName:(NSString *)n;

@end

@implementation HHHuman

+ (instancetype) humanWithName:(NSString *)n
{
    HHHuman *human = [[HHHuman alloc] init];
    human.name = n;
    human.family = [NSHashTable hashTableWithOptions:NSHashTableWeakMemory];
    [human.family addObject:human];

    return [human autorelease];
}
- (NSString *)description
{
    return [NSString stringWithFormat:@"%@'s retainCount is %lu",self.name,[self retainCount]];
}
- (void)dealloc
{
    self.name = nil;
    self.family = nil;
    [super dealloc];
}

@end

NSHashTable可以使用的函数

typedef struct {NSUInteger _pi; NSUInteger _si; void *_bs;} NSHashEnumerator;

FOUNDATION_EXPORT void NSFreeHashTable(NSHashTable *table);
FOUNDATION_EXPORT void NSResetHashTable(NSHashTable *table);
FOUNDATION_EXPORT BOOL NSCompareHashTables(NSHashTable *table1, NSHashTable *table2);
FOUNDATION_EXPORT NSHashTable *NSCopyHashTableWithZone(NSHashTable *table, NSZone *zone);
FOUNDATION_EXPORT void *NSHashGet(NSHashTable *table, const void *pointer);
FOUNDATION_EXPORT void NSHashInsert(NSHashTable *table, const void *pointer);
FOUNDATION_EXPORT void NSHashInsertKnownAbsent(NSHashTable *table, const void *pointer);
FOUNDATION_EXPORT void *NSHashInsertIfAbsent(NSHashTable *table, const void *pointer);
FOUNDATION_EXPORT void NSHashRemove(NSHashTable *table, const void *pointer);
FOUNDATION_EXPORT NSHashEnumerator NSEnumerateHashTable(NSHashTable *table);
FOUNDATION_EXPORT void *NSNextHashEnumeratorItem(NSHashEnumerator *enumerator);
FOUNDATION_EXPORT void NSEndHashTableEnumeration(NSHashEnumerator *enumerator);
FOUNDATION_EXPORT NSUInteger NSCountHashTable(NSHashTable *table);
FOUNDATION_EXPORT NSString *NSStringFromHashTable(NSHashTable *table);
FOUNDATION_EXPORT NSArray *NSAllHashTableObjects(NSHashTable *table);

NSMapTable

NSMapTable
- 构造函数
  - - (instancetype)initWithKeyOptions:(NSPointerFunctionsOptions)keyOptions valueOptions:(NSPointerFunctionsOptions)valueOptions capacity:(NSUInteger)initialCapacity;
  - - (instancetype)initWithKeyPointerFunctions:(NSPointerFunctions *)keyFunctions valuePointerFunctions:(NSPointerFunctions *)valueFunctions capacity:(NSUInteger)initialCapacity;
  - + (NSMapTable *)mapTableWithKeyOptions:(NSPointerFunctionsOptions)keyOptions valueOptions:(NSPointerFunctionsOptions)valueOptions;
  - + (NSMapTable *)strongToStrongObjectsMapTable;
  - + (NSMapTable *)weakToStrongObjectsMapTable;
  - + (NSMapTable *)strongToWeakObjectsMapTable;
  - + (NSMapTable *)weakToWeakObjectsMapTable;

NSMapTable对象类似与NSDictionary的数据结构，但是NSMapTable功能比NSDictionary对象要多的功能就是可以设置key和value的NSPointerFunctionsOptions特性!其他的用法与NSDictionary相同.

NSMapTable可以使用的函数

FOUNDATION_EXPORT void NSFreeMapTable(NSMapTable *table);
FOUNDATION_EXPORT void NSResetMapTable(NSMapTable *table);
FOUNDATION_EXPORT BOOL NSCompareMapTables(NSMapTable *table1, NSMapTable *table2);
FOUNDATION_EXPORT NSMapTable *NSCopyMapTableWithZone(NSMapTable *table, NSZone *zone);
FOUNDATION_EXPORT BOOL NSMapMember(NSMapTable *table, const void *key, void **originalKey, void **value);
FOUNDATION_EXPORT void *NSMapGet(NSMapTable *table, const void *key);
FOUNDATION_EXPORT void NSMapInsert(NSMapTable *table, const void *key, const void *value);
FOUNDATION_EXPORT void NSMapInsertKnownAbsent(NSMapTable *table, const void *key, const void *value);
FOUNDATION_EXPORT void *NSMapInsertIfAbsent(NSMapTable *table, const void *key, const void *value);
FOUNDATION_EXPORT void NSMapRemove(NSMapTable *table, const void *key);
FOUNDATION_EXPORT NSMapEnumerator NSEnumerateMapTable(NSMapTable *table);
FOUNDATION_EXPORT BOOL NSNextMapEnumeratorPair(NSMapEnumerator *enumerator, void **key, void **value);
FOUNDATION_EXPORT void NSEndMapTableEnumeration(NSMapEnumerator *enumerator);
FOUNDATION_EXPORT NSUInteger NSCountMapTable(NSMapTable *table);
FOUNDATION_EXPORT NSString *NSStringFromMapTable(NSMapTable *table);
FOUNDATION_EXPORT NSArray *NSAllMapTableKeys(NSMapTable *table);
FOUNDATION_EXPORT NSArray *NSAllMapTableValues(NSMapTable *table);

    原文作者：肖浩呗
    原文地址: https://www.jianshu.com/p/de71385930ba
    本文转自网络文章，转载此文章仅为分享知识，如有侵权，请联系博主进行删除。