不常見(jiàn)集合NSHashTable和NSMapTable

像NSArray,NSSet,NSDictionary是平時(shí)常用的數(shù)據(jù)類(lèi)型，今天想說(shuō)的是另外兩個(gè)比較高階的集合NSHashTable和NSMapTable。

NSHashTable

首先我們看下官方的解釋?zhuān)?/p> 

A collection similar to a set, but with broader range of available memory semantics.
The hash table is modeled after NSSet with the following differences:
*   It can hold weak references to its members.
*   Its members may be copied on input or may use pointer identity for equality and hashing.
*   It can contain arbitrary pointers (its members are not constrained to being objects).

大概意思就是說(shuō)NSHashTable是一種類(lèi)似NSSet一樣的集合。但是它具有更廣泛的可用內(nèi)存語(yǔ)義。能夠?qū)Τ钟袑?duì)象以弱引用的方式存儲(chǔ)。大家都知道平時(shí)用的NSArray和NSSet都是對(duì)對(duì)應(yīng)的強(qiáng)持有(強(qiáng)引用),結(jié)果就是在某些場(chǎng)合達(dá)不到理想效果。

那么NSHashTable有哪些使用場(chǎng)景呢？

不知道同學(xué)們有沒(méi)有遇到過(guò)類(lèi)似場(chǎng)景需求，某工具類(lèi)需要持有多個(gè)代理對(duì)象，方便后續(xù)逐一回調(diào)。比如某個(gè)訂閱器訂閱了某個(gè)通知，然后通知到來(lái)時(shí)需要下發(fā)給每一個(gè)需要響應(yīng)的頁(yè)面，這些頁(yè)面肯定是要實(shí)現(xiàn)訂閱器的代理方法的。所以，遇到這種場(chǎng)景時(shí)，我們可能要注意了。不能使用常用數(shù)據(jù)類(lèi)型來(lái)管理多個(gè)代理者了(因?yàn)榇碚卟荒鼙粡?qiáng)引用，會(huì)有循環(huán)引用問(wèn)題),此時(shí)我們可以采用NSHashTable的弱引用特性。好了，不多說(shuō)了，直接上代碼解釋吧。
有這么個(gè)單例類(lèi)：

// SharedObject.h
#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface SharedObject : NSObject
@property (nonatomic,strong,readonly)NSArray *delegates;
+ (instancetype)shared;
- (void)addDelegate:(id)delegate;
@end

NS_ASSUME_NONNULL_END

.m文件實(shí)現(xiàn)如下：

#import "SharedObject.h"

@implementation SharedObject
{
    NSHashTable *_hashTable;
}

+ (instancetype)shared {
    static SharedObject *object = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        object = [[self alloc] init];
    });
    return object;
};

- (instancetype)init {
    if (self=[super init]) {
        _hashTable = [NSHashTable weakObjectsHashTable];
    }
    return self;;
}

- (void)addDelegate:(id)delegate {
    if (delegate) {
        [_hashTable addObject:delegate];
    }
}

- (NSArray *)delegates {
     return _hashTable.allObjects;
}
@end

看到了沒(méi)，這里我們使用的是weakObjectsHashTable來(lái)實(shí)現(xiàn)。
然后代理者地方實(shí)現(xiàn)：

self.sharedObject = [SharedObject shared];
[self.sharedObject addDelegate:self];

大家可以試試，把weakObjectsHashTable換成NSArray看看什么效果？(結(jié)果應(yīng)該是循環(huán)引用，導(dǎo)致代理者無(wú)法被釋放)

NSHashTable使用介紹

大家可以看到，NSHashTable有如下幾個(gè)初始化方法：

- (instancetype)initWithOptions:(NSPointerFunctionsOptions)options capacity:(NSUInteger)initialCapacity NS_DESIGNATED_INITIALIZER;
- (instancetype)initWithPointerFunctions:(NSPointerFunctions *)functions capacity:(NSUInteger)initialCapacity NS_DESIGNATED_INITIALIZER;

+ (NSHashTable<ObjectType> *)hashTableWithOptions:(NSPointerFunctionsOptions)options;
+ (NSHashTable<ObjectType> *)weakObjectsHashTable API_AVAILABLE(macos(10.8), ios(6.0), watchos(2.0), tvos(9.0));

使用init初始化時(shí)我們看到有個(gè)NSPointerFunctionsOptions參數(shù)，它有如下集中枚舉值：

typedef NS_OPTIONS(NSUInteger, NSPointerFunctionsOptions) {
    // Memory options are mutually exclusive
    
    // default is strong
    NSPointerFunctionsStrongMemory API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (0UL << 0),       // use strong write-barrier to backing store; use GC memory on copyIn
    NSPointerFunctionsZeroingWeakMemory API_DEPRECATED("GC no longer supported", macos(10.5, 10.8)) API_UNAVAILABLE(ios, watchos, tvos) = (1UL << 0),  // deprecated; uses GC weak read and write barriers, and dangling pointer behavior otherwise
    NSPointerFunctionsOpaqueMemory API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (2UL << 0),
    NSPointerFunctionsMallocMemory API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (3UL << 0),       // free() will be called on removal, calloc on copyIn
    NSPointerFunctionsMachVirtualMemory API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (4UL << 0),
    NSPointerFunctionsWeakMemory API_AVAILABLE(macos(10.8), ios(6.0), watchos(2.0), tvos(9.0)) = (5UL << 0),         // uses weak read and write barriers appropriate for ARC
    
    // Personalities are mutually exclusive
    // default is object.  As a special case, 'strong' memory used for Objects will do retain/release under non-GC
    NSPointerFunctionsObjectPersonality API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (0UL << 8),         // use -hash and -isEqual, object description
    NSPointerFunctionsOpaquePersonality API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (1UL << 8),         // use shifted pointer hash and direct equality
    NSPointerFunctionsObjectPointerPersonality API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (2UL << 8),  // use shifted pointer hash and direct equality, object description
    NSPointerFunctionsCStringPersonality API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (3UL << 8),        // use a string hash and strcmp, description assumes UTF-8 contents; recommended for UTF-8 (or ASCII, which is a subset) only cstrings
    NSPointerFunctionsStructPersonality API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (4UL << 8),         // use a memory hash and memcmp (using size function you must set)
    NSPointerFunctionsIntegerPersonality API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (5UL << 8),        // use unshifted value as hash & equality

    NSPointerFunctionsCopyIn API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (1UL << 16),      // the memory acquire function will be asked to allocate and copy items on input
};

我們可以發(fā)現(xiàn)，默認(rèn)是NSPointerFunctionsStrongMemory,即內(nèi)存強(qiáng)引用。效果和NSSet類(lèi)似，會(huì)造成元素強(qiáng)引用，聰明的你當(dāng)然能發(fā)現(xiàn)，剛好有一個(gè)NSPointerFunctionsWeakMemory。其他方法和常規(guī)NSSet使用類(lèi)似。

- (void)addObject:(nullable ObjectType)object;
- (void)removeObject:(nullable ObjectType)object;
- (void)removeAllObjects;

NSMapTable介紹

官方介紹是：

A collection similar to a dictionary, but with a broader range of available memory semantics.
The map table is modeled after [NSDictionary] with the following differences:

*   Keys and/or values are optionally held “weakly” such that entries are removed when one of the objects is reclaimed.

*   Its keys or values may be copied on input or may use pointer identity for equality and hashing.

*   It can contain arbitrary pointers (its contents are not constrained to being objects).

大概意思就是：NSMapTable和NSDictionary類(lèi)似，也擁有強(qiáng)大的內(nèi)存管理能力。分別對(duì)key和value都可以進(jìn)行不同內(nèi)存引用管理。
還是拿上面那個(gè)例子說(shuō)明：新增一個(gè)需求，能夠添加代理者和回調(diào)線程。
此時(shí)我們不好用NSHashTable來(lái)實(shí)現(xiàn)了，因?yàn)?code>NSHashTable只能夠添加一個(gè)參數(shù)(當(dāng)然要實(shí)現(xiàn)也是可以的，采用中間件思想，用一個(gè)新對(duì)象來(lái)分別持有這兩個(gè)參數(shù))。 然而也有另外一種思路是采用NSMapTable我們剛好可以把兩個(gè)參數(shù)分別作為key-value存儲(chǔ)起來(lái)。好了，下面直接上代碼吧。

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface SharedObject : NSObject
@property (nonatomic,strong,readonly)NSArray *delegates;
+ (instancetype)shared;
- (void)addDelegate:(id)delegate dispathQueue:(dispatch_queue_t)queue_t;
@end

NS_ASSUME_NONNULL_END

新增一個(gè)接口方法支持傳兩個(gè)參數(shù)。

#import "SharedObject.h"

@implementation SharedObject
{
    NSMapTable *_mapTable;
}

+ (instancetype)shared {
    static SharedObject *object = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        object = [[self alloc] init];
    });
    return object;
};

- (instancetype)init {
    if (self=[super init]) {
        _mapTable = [NSMapTable weakToStrongObjectsMapTable];
    }
    return self;;
}

- (void)addDelegate:(id)delegate dispathQueue:(dispatch_queue_t)queue_t {
    if (delegate) {
        //這里需要在delegate上包一層作為key，因?yàn)閗ey需要能夠?qū)崿F(xiàn)NSCoping協(xié)議,同NSDictiony類(lèi)似。
        NSMutableOrderedSet *orderSet = [NSMutableOrderedSet orderedSet];
        [orderSet addObject:delegate];
        [_mapTable setObject:queue_t?queue_t:dispatch_get_main_queue() forKey:orderSet.copy];
    }
}

- (NSArray *)delegates {
    return _mapTable.dictionaryRepresentation.allKeys;
}

@end

代理者使用地方

    self.sharedObject = [SharedObject shared];
    [self.sharedObject addDelegate:self dispathQueue:dispatch_get_main_queue()];

NSMapTable使用介紹

我們可以看到NSMapTable有下面幾種初始化方法：

- (instancetype)initWithKeyOptions:(NSPointerFunctionsOptions)keyOptions valueOptions:(NSPointerFunctionsOptions)valueOptions capacity:(NSUInteger)initialCapacity NS_DESIGNATED_INITIALIZER;
- (instancetype)initWithKeyPointerFunctions:(NSPointerFunctions *)keyFunctions valuePointerFunctions:(NSPointerFunctions *)valueFunctions capacity:(NSUInteger)initialCapacity NS_DESIGNATED_INITIALIZER;
+ (NSMapTable<KeyType, ObjectType> *)mapTableWithKeyOptions:(NSPointerFunctionsOptions)keyOptions valueOptions:(NSPointerFunctionsOptions)valueOptions;

+ (NSMapTable<KeyType, ObjectType> *)strongToStrongObjectsMapTable API_AVAILABLE(macos(10.8), ios(6.0), watchos(2.0), tvos(9.0));
+ (NSMapTable<KeyType, ObjectType> *)weakToStrongObjectsMapTable API_AVAILABLE(macos(10.8), ios(6.0), watchos(2.0), tvos(9.0)); // entries are not necessarily purged right away when the weak key is reclaimed
+ (NSMapTable<KeyType, ObjectType> *)strongToWeakObjectsMapTable API_AVAILABLE(macos(10.8), ios(6.0), watchos(2.0), tvos(9.0));
+ (NSMapTable<KeyType, ObjectType> *)weakToWeakObjectsMapTable API_AVAILABLE(macos(10.8), ios(6.0), watchos(2.0), tvos(9.0)); // entries are not necessarily purged right away when the weak key or object is reclaimed

我們?cè)谶x擇時(shí)根據(jù)需求來(lái)選擇，這里有個(gè)NSPointerFunctionsOptions提供的值類(lèi)型也是和內(nèi)存相關(guān)。

typedef NS_OPTIONS(NSUInteger, NSPointerFunctionsOptions) {
    // Memory options are mutually exclusive
    
    // default is strong
    NSPointerFunctionsStrongMemory API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (0UL << 0),       // use strong write-barrier to backing store; use GC memory on copyIn
    NSPointerFunctionsZeroingWeakMemory API_DEPRECATED("GC no longer supported", macos(10.5, 10.8)) API_UNAVAILABLE(ios, watchos, tvos) = (1UL << 0),  // deprecated; uses GC weak read and write barriers, and dangling pointer behavior otherwise
    NSPointerFunctionsOpaqueMemory API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (2UL << 0),
    NSPointerFunctionsMallocMemory API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (3UL << 0),       // free() will be called on removal, calloc on copyIn
    NSPointerFunctionsMachVirtualMemory API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (4UL << 0),
    NSPointerFunctionsWeakMemory API_AVAILABLE(macos(10.8), ios(6.0), watchos(2.0), tvos(9.0)) = (5UL << 0),         // uses weak read and write barriers appropriate for ARC
    
    // Personalities are mutually exclusive
    // default is object.  As a special case, 'strong' memory used for Objects will do retain/release under non-GC
    NSPointerFunctionsObjectPersonality API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (0UL << 8),         // use -hash and -isEqual, object description
    NSPointerFunctionsOpaquePersonality API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (1UL << 8),         // use shifted pointer hash and direct equality
    NSPointerFunctionsObjectPointerPersonality API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (2UL << 8),  // use shifted pointer hash and direct equality, object description
    NSPointerFunctionsCStringPersonality API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (3UL << 8),        // use a string hash and strcmp, description assumes UTF-8 contents; recommended for UTF-8 (or ASCII, which is a subset) only cstrings
    NSPointerFunctionsStructPersonality API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (4UL << 8),         // use a memory hash and memcmp (using size function you must set)
    NSPointerFunctionsIntegerPersonality API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (5UL << 8),        // use unshifted value as hash & equality

    NSPointerFunctionsCopyIn API_AVAILABLE(macos(10.5), ios(6.0), watchos(2.0), tvos(9.0)) = (1UL << 16),      // the memory acquire function will be asked to allocate and copy items on input
};

可以看到，默認(rèn)是NSPointerFunctionsStrongMemory強(qiáng)引用值，這里我們推薦使用NSPointerFunctionsWeakMemory來(lái)搭配使用。
然后幾個(gè)類(lèi)方法也分別對(duì)應(yīng)了四種搭配。如下：
strongToStrongObjectsMapTable相當(dāng)于

[NSMapTable mapTableWithKeyOptions:NSPointerFunctionsStrongMemory valueOptions: NSPointerFunctionsStrongMemory];

weakToStrongObjectsMapTable相當(dāng)于：

[NSMapTable mapTableWithKeyOptions:NSPointerFunctionsWeakMemory valueOptions:NSPointerFunctionsStrongMemory];

strongToWeakObjectsMapTable相當(dāng)于：

[NSMapTable mapTableWithKeyOptions:NSPointerFunctionsStrongMemory valueOptions:NSPointerFunctionsWeakMemory];

weakToWeakObjectsMapTable相當(dāng)于：

[NSMapTable mapTableWithKeyOptions:NSPointerFunctionsWeakMemory valueOptions: NSPointerFunctionsWeakMemory];

好了，簡(jiǎn)單的梳理就介紹到這了。后面有更深入的理解時(shí)再來(lái)完成補(bǔ)充，謝謝。

最后編輯于 ：
?著作權(quán)歸作者所有,轉(zhuǎn)載或內(nèi)容合作請(qǐng)聯(lián)系作者
【社區(qū)內(nèi)容提示】社區(qū)部分內(nèi)容疑似由AI輔助生成，瀏覽時(shí)請(qǐng)結(jié)合常識(shí)與多方信息審慎甄別。
平臺(tái)聲明：文章內(nèi)容（如有圖片或視頻亦包括在內(nèi)）由作者上傳并發(fā)布，文章內(nèi)容僅代表作者本人觀點(diǎn)，簡(jiǎn)書(shū)系信息發(fā)布平臺(tái)，僅提供信息存儲(chǔ)服務(wù)。

友情鏈接更多精彩內(nèi)容