SDWebImage学习笔记之NSMapTable

NSDictionary/NSMutableArray浅析

我们在使用NSDictionary/NSMutableArray时，通常会使用NSString对象作为key，因为key必须遵循NSCopying协议，见NSMutableArray中的方法：

- (void)setObject:(ObjectType)anObject forKey:(KeyType <NSCopying>)aKey;

在NSDictionary/NSMutableArray对象中，aKey对象被copy一份后存入，anObject对象则被强引用。来看一段代码：

NSMutableDictionary *aDictionary = [[NSMutableDictionary alloc] initWithCapacity:0]; { NSString *aKey = @"akey"; NSObject *aObject = [[NSObject alloc] init]; [aDictionary setObject:aObject forKey:aKey]; } NSLog(@"dictionary: %@", aDictionary);

打印日志：

dictionary: { akey = "<NSObject: 0x60400001d3b0>"; }

本来作用于结束后，aKey变量指向的NSString对象（简称aKey对象）和aObject变量指向的NSObject对象（简称aObject对象）应该被自动释放，但是aDictionary变量指向的NSMutableDictionary对象（简称aDictionary对象）持有一份aObject对象的强引用，所以打印日志时，aDictionary对象不为空。

现在有一个Teacher类表示班主任信息，包含姓名name属性和年龄age属性，另有一个Student类表示学生信息，也包含姓名name属性和年龄age属性。那么一个班包含一个班主任（Teacher对象）和n个学生（Student数组），为了统计一个班的信息，需要把班主任和学生的信息及对应关系保存下来。

// Teacher.h @interface Teacher : NSObject @property (nonatomic, copy) NSString *name; @property (nonatomic, assign) NSInteger age; @end // Teacher.m @implementation Teacher @end

// Student.h @interface Student : NSObject @property (nonatomic, copy) NSString *name; @property (nonatomic, assign) NSInteger age; @end // Student.m @implementation Student @end

// ViewController.m { NSMutableDictionary *aDictionary = [[NSMutableDictionary alloc] initWithCapacity:0]; Teacher *teacher = [[Teacher alloc] init]; teacher.name = @"teacher"; teacher.age = 30; NSMutableArray *aArray = [[NSMutableArray alloc] initWithCapacity:0]; for (int i = 0; i < 3; i++) { Student *student = [[Student alloc] init]; student.name = [NSString stringWithFormat:@"student%d", i]; student.age = i; [aArray addObject:student]; } [aDictionary setObject:aArray forKey:teacher.name]; NSLog(@"%@", aDictionary); }

打印日志：

 dictionary:{ teacher = ( "<Student: 0x60000003aa00>", "<Student: 0x60000003a9c0>", "<Student: 0x60000003aa80>" ); }

这里将班主任的姓名作为key，这样会损失其他信息。如果想要将teacher对象作为key，则需要让Teacher类遵循NSCopying协议，而且NSDictionary/NSMutable使用hash表来实现key和value之间的映射和存储，所以作为key值的类型必须重写
++- (NSUInteger)hash++
和
++- (BOOL)isEqual:(id)object++
两个方法，其中hash方法计算该对象的hash值，hash值决定该对象在hash表中存储的位置，isEqual方法通过hash值来定位对象在hash表中的位置。具体代码如下:

// Teacher.h @interface Teacher : NSObject<NSCopying> @property (nonatomic, copy) NSString *name; @property (nonatomic, assign) NSInteger age; @end // Teacher.m @implementation Teacher - (id)copyWithZone:(NSZone *)zone { Teacher *teacher = [[Teacher allocWithZone:zone] init]; teacher.name = self.name; teacher.age = self.age; return teacher; } - (BOOL)isEqual:(id)object { // 比较hash值是否相等 return [self hash] == [object hash]; } - (NSUInteger)hash { // 调用父类的hash方法，也可以自定义 return [super hash]; }

打印日志：

dictionary:{ "<Teacher: 0x600000027940>" = ( "<Student: 0x60c00022fa20>", "<Student: 0x60c00022fa00>", "<Student: 0x60c00022f960>" ); }

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NSMapTable浅析

NSMapTable继承自NSObject，自iOS6.0开始使用，NSMapTable是可变的。

NS_CLASS_AVAILABLE(10_5, 6_0) @interface NSMapTable<KeyType, ObjectType> : NSObject <NSCopying, NSCoding, NSFastEnumeration>

NSMapTable有两个指定初始化方法和一个便捷初始化方法：

// 指定初始化方法 - (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; // 便捷初始化方法 + (NSMapTable<KeyType, ObjectType> *)mapTableWithKeyOptions:(NSPointerFunctionsOptions)keyOptions valueOptions:(NSPointerFunctionsOptions)valueOptions;

初始化方法方法中有两个参数keyOptions和valueOptions，都是NSPointerFunctionsOptions类型，NSPointerFunctionsOptions是一个枚举类型，

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 #if (TARGET_OS_MAC && !(TARGET_OS_EMBEDDED || TARGET_OS_IPHONE)) || TARGET_OS_WIN32 NSPointerFunctionsZeroingWeakMemory NS_ENUM_DEPRECATED_MAC(10_5, 10_8) = (1UL << 0), // deprecated; uses GC weak read and write barriers, and dangling pointer behavior otherwise #endif 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 };

常用的枚举值及对应的含义如下：

1. NSPointerFunctionsStrongMemory: 强引用存储对象
2. NSPointerFunctionsWeakMemory： 弱引用存储对象
3. NSPointerFunctionsCopyIn：copy存储对象

就是说，如果NSMapTable的初始化方法为：

NSMapTable *aMapTable = [[NSMapTable alloc] initWithKeyOptions:NSPointerFunctionsCopyIn valueOptions:NSPointerFunctionsStrongMemory capacity:0]; 或 NSMapTable *aMapTable = [NSMapTable mapTableWithKeyOptions:NSPointerFunctionsCopyIn valueOptions:NSPointerFunctionsStrongMemory];

那么就等同于NSMutableDictionay的初始化方法：

NSMutableDictionary *aDictionary = [[NSMutableDictionary alloc] initWithCapacity:0]; 或 NSMutableDictionary *aDictionary = [NSMutableDictionary dictionary];

若初始方法修改为

NSMapTable *aMapTable = [NSMapTable mapTableWithKeyOptions:NSPointerFunctionsWeakMemory valueOptions:NSPointerFunctionsStrongMemory];

即对key值进行弱引用，就可以不用让Teacher类遵循NSCopying协议和重新跟hash有关的两个方法，代码如下：

// Teacher.h @interface Teacher : NSObject @property (nonatomic, copy) NSString *name; @property (nonatomic, assign) NSInteger age; @end // Teacher.m @implementation Teacher @end // ViewController.m Teacher *teacher = [[Teacher alloc] init]; teacher.name = @"teacher"; teacher.age = 30;NSMutableArray *aArray = [[NSMutableArray alloc] initWithCapacity:0]; for (int i = 0; i < 3; i++) { Student *student = [[Student alloc] init]; student.name = [NSString stringWithFormat:@"student%d", i]; student.age = i; [aArray addObject:student]; } NSMapTable *aMapTable = [NSMapTable mapTableWithKeyOptions:NSPointerFunctionsWeakMemory valueOptions:NSPointerFunctionsStrongMemory]; [aMapTable setObject:aArray forKey:teacher]; NSLog(@"%@", aMapTable);

打印日志：

NSMapTable { [10] <Teacher: 0x604000038a40> -> ( "<Student: 0x60400003c640>", "<Student: 0x60400003c660>", "<Student: 0x60400003c620>" ) }

这样的方法可以快速的将NSObject对象作为key存入到“字典”中。

由于NSDictionary/NSMutableArray会强引用value，使得value的引用计数+1，加入不希望怎么做，可以用NSMapTable来实现。

NSMapTable *aMapTable = [NSMapTable mapTableWithKeyOptions:NSPointerFunctionsStrongMemory valueOptions:NSPointerFunctionsWeakMemory]; { NSObject *keyObject = [[NSObject alloc] init]; NSObject *valueObject = [[NSObject alloc] init]; [aMapTable setObject:valueObject forKey:keyObject]; NSLog(@"NSMapTable:%@", aMapTable); } NSLog(@"NSMapTable:%@", aMapTable);

打印日志：

NSMapTable:NSMapTable { [6] <NSObject: 0x60c00000c690> -> <NSObject: 0x60c00000c730> } NSMapTable:NSMapTable { } 

第一个NSLog打印出了key-value值，等到object对象指向的NSObject对象超出作用域，释放该对象，由于aMapTable弱引用object对象，aMapTable的中的key-value值会被安全的删除，第二个NSLog打印出的值为空。

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NSMapTable与NSDictionary/NSMutableDictionary对比

1. NSDcitionary有一个可变类型NSMutableDictionary，NSMapTable没有可变类型，它本身就是可变的;
2. NSDcitionary/NSMutableDictionary中对于key和value的内存管理方法唯一，即对key进行copy，对value进行强引用，而NSMapTable没有限制；
3. NSDcitionary中对key值进行copy，不可改变，通常用字符串作为key值，只是key->object的映射，而NSMapTable的key是可变的对象，既可以实现key->object的映射，又可以实现object->object的映射。

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遗留的问题

笔者才疏学浅，对NSMapTable与NSDictionary的内部结构了解不是很深，不清楚key-value是通过怎么样的方式绑定起来的，假如看到这篇文章的朋友有所了解，希望可以指点一二。