版本记录

版本号	时间
V1.0	2017.08.27

前言

NSDictionary是不可变字典，它的不可变性可以参考NSArray数组，但是它与数组还是有很大不同，尽管他们都属于集合类，下面这几篇我们继续来将一下基础类的知识。还是老规矩从整体到局部，从浅入深进行讲解，谢谢大家。

基本了解

NSDictionary字典类非常重要，它有键key，对应每一个键都有值value，键之间是不会重复的，字典的查询和赋值也是通过键找到的值，然后进行的操作。

在具体工程实践中，用的比较多的情况就是，从网络请求的数据，经过json解析，解析后称为一个个的字典，然后在字典转化为模型，然后将模型添加到数组中，用这个数组作为数据源，给页面相应的元素进行赋值。

API

下面我们就看一下NSDictionary的API接口。

#import <Foundation/NSObject.h>
#import <Foundation/NSEnumerator.h>

@class NSArray<ObjectType>, NSSet<ObjectType>, NSString, NSURL;

/****************   Immutable Dictionary    ****************/

NS_ASSUME_NONNULL_BEGIN

@interface NSDictionary<__covariant KeyType, __covariant ObjectType> : NSObject <NSCopying, NSMutableCopying, NSSecureCoding, NSFastEnumeration>

@property (readonly) NSUInteger count;
- (nullable ObjectType)objectForKey:(KeyType)aKey;
- (NSEnumerator<KeyType> *)keyEnumerator;
- (instancetype)init NS_DESIGNATED_INITIALIZER;
#if TARGET_OS_WIN32
- (instancetype)initWithObjects:(const ObjectType _Nonnull [_Nullable])objects forKeys:(const KeyType _Nonnull [_Nullable])keys count:(NSUInteger)cnt;
#else
- (instancetype)initWithObjects:(const ObjectType _Nonnull [_Nullable])objects forKeys:(const KeyType <NSCopying> _Nonnull [_Nullable])keys count:(NSUInteger)cnt NS_DESIGNATED_INITIALIZER;
#endif
- (nullable instancetype)initWithCoder:(NSCoder *)aDecoder NS_DESIGNATED_INITIALIZER;

@end

@interface NSDictionary<KeyType, ObjectType> (NSExtendedDictionary)

@property (readonly, copy) NSArray<KeyType> *allKeys;
- (NSArray<KeyType> *)allKeysForObject:(ObjectType)anObject;
@property (readonly, copy) NSArray<ObjectType> *allValues;
@property (readonly, copy) NSString *description;
@property (readonly, copy) NSString *descriptionInStringsFileFormat;
- (NSString *)descriptionWithLocale:(nullable id)locale;
- (NSString *)descriptionWithLocale:(nullable id)locale indent:(NSUInteger)level;
- (BOOL)isEqualToDictionary:(NSDictionary<KeyType, ObjectType> *)otherDictionary;
- (NSEnumerator<ObjectType> *)objectEnumerator;
- (NSArray<ObjectType> *)objectsForKeys:(NSArray<KeyType> *)keys notFoundMarker:(ObjectType)marker;
- (BOOL)writeToFile:(NSString *)path atomically:(BOOL)useAuxiliaryFile;
- (BOOL)writeToURL:(NSURL *)url atomically:(BOOL)atomically; // the atomically flag is ignored if url of a type that cannot be written atomically.

- (NSArray<KeyType> *)keysSortedByValueUsingSelector:(SEL)comparator;
// count refers to the number of elements in the dictionary
- (void)getObjects:(ObjectType __unsafe_unretained [])objects andKeys:(KeyType __unsafe_unretained [])keys count:(NSUInteger)count NS_AVAILABLE(10_7, 5_0) NS_SWIFT_UNAVAILABLE("Use 'allKeys' and/or 'allValues' instead");

- (nullable ObjectType)objectForKeyedSubscript:(KeyType)key NS_AVAILABLE(10_8, 6_0);

- (void)enumerateKeysAndObjectsUsingBlock:(void (NS_NOESCAPE ^)(KeyType key, ObjectType obj, BOOL *stop))block NS_AVAILABLE(10_6, 4_0);
- (void)enumerateKeysAndObjectsWithOptions:(NSEnumerationOptions)opts usingBlock:(void (NS_NOESCAPE ^)(KeyType key, ObjectType obj, BOOL *stop))block NS_AVAILABLE(10_6, 4_0);

- (NSArray<KeyType> *)keysSortedByValueUsingComparator:(NSComparator NS_NOESCAPE)cmptr NS_AVAILABLE(10_6, 4_0);
- (NSArray<KeyType> *)keysSortedByValueWithOptions:(NSSortOptions)opts usingComparator:(NSComparator NS_NOESCAPE)cmptr NS_AVAILABLE(10_6, 4_0);

- (NSSet<KeyType> *)keysOfEntriesPassingTest:(BOOL (NS_NOESCAPE ^)(KeyType key, ObjectType obj, BOOL *stop))predicate NS_AVAILABLE(10_6, 4_0);
- (NSSet<KeyType> *)keysOfEntriesWithOptions:(NSEnumerationOptions)opts passingTest:(BOOL (NS_NOESCAPE ^)(KeyType key, ObjectType obj, BOOL *stop))predicate NS_AVAILABLE(10_6, 4_0);

@end

@interface NSDictionary<KeyType, ObjectType> (NSDeprecated)
/// This method is unsafe because it could potentially cause buffer overruns. You should use -getObjects:andKeys:count:
- (void)getObjects:(ObjectType __unsafe_unretained [])objects andKeys:(KeyType __unsafe_unretained [])keys  NS_SWIFT_UNAVAILABLE("Use 'allKeys' and/or 'allValues' instead");
@end

@interface NSDictionary<KeyType, ObjectType> (NSDictionaryCreation)

+ (instancetype)dictionary;
+ (instancetype)dictionaryWithObject:(ObjectType)object forKey:(KeyType <NSCopying>)key;
#if TARGET_OS_WIN32
+ (instancetype)dictionaryWithObjects:(const ObjectType [])objects forKeys:(const KeyType [])keys count:(NSUInteger)cnt;
#else
+ (instancetype)dictionaryWithObjects:(const ObjectType [])objects forKeys:(const KeyType <NSCopying> [])keys count:(NSUInteger)cnt;
#endif

+ (instancetype)dictionaryWithObjectsAndKeys:(id)firstObject, ... NS_REQUIRES_NIL_TERMINATION NS_SWIFT_UNAVAILABLE("Use dictionary literals instead");

+ (instancetype)dictionaryWithDictionary:(NSDictionary<KeyType, ObjectType> *)dict;
+ (instancetype)dictionaryWithObjects:(NSArray<ObjectType> *)objects forKeys:(NSArray<KeyType <NSCopying>> *)keys;

- (instancetype)initWithObjectsAndKeys:(id)firstObject, ... NS_REQUIRES_NIL_TERMINATION;
- (instancetype)initWithDictionary:(NSDictionary<KeyType, ObjectType> *)otherDictionary;
- (instancetype)initWithDictionary:(NSDictionary<KeyType, ObjectType> *)otherDictionary copyItems:(BOOL)flag;
- (instancetype)initWithObjects:(NSArray<ObjectType> *)objects forKeys:(NSArray<KeyType <NSCopying>> *)keys;

+ (nullable NSDictionary<KeyType, ObjectType> *)dictionaryWithContentsOfFile:(NSString *)path;
+ (nullable NSDictionary<KeyType, ObjectType> *)dictionaryWithContentsOfURL:(NSURL *)url;
- (nullable NSDictionary<KeyType, ObjectType> *)initWithContentsOfFile:(NSString *)path;
- (nullable NSDictionary<KeyType, ObjectType> *)initWithContentsOfURL:(NSURL *)url;

@end

@interface NSDictionary<KeyType, ObjectType> (NSSharedKeySetDictionary)

/*  Use this method to create a key set to pass to +dictionaryWithSharedKeySet:.
 The keys are copied from the array and must be copyable.
 If the array parameter is nil or not an NSArray, an exception is thrown.
 If the array of keys is empty, an empty key set is returned.
 The array of keys may contain duplicates, which are ignored (it is undefined which object of each duplicate pair is used).
 As for any usage of hashing, is recommended that the keys have a well-distributed implementation of -hash, and the hash codes must satisfy the hash/isEqual: invariant.
 Keys with duplicate hash codes are allowed, but will cause lower performance and increase memory usage.
 */
+ (id)sharedKeySetForKeys:(NSArray<KeyType <NSCopying>> *)keys NS_AVAILABLE(10_8, 6_0);

@end

从API文档中我们可以看到，它有一个本类，还有四个分类，分别是NSExtendedDictionary、NSDeprecated、NSDictionaryCreation和NSSharedKeySetDictionary。

除了上面列出的文档外，NSFileManager.h里面还有一个字典的分类，如下所示。

@interface NSDictionary<KeyType, ObjectType> (NSFileAttributes)

- (unsigned long long)fileSize;
- (nullable NSDate *)fileModificationDate;
- (nullable NSString *)fileType;
- (NSUInteger)filePosixPermissions;
- (nullable NSString *)fileOwnerAccountName;
- (nullable NSString *)fileGroupOwnerAccountName;
- (NSInteger)fileSystemNumber;
- (NSUInteger)fileSystemFileNumber;
- (BOOL)fileExtensionHidden;
- (OSType)fileHFSCreatorCode;
- (OSType)fileHFSTypeCode;
- (BOOL)fileIsImmutable;
- (BOOL)fileIsAppendOnly;
- (nullable NSDate *)fileCreationDate;
- (nullable NSNumber *)fileOwnerAccountID;
- (nullable NSNumber *)fileGroupOwnerAccountID;
@end