前言

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

• 字典只能存储 OC 对象，不能存储 C 语言中的基本数据类型，也不能存储 nil 。字典中的键必须是单值的，通常它们是字符串。

• 字典对象是无序的，第一个放到词典中的键-对象对并不一定是第一个取出。

• Xcode 7 对系统中常用的一系列容器类型都增加了泛型支持（），有了泛型后就可以指定容器类中对象的类型了。假如向泛型容器中加入错误的对象，编译器会报警告。

      __covariant：协变性，子类型可以强转到父类型（里氏替换原则）。
      __contravariant：逆变性，父类型可以强转到子类型。

• 不指定泛型类型的对象可以和任意泛型类型转化，但指定了泛型类型后，两个不同类型间是不可以强转的，假如你希望主动控制转化关系，就需要使用泛型的协变性和逆变性修饰符。

1、不可变字典的创建

    // 直接创建不可变字典
        
        // 字典中的 key 和 value 成对出现
        NSDictionary *dic1 = @{@"key1":@"value1", @"key2":@"value2"};
        
    // 对象方法创建
        
        NSDictionary *dic2 = [[NSDictionary alloc] init];
            
        // 和 dic1 的地址不相同
        NSDictionary *dic3 = [[NSDictionary alloc] initWithDictionary:dic1];
    
        NSDictionary *dic4 = [[NSDictionary alloc] initWithObjectsAndKeys:@"value1", @"key1", @"value2", @"key2", nil];
        NSDictionary *dic5 = [[NSDictionary alloc] initWithObjects:@[@"value1", @"value2", @"value3",] 
                                                           forKeys:@[@"key1", @"key2", @"key3"]];
        
    // 类方法创建
            
        NSDictionary *dic6 = [NSDictionary dictionary];
            
        // 和 dic1 的地址不相同
        NSDictionary *dic7 = [NSDictionary dictionaryWithDictionary:dic1];
    
        NSDictionary *dic8 = [NSDictionary dictionaryWithObjectsAndKeys:@"value1", @"key1", @"value2", @"key2", nil];
        NSDictionary *dic9 = [NSDictionary dictionaryWithObject:@"value1" forKey:@"key1"];
        NSDictionary *dic10 = [NSDictionary dictionaryWithObjects:@[@"value1", @"value2", @"value3",] 
                                                          forKeys:@[@"key1", @"key2", @"key3"]];
        
    // 从 文件 创建字符串
    
        NSString *path = [NSHomeDirectory() stringByAppendingString:@"/Desktop/test.txt"];
            
        NSDictionary *dic11 = [[NSDictionary alloc] initWithContentsOfFile:path];
        NSDictionary *dic12 = [NSDictionary dictionaryWithContentsOfFile:path];
            
    // 从 Url 创建字符串
    
        /*
            file:// 文件前缀
        */
    
        NSURL *url = [NSURL URLWithString:[@"file://" stringByAppendingString:[NSHomeDirectory() 
                                                      stringByAppendingString:@"/Desktop/test.txt"]]];
            
        NSDictionary *dic13 = [[NSDictionary alloc] initWithContentsOfURL:url];
        NSDictionary *dic14 = [NSDictionary dictionaryWithContentsOfURL:url];
        
    // 泛型创建
    
        NSDictionary<NSString *, NSNumber *> *dic15 = @{@"key1":@3, @"key2":@5};

2、字典中元素个数的计算

    NSDictionary *dic = [NSDictionary dictionaryWithObjectsAndKeys:@"value1", @"key1", @"value2", @"key2", nil];
        
    NSUInteger length = [dic count];

3、通过 key 找对应的 value

    NSDictionary *dic = [NSDictionary dictionaryWithObjectsAndKeys:@"value1", @"key1", @"value2", @"key2", nil];
        
    // 通过 key 找对应的 value
    NSLog(@"%@", [dic objectForKey:@"key1"]);
    
    // 以下标的方式找 key 找对应的 value
    NSLog(@"%@", dic[@"key2"]);
        
    // key 没有对应的 value 时用 notFoundMarker 参数代替
    NSArray *arr = [dic objectsForKeys:@[@"key1", @"key1", @"key3"] notFoundMarker:@"not-Found"];

4、通过 value 找对应的 key

    NSDictionary *dic = [NSDictionary dictionaryWithObjectsAndKeys:@"value1", @"key1", @"value2", 
                                                                   @"key2", @"value2", @"key3", nil];
        
    NSLog(@"%@", [dic allKeysForObject:@"value2"]);

5、字典中所有 key 的获取

    NSDictionary *dic = [NSDictionary dictionaryWithObjectsAndKeys:@"value1", @"key1", @"value2", @"key2", nil];
        
    NSLog(@"%@", [dic allKeys]);

6、字典中所有 value 的获取

    NSDictionary *dic = [NSDictionary dictionaryWithObjectsAndKeys:@"value1", @"key1", @"value2", @"key2", nil];
        
    NSLog(@"%@", [dic allValues]);

7、可变字典 NSMutableDictionary 的创建

    // 对象方法创建
        
        // 创建指定长度的字典，预先分配空间，提高效率，实际长度可大于指定长度
        NSMutableDictionary *dic1 = [[NSMutableDictionary alloc] initWithCapacity:0];
        
    // 类方法创建
        
        // 创建指定长度的字典，预先分配空间，提高效率，实际长度可大于指定长度
        NSMutableDictionary *dic2 = [NSMutableDictionary dictionaryWithCapacity:0];

8、字典中键值对的追加

    NSMutableDictionary *dic = [NSMutableDictionary dictionaryWithObjectsAndKeys:@"value1", @"key1", nil];
        
    // 向字典末尾追加一个元素（key 值为新增的，若 key 值已存在则为修改对应的 value 值）
    [dic setObject:@"value2" forKey:@"key2"];

    // 向字典末尾追加一个字典
    NSDictionary *dicAdd = @{@"key3": @"value3"};
    [dic addEntriesFromDictionary:dicAdd];

9、字典中元素的删除

    NSMutableDictionary *dic = [NSMutableDictionary dictionaryWithObjectsAndKeys:@"value1", @"key1", @"value2", 
                                                                                 @"key2", @"value3", @"key3", 
                                                                                 @"value4", @"key4", nil];
        
    // 删除指定的 key 对应的 value 和 key 值
    [dic removeObjectForKey:@"key3"];
    
    // 删除所有的 value 和 key 值
    [dic removeAllObjects];

10、字典中元素的修改

    NSMutableDictionary *dic = [NSMutableDictionary dictionaryWithObjectsAndKeys:@"value1", @"key1", @"value2", 
                                                                                 @"key2", @"value3", @"key3", 
                                                                                 @"value4", @"key4", nil];
        
    // 修改已存在的键值（key 值为已经存在的，若 key 值为新增的则为追加对应的 value 值）
    [dic setObject:@"value5" forKey:@"key1"];
    
    // 修改整个字典（覆盖重写)
    NSDictionary *dic1 = @{@"key10":@"value10"};
    [dic setDictionary:dic1];

11、字典遍历

• for 循环

      NSDictionary *dic = [NSDictionary dictionaryWithObjectsAndKeys:@"value1", @"key1", @"value2", @"key2", nil];
  
      for (int i = 0; i < [dic count]; i++) {
  
          NSLog(@"%@ = %@", [[dic allKeys] objectAtIndex:i], [dic objectForKey:[[dic allKeys] objectAtIndex:i]]);
      }

• for...in 循环

      NSDictionary *dic = [NSDictionary dictionaryWithObjectsAndKeys:@"value1", @"key1", @"value2", @"key2", nil];
  
      // 遍历所取出的为 key
      for(id key in dic){
  
          NSLog(@"%@ = %@", key, [dic objectForKey:key]);
      }

• Block 遍历

      NSDictionary *dic = [NSDictionary dictionaryWithObjectsAndKeys:@"value1", @"key1", @"value2", @"key2", nil];
  
      [dic enumerateKeysAndObjectsUsingBlock:^(id  _Nonnull key, id  _Nonnull obj, BOOL * _Nonnull stop) {
  
          NSLog(@"%@ = %@", key, obj);
      }];

• 迭代器遍历

      NSDictionary *dic = [NSDictionary dictionaryWithObjectsAndKeys:@"value1", @"key1", @"value2", @"key2", nil];
  
      // key 迭代器
      NSEnumerator *keyEnumer = [dic keyEnumerator];
  
      id key = nil;
      while (key = [keyEnumer nextObject]) {
          NSLog(@"%@ = %@", key, [dic objectForKey:key]);
      }
  
      // 对象迭代器
      NSEnumerator *objEnumer = [dic objectEnumerator];
  
      id obj = nil;
      while (obj = [objEnumer nextObject]) {
          NSLog(@"%@", obj);
      }

12、字典排序

• 用指定的方法排序

      NSDictionary *dic = [NSDictionary dictionaryWithObjectsAndKeys:@"value1", @"key1", @"value2", @"key2", 
                                                                     @"value3", @"key3", @"value4", @"key4", nil];
  
      // 先对 key 进行排序，再遍历数组
      id keys1 = [dic keysSortedByValueUsingSelector:@selector(compare:)];
  
      for (id key in keys1) {
          NSLog(@"%@ = %@", key, dic[key]);
      }

• 用 Block 排序

      NSDictionary *dic = [NSDictionary dictionaryWithObjectsAndKeys:@"value1", @"key1", @"value2", @"key2", 
                                                                     @"value3", @"key3", @"value4", @"key4", nil];
  
      id keys2 = [dic keysSortedByValueUsingComparator:^NSComparisonResult(id  _Nonnull obj1, id  _Nonnull obj2) {
          return [obj1 compare:obj2];
      }];
  
      for (id key in keys2) {
          NSLog(@"%@ = %@", key, dic[key]);
      }