NSDictionary基本概念

本小节知识点:

1. 【理解】NSDictionar基本概念
2. 【理解】NSDictionary的创建
3. 【理解】NSDictionary的遍历
4. 【理解】NSDictionary文件操作

1.NSDictionar基本概念

• 什么是NSDictionary
  • NSDictionary翻译过来叫做”字典”
  • 日常生活中,“字典”的作用:通过一个拼音或者汉字,就能找到对应的详细解释
  • NSDictionary的作用类似:通过一个key,就能找到对应的value
  • NSDictionary是不可变的, 一旦初始化完毕, 里面的内容就无法修改

2.NSDictionary的创建

+ (instancetype)dictionary;
+ (instancetype)dictionaryWithObject:(id)object forKey:(id <NSCopying>)key;
+ (instancetype)dictionaryWithObjectsAndKeys:(id)firstObject, ...;
+ (id)dictionaryWithContentsOfFile:(NSString *)path;
+ (id)dictionaryWithContentsOfURL:(NSURL *)url;

• NSDictionary创建简写

  • 以前

    NSDictionary *dict = [NSDictionary dictionaryWithObjectsAndKeys:@"lnj", @"name", @"12345678", @"phone", @"天朝", @"address", nil];
    

  • 现在

    NSDictionary *dict = @{@"name":@"lnj", @"phone":@"12345678", @"address":@"天朝"};
    

• NSDictionary获取元素简写

  • 以前

    [dict objectForKey:@"name”];
    

  • 现在

    dict[@"name”];
    

• 键值对集合的特点
  • 字典存储的时候,必须是"键值对"的方式来存储(同时键不要重复)
  • 键值对中存储的数据是"无序的".
  • 键值对集合可以根据键, 快速获取数据.

3.NSDictionary的遍历

• - (NSUInteger)count;

  • 返回字典的键值对数目

• - (id)objectForKey:(id)aKey;

  • 根据key取出value

• 快速遍历

    NSDictionary *dict = @{@"name":@"lnj", @"phone":@"12345678", @"address":@"天朝"};
    for (NSString *key in dict) {
        NSLog(@"key = %@, value = %@", key, dict[key]);
    }

• Block遍历

    [dict enumerateKeysAndObjectsUsingBlock:^(NSString *key, NSString *obj, BOOL *stop) {
        NSLog(@"key = %@, value = %@", key, obj);
    }];

4.NSDictionary文件操作

• 将字典写入文件中

  • - (BOOL)writeToFile:(NSString *)path atomically:(BOOL)useAuxiliaryFile;
  • - (BOOL)writeToURL:(NSURL *)url atomically:(BOOL)atomically;
  • 存结果是xml文件格式,但苹果官方推荐为plist后缀。

• 示例

    NSDictionary *dict = @{@"name":@"lnj", @"phone":@"12345678", @"address":@"天朝"};
    BOOL flag = [dict writeToFile:@"/Users/LNJ/Desktop/dict.plist" atomically:YES];
    NSLog(@"flag = %i", flag);

• 从文件中读取字典

NSDictionary *newDict = [NSDictionary dictionaryWithContentsOfFile:@"/Users/LNJ/Desktop/dict.plist"];
    NSLog(@"newDict = %@", newDict);