据说，Foundation是支撑整个Objective-C开发的基础库，重要性不言而喻。
偷来一副图，表述一下Foundation的位置。

Cocoa和UIKit主要关注于应用程序、UI及用户输入，而Foundation主要关注更底层的组织数据的任务。

本文为菜鸟所做，简单的记录自己使用NSString、NSArray、NSDictionary

NSString

官方文档：https://developer.apple.com/reference/foundation/nsstring
分OC和Swift两个版本讲解。
1）创建字符串

  NSString* constantString = @"Text of the string";

2)大小写转换

    //变成大写
    NSString* uppercaseString = [constantString uppercaseString]; 
    //变成小写
    NSString* lowercaseString = [constantString lowercaseString]; 
    //首字母大写
    NSString* capitalizedString = [constantString capitalizedString]; 

3）获取子字符串

    //获取前5个字符串
    NSString* startSubstring = [constantString substringToIndex:5]; 
    //获取第五个字符之后的字符串
    NSString* endSubstring = [constantString substringFromIndex:5]; 
    //获取index为2，长度是5的子字符串 
    NSRange range = NSMakeRange(2, 5); 
    NSString* substring = [constantString substringWithRange:range]; 

4）比较字符串

    if([uppercaseString isEqualToString:lowercaseString]) { 
        NSLog(@"两个字符串相同"); 
    } else { 
        NSLog(@"两个字符串不相同"); 
    }

5）查找字符串

    NSRange result = [constantString rangeOfString:@"the" options:NSCaseInsensitiveSearch]; 
    if(result.location == NSNotFound) { 
        NSLog(@"string not found"); 
    } else { 
        NSLog(@"result found is from : %lu",(unsigned long)result.location); 
    }

NSArray

官方文档：https://developer.apple.com/reference/foundation/nsarray
1）定义一个不可变数组

    NSArray* myArray = @[@"one",@"two",@"three",[NSNull null]]; 
    NSString* oneString = myArray[0]; 
    NSString* nullString = myArray[3]; 
    NSLog(@"null string: %@",nullString); 
     
    //数组元素的索引
    int index = [myArray indexOfObject:@"two"]; 
    if(index == NSNotFound) { 
        //do something 
    }

2）创建一个子字符串

    NSRange subArrayRange = NSMakeRange(1, 2); 
    NSArray* subArray = [myArray subarrayWithRange:subArrayRange];

3)快速枚举

    for(NSString* string in myArray) { 
        //do something 
    }

4)可变数组

    NSMutableArray* mutableArray = [NSMutableArray arrayWithArray:@[@"linc",@"lily"]]; 
    [mutableArray addObject:@"lucy"]; 
    [mutableArray insertObject:@"lee" atIndex:2]; 
     
    [mutableArray removeObject:@"lily"]; 
    [mutableArray removeObjectAtIndex:1]; 
    [mutableArray replaceObjectAtIndex:1 withObject:@"linda"];

NSDictionary

官方文档：https://developer.apple.com/reference/foundation/nsdictionary?language=objc
字典就是存储键值对的容器，与Java中的Map类似。
1）创建一个字典

    NSDictionary* dictionary = @{@"name":@"linc", 
                                 @"age":@"18"}; 
    NSString* name = dictionary[@"name"]; 
    NSLog(@"name is %@",name);

2)NSValue和NSNumber
NSArray和NSDictionary中只能包含OC对象，那么其他非OC对象的值该怎么活呢？
这就引入了NSValue，它允许你存储大量非对象的类型。
而NSNumber就是其子类，专门存储数字。用法如下。

    NSNumber* number = @124; 
    int value = [number intValue]; 
    NSNumber* number2 = @(value+20);

参考：
0、《Coaco入门–使用Objective-C》
1、http://www.cnblogs.com/kenshincui/p/3885689.html
2、https://developer.apple.com/reference/foundation