1.Foundation框架
Cocoa可以看作许多框架的集合，在iOS中包括Foundation框架和UIKit框架，Foundation框架里主要是一些API供程序开发使用，UIKit则包括了许多UI绘制的函数。
2.字符串操作
初始化字符串：
NSString *str1 = [[NSString alloc] init];
str1 = @"Hello,iOS.";
NSString *str2 = [[NSString alloc] initWithString:@"Hello,iOS."];
NSString *str3 = [[NSString alloc] initWithFormat:@"Age is %i,name is %.2f",20,56.0f];
NSString *str4 = [[NSString alloc] initWithUTF8String:@"Hello,iOS."];
格式化大小写：
//转换成小写
NSLog(@"%@",[@"Hello,iOS." lowercaseString]);
//转换成大写
NSLog(@"%@",[@"Hello,iOS." uppercaseString]);
//首字母大写，其他字母小写
NSLog(@"%@",[@"Hello,iOS." capitalizedString]);
比较字符串：
//判断是否相等
BOOL res = [@"abc" isEqualToString:@"abc"];
//大小比较，有三种结果：NSOrderedAscending/NSOrderedDescending/NSOrderedSame
NSComparisonResult res = [@"abc" compare:@"ABC"];
if(res == NSOrderedAscending)
NSLog("Left < Right");
else if(res == NSOrderedDescending)
NSLog("Left > Right");
else
NSLog("Left = Right");
字符串前缀和后缀的判断：
[@"abcdef" hasPrefix:@"abc"];
[@"abcdef" hasSuffix:@"def"];
查找字符串：
NSRange range = [@"abcdefg" rangeOfString:@"cd" ];
if(range.location == NSNotFound)
NSLog(@"Not found.");
else
NSLog(@"range is %@",NSStringFromRange(range));//range is {2,2}
字符串分割：
NSLog(@"%@",[@"abcdef" substringFromIndex:3]);//def
NSLog(@"%@",[@"abcdef" substringToIndex:3]);//abc
NSLog(@"%@",[@"abcdef" substringWithRange:NSMakeRange(2,3)]);//cde
NSString *str = @"a.b.c.d.e.f";
NSArray *array = [str componentsSeparatedByString:@"."];
NSLog(@"%@",array);//(a,b,c,d,e,f);
类型转换：
NSLog(@"%i",[@"12" intValue]);//12
NSLog(@"%zi",[@"Hello,iOS." length]);//10
NSLog(@"%c",[@"Hello,iOS." characterAtIndex:0]);//H
const char *s = [@"Hello,iOS." UTF8String];
NSLog(@"%s",s);//Hello,iOS.
文件操作：
//读取文件内容
NSString *path = @"/Users/Shvier/Desktop/Demo.txt";
NSString *str = [NSString stringWithContentsOfFile:path encoding:NSUTF8StringEncoding error:nil];
NSLog(@"%@",str);//Hello,iOS.
//写入文件内容
NSString *path = @"/Users/Shvier/Desktop/Demo.txt";
NSString *str = @"Hello,iOS";
[str writeToFile:path atomically:YES encoding:NSUTF8StringEncoding error:nil];//atomically表示一次性写入，如果写入过程中出错，就全部都不要写入了。