iOS字符串处理

一、NSCharacterSet的使用

NSCharacterSet是一个字符集合，利用好这个类，可以更加方便的对字符串进行操作。

比如"abcdefghijklmnfwafajkfjawifa"，需求要将这段字符中的"f"、"a"去掉，我们大概会这么做:

    NSString *str = @"abcdefghijklmnfwafajkfjawifa";
    NSInteger length = str.length;
    
    NSString *removeStr = @"af";
    NSMutableString *resultStr = [NSMutableString string];
    for (int i = 0; i < length; i ++) {
        NSString *indexStr = [str substringWithRange:NSMakeRange(i, 1)];
        if (![removeStr containsString:indexStr]) {
            [resultStr appendString:indexStr];
        }
    }
    
    NSLog(@"%@",resultStr);

有心的可以算一下时间复杂度，这真的是一段看起来就很烦躁的代码。

如果用NSCharacterSet进行处理：

    NSString *str = @"abcdefghijklmnfwafajkfjawifa";
    NSCharacterSet *set = [NSCharacterSet characterSetWithCharactersInString:@"af"];
    
    NSString *resultStr = [str stringByTrimmingCharactersInSet:set];
    
    NSLog(@"%@",resultStr);

stringByTrimmingCharactersInSet：缩减NSCharacterSet里面的字符。

NSCharacterSet的创建：

除了自己拼接string的方式，还可以使用以下类方法，直接获取一个想要的字符集合

/** 常用快捷方法集合  */ 
+ controlCharacterSet 
+ whitespaceCharacterSet //空格 
+ whitespaceAndNewlineCharacterSet //空格和换行符 
+ decimalDigitCharacterSet //0-9的数字 
+ letterCharacterSet //所有字母 
+ lowercaseLetterCharacterSet //小写字母 
+ uppercaseLetterCharacterSet //大写字母 
+ alphanumericCharacterSet //所有数字和字母（大小写不分） 
+ punctuationCharacterSet //标点符号 
+ newlineCharacterSet //换行

/** URL相关快捷方法集合  */ 
+ URLUserAllowedCharacterSet 
+ URLPasswordAllowedCharacterSet  
+ URLHostAllowedCharacterSet 
+ URLPathAllowedCharacterSet //路径允许使用的字符集合
+ URLQueryAllowedCharacterSet //参数允许使用的字符集合
+ URLFragmentAllowedCharacterSet 

二、rangeOfComposedCharacterSequencesForRange、rangeOfComposedCharacterSequenceAtIndex

每一个中文或者英文在NSString中的length均为1，但是一个Emoji的length的长度为2或者4，如果使用substringToIndex可能存在把Emoji截断而导致乱码的情况。

这两个方法，可以获取当前范围或者当前index下的完整字符，避免乱码情况的出现。

---

URL的编码就是一个很好地字符串处理的例子：

我们先来回顾一下URL的编码规则：

根据2005年发布的RFC3986“%编码”规范：对URL中属于ASCII字符集的非保留字不做编码；对URL中的保留字需要取其ASCII内码，然后加上“%”前缀将该字符进行替换（编码）；对于URL中的非ASCII字符需要取其Unicode内码，然后加上“%”前缀将该字符进行替换（编码）。由于这种编码是采用“%”加上字符内码的方式，所以，有些地方也称其为“百分号编码”。

URL中的保留字符： ! * ' ( ) ; : @ & = + $ , / ? # [ ]
其中分隔符：/ ?

AFNetworking提供的编码方法

/**
 Returns a percent-escaped string following RFC 3986 for a query string key or value.
 RFC 3986 states that the following characters are "reserved" characters.
    - General Delimiters: ":", "#", "[", "]", "@", "?", "/"
    - Sub-Delimiters: "!", "$", "&", "'", "(", ")", "*", "+", ",", ";", "="

 In RFC 3986 - Section 3.4, it states that the "?" and "/" characters should not be escaped to allow
 query strings to include a URL. Therefore, all "reserved" characters with the exception of "?" and "/"
 should be percent-escaped in the query string.
    - parameter string: The string to be percent-escaped.
    - returns: The percent-escaped string.
 */
NSString * AFPercentEscapedStringFromString(NSString *string) {
    static NSString * const kAFCharactersGeneralDelimitersToEncode = @":#[]@"; // does not include "?" or "/" due to RFC 3986 - Section 3.4
    static NSString * const kAFCharactersSubDelimitersToEncode = @"!$&'()*+,;=";

    NSMutableCharacterSet * allowedCharacterSet = [[NSCharacterSet URLQueryAllowedCharacterSet] mutableCopy];
    [allowedCharacterSet removeCharactersInString:[kAFCharactersGeneralDelimitersToEncode stringByAppendingString:kAFCharactersSubDelimitersToEncode]];

    // FIXME: https://github.com/AFNetworking/AFNetworking/pull/3028
    // return [string stringByAddingPercentEncodingWithAllowedCharacters:allowedCharacterSet];

    static NSUInteger const batchSize = 50;

    NSUInteger index = 0;
    NSMutableString *escaped = @"".mutableCopy;

    while (index < string.length) {
        NSUInteger length = MIN(string.length - index, batchSize);
        NSRange range = NSMakeRange(index, length);

        // To avoid breaking up character sequences such as 
        range = [string rangeOfComposedCharacterSequencesForRange:range];

        NSString *substring = [string substringWithRange:range];
        NSString *encoded = [substring stringByAddingPercentEncodingWithAllowedCharacters:allowedCharacterSet];
        [escaped appendString:encoded];

        index += range.length;
    }

    return escaped;
}