1.创建初始化（Initialization&Creation）

1.1 常量字符串（literal string）

NSString* constString = @"Hello, World!"; // 此处使用了字面量语法，否则需要使用alloc initWithUTF8String或stringWithUTF8String来从双引号的C String初始化NSString对象

变量constString并不是真正包含一个字符串对象，而指向内存中字符串对象的指针（地址），我们称之为对象标识符。

以下示例中，字面量@“Hello, World!”存储在文字常量区。指针constString1和constString2都指向它，它们在编译时生成的真实类型是NSConstantString(继承关系链——:NSSimpleCString:NSString)。

Objective-C里没有包或者命名空间，靠前缀来区分，NS是“NeXTSTEP”的缩写。CF是“CoreFoundation”的缩写。CFString可以看做是NSString的C底层实现。

Foundation库（Foundation.framework/Foundation.h）是有Cocoa框架提供的基本数据管理和服务功能的Objective-C接口，而Core Foundation库 (CoreFoundation.framework/CoreFoundation.h) 则是Cocoa底层实现，提供了C语言层面的接口。

以下介绍不可变字符串两种初始化创建方法。

After an immutable string has been initialized in the following way, it cannot be modified.

1.2 Initializing an String(NS_DESIGNATED_INITIALIZER)

- (instancetype)init NS_DESIGNATED_INITIALIZER;  
- (instancetype)initWithCoder:(NSCoder*)aDecoder NS_DESIGNATED_INITIALIZER;  
   
// 从UNICODE C String中选取定长字符初始化NSString对象  
- (instancetype)initWithCharacters:(const unichar*)characters length:(NSUInteger)length;  
// 从UTF8 C String初始化NSString对象  
- (instancetype)initWithUTF8String:(const char*)nullTerminatedCString;  
// 从C String初始化NSString对象，指定编码格式。  
- (instancetype)initWithCString:(const char*)nullTerminatedCString encoding:(NSStringEncoding)encoding;  
   
// 从另一个NSString初始化新的NSString对象  
- (instancetype)initWithString:(NSString*)aString;  
// 从NSData指定编码格式初始化NSString对象  
- (instancetype)initWithData:(NSData*)data encoding:(NSStringEncoding)encoding;  
// 从C Buffer指定编码格式初始化NSString对象  
- (instancetype)initWithBytes:(const void*)bytes length:(NSUInteger)len encoding:(NSStringEncoding)encoding;  
// 可变参格式化初始化NSString对象  
- (instancetype)initWithFormat:(NSString*)format, ... NS_FORMAT_FUNCTION(1,2);

比较常用的有以下几个：

// 从UTF8 C String初始化NSString对象

- (instancetype)initWithUTF8String:(constchar *)nullTerminatedCString;

// 从C String初始化NSString对象，指定编码格式（例如NSUTF8StringEncoding）。

- (instancetype)initWithCString:(constchar *)nullTerminatedCString encoding:(NSStringEncoding)encoding;

// 从C Buffer指定编码格式初始化NSString对象

- (instancetype)initWithBytes:(constvoid *)bytes length:(NSUInteger)len encoding:(NSStringEncoding)encoding;

// 可变参格式化初始化NSString对象

- (instancetype)initWithFormat:(NSString *)format, ...NS_FORMAT_FUNCTION(1,2);

示例：

//NSString* string = [[NSString alloc] initWithFormat:@"Hello, %s","World!"];  
NSString* string = [[NSString alloc] initWithFormat:@"Hello, %@", @"World!"];  
NSLog(@"string= %@", string);

关于格式化规范，参考String Format Specifiers。

%@：OC对象描述（description）。
%d/%D：带符号32位整数（int），NSInteger：%ld，size_t：%zd。
%u/%U：无符号32位整数（unsigned int），NSUInteger：%lu。
%o/%O：无符号32位整数（unsigned int）的八进制格式。
%x/%X：无符号32位整数（unsigned int）的十六进制格式。
%c：8位无符号字符（unsigned char）。如果非ASCII码则以八进制“\\ddd”或十六进制“\\udddd”格式显示字符值。
%C：16位UNICODE字符（unichar）。如果非ASCII码则以八进制“\\ddd”或十六进制“\\udddd”格式显示字符值。
%f：64位浮点数（double）

1.3 Creating an String(autorelease)

// 创建一个字符串独享，其值为空  
+ (instancetype)string;

示例：

NSString* constString = @"";  
NSString* string = [NSString string];  
BOOL bPointerEqual = constString==string; // NO  
BOOL bContentEqualTo = [constString isEqualTo:string]; // YES  
BOOLbEqualToString = [constString isEqualToString:string]; // YES

以下是一些便利构造方法：

// initWithString对应的类方法  
+ (instancetype)stringWithString:(NSString *)string;  
  
// initWithCString:encoding对应的类方法  
+ (instancetype)stringWithCString:(const charchar *)cString encoding:(NSStringEncoding)enc;  
  
// initWithCharacters:length: 对应的类方法  
+ (instancetype)stringWithCharacters:(const unichar *)characters length:(NSUInteger)length;  
  
// initWithUTF8String对应的类方法  
+ (instancetype)stringWithUTF8String:(const charchar *)nullTerminatedCString;  
  
// initWithFormat对应的类方法  
+ (instancetype)stringWithFormat:(NSString *)format, ... NS_FORMAT_FUNCTION(1,2);  
+ (instancetype)localizedStringWithFormat:(NSString *)format, ... NS_FORMAT_FUNCTION(1,2);

比较常用的有以下几个：

// initWithCString:encoding对应的类方法

+ (instancetype)stringWithCString:(constchar *)cString encoding:(NSStringEncoding)enc;

// initWithUTF8String对应的类方法

+ (instancetype)stringWithUTF8String:(constchar *)nullTerminatedCString;

// initWithFormat对应的类方法

+ (instancetype)stringWithFormat:(NSString *)format, ...NS_FORMAT_FUNCTION(1,2);

2.访问字符串（Querying）

2.1 字符串长度（字符数组大小）

// Thenumber of Unicode characters in the receiver.  
@property (readonly) NSUInteger length;

示例：

NSString* constString1 = nil;  
NSString* constString2 = @"";  
NSString* constString3 = @"Hello, World!";  
NSString* constString4 = @"哈喽,世界!"; // 汉字+半角标点混合  
  
NSLog(@"constString1[size,length] = [%zd, %zd]", sizeof(constString1),constString1.length); // [8,0]  
NSLog(@"constString2[size,length] = [%zd, %zd]", sizeof(constString2),constString2.length); // [8,0]  
NSLog(@"constString3[size,length] = [%zd, %zd]", sizeof(constString3),constString3.length); // [8,13]  
NSLog(@"constString4[size,length] = [%zd, %zd]", sizeof(constString4),constString4.length); // [8,6]

以上可知string.length可用来判空：如果length为零，则表示字符串对象为nil或为不包含任何字符。

2.2 字符（字符数组元素）

// 获取指定索引位置/范围的字符（集）  
- (unichar)characterAtIndex:(NSUInteger)index;  
- (void)getCharacters:(unichar *)buffer range:(NSRange)aRange;

示例：

unichar ch = [constString3 characterAtIndex:7];  
NSLog(@"ch = %c", ch); // W  
unichar* cBuf = malloc(sizeof(unichar)*constString3.length);  
[constString3 getCharacters:cBuf];  
NSString* stringFromCharacters1 = [[NSString alloc] initWithCharacters:cBuf length:constString3.length];  
NSLog(@"stringFromCharacters1 = %@", stringFromCharacters1); // @"Hello, World!"  
[stringFromCharacters1 release];  
  
[constString3 getCharacters:cBuf range:NSMakeRange(7, 6)];  
NSString* stringFromCharacters2 = [NSString stringWithCharacters:cBuf length:constString3.length];  
NSLog(@"stringFromCharacters2 = %@", stringFromCharacters2);  // @"World! World!"

2.3 取C String

//Convenience to return null-terminated UTF8 representation  
@property (readonly) __strong const charchar *UTF8String NS_RETURNS_INNER_POINTER;

2.4 取字面值

类似cstdlib中的atoi/atol/strtol/atof。

/* convenience methods all skip initial space characters (whitespaceSet)and ignore trailing characters.  
    <strong>NSScanner</strong> can be used for more"exact" parsing of numbers. */  
@property (readonly) double doubleValue;  
@property (readonly) float floatValue;  
@property (readonly) int intValue;  
@property (readonly) NSInteger integerValue NS_AVAILABLE(10_5, 2_0);  
@property (readonly) long long longLongValue NS_AVAILABLE(10_5, 2_0);  
@property (readonly) BOOL boolValue NS_AVAILABLE(10_5, 2_0);

2.5 大小写转换

// 所有字符转换成大写  
@property (readonly, copy) NSString *uppercaseString;  
// 所有字符转换成小写  
@property (readonly, copy) NSString *lowercaseString;  
// 每个单词首字母大写，其余字母小写。  
@property (readonly, copy) NSString *capitalizedString;

3.查询字符串（Finding）

3.1 定位子串（rangeOf）

// 返回查找到包含子串的范围  
- (NSRange)rangeOfString:(NSString *)aString;  
// 返回查找到包含子串的范围，可指定查询选项（忽略大小写、逆序）  
- (NSRange)rangeOfString:(NSString *)aString options:(NSStringCompareOptions)mask;  
// 返回查找到包含子串的范围，可指定查询选项（忽略大小写、逆序），可进一步指定查找范围  
- (NSRange)rangeOfString:(NSString *)aString options:(NSStringCompareOptions)mask range:(NSRange)searchRange;

3.2 取子串（subString）

// 返回从指定索引到结尾的子串  
- (NSString *)substringFromIndex:(NSUInteger)from;  
// 返回从开头到指定索引的子串  
- (NSString *)substringToIndex:(NSUInteger)to;  
// 返回从指定范围（开始索引+长度）界定的子串  
- (NSString *)substringWithRange:(NSRange)range;   // Hint: Use withrangeOfComposedCharacterSequencesForRange: to avoid breaking up composedcharacters

示例：

NSString* prefix = [constString3 substringToIndex:7]; // @"Hello, "  
NSString* suffix = [constString3 substringFromIndex:7]; // @"World!"  
NSString* substr =[constString3 substringWithRange:NSMakeRange(3,6)]; // @"lo, Wo"

3.3 是否包含子串（prefix/suffix/contains）

// 是否以指定子串开头  
- (BOOL)hasPrefix:(NSString *)aString;  
// 是否以指定子串结尾  
- (BOOL)hasSuffix:(NSString *)aString;  
// 是否包含子串，注意只适用于iOS8以上！  
- (BOOL)containsString:(NSString *)aString NS_AVAILABLE(10_10, 8_0);

示例：

BOOL bHasPrefix = [constString3 hasPrefix:@"Hello,"]; // YES  
BOOL bHasSuffix = [constString3 hasSuffix:@"World!"]; // YES  
BOOL bContain= [constString3 containsString:@"lo, Wo"]; // YES

以下为NSString扩展类别（NSStringUtilities）判断是否包含子串的适配接口：

@implementation NSString (NSStringUtilities)  
- (BOOL)hasContainStr:(NSString*)subStr  
{  
    if(!subStr) {  
        return NO;  
    } 
    if([self respondsToSelector:@selector(containsString:)]) { // ≥iOS8  
        return [self containsString:subStr];  
    } else { // <iOS8  
        NSRange range = [self rangeOfString:subStr];  
        return (range.location!=NSNotFound ? YES : NO); // return (range.length>0 ? YES : NO);  
    }  
}  
@end

3.4 查询字符集

/* These return the range of the first character from the set in the string, not the range of a sequence of characters. 
*/  
- (NSRange)rangeOfCharacterFromSet:(NSCharacterSet *)aSet;  
- (NSRange)rangeOfCharacterFromSet:(NSCharacterSet *)aSet options:(NSStringCompareOptions)mask;  
- (NSRange)rangeOfCharacterFromSet:(NSCharacterSet *)aSet options:(NSStringCompareOptions)mask range:(NSRange)searchRange;  
   
- (NSRange)rangeOfComposedCharacterSequenceAtIndex:(NSUInteger)index;  
- (NSRange)rangeOfComposedCharacterSequencesForRange:(NSRange)range NS_AVAILABLE(10_5, 2_0);

4.比较字符串（Comparing）

// 判断两个字符串内容是否相等  
- (BOOL)isEqualToString:(NSString *)aString;  
   
/* In the compare: methods, the range argument specifies the subrange,rather than the whole, of the receiver to use in the comparison. 
The range is not applied to the search string.  For example, [@"AB"compare:@"ABC" options:0 range:NSMakeRange(0,1)] compares"A" to "ABC", not "A" to "A", and will return NSOrderedAscending. 
*/  
// 比较字符串，如果相同返回NSOrderedSame；如果不相同，返回第一个不相同的字符值比较结果（NSOrderedAscending、NSOrderedDescending）  
- (NSComparisonResult)compare:(NSString *)string;  
// 比较字符串，可指定比较选项（忽略大小写、逆序、按十进制值）  
- (NSComparisonResult)compare:(NSString *)string options:(NSStringCompareOptions)mask;  
// 比较字符串，可指定比较选项（忽略大小写、逆序、按十进制值），可进一步指定查找范围  
- (NSComparisonResult)compare:(NSString *)string options:(NSStringCompareOptions)mask range:(NSRange)compareRange;  
// 比较字符串，可指定查询选项（忽略大小写、逆序、按十进制值），可进一步指定查找范围，可进一步按照本地化比较  
- (NSComparisonResult)compare:(NSString *)string options:(NSStringCompareOptions)mask range:(NSRange)compareRange locale:(id)locale; //locale arg used to be a dictionary pre-Leopard. We now accept NSLocale. Assumes the current locale if non-nil and non-NSLocale. nil continues to mean canonical compare, which doesn't depend on user's locale choice.  
   
// 相当于compare:string options:NSCaseInsensitiveSearch  
- (NSComparisonResult)caseInsensitiveCompare:(NSString *)string;  
// 按照本地化比较  
- (NSComparisonResult)localizedCompare:(NSString *)string;  
// 以上两个版本组合  
- (NSComparisonResult)localizedCaseInsensitiveCompare:(NSString *)string;

5.替换子串（Replacing）

- (NSString *)stringByReplacingOccurrencesOfString:(NSString *)target withString:(NSString *)replacement options:(NSStringCompareOptions)options range:(NSRange)searchRange NS_AVAILABLE(10_5, 2_0);  
   
/* Replace all occurrences of the target string with replacement. Invokes the above method with 0 options and range of the whole string.*/  
- (NSString *)stringByReplacingOccurrencesOfString:(NSString *)target withString:(NSString *)replacement NS_AVAILABLE(10_5, 2_0);  
   
/* Replace characters in range with the specified string, returning new string.*/  
- (NSString *)stringByReplacingCharactersInRange:(NSRange)range withString:(NSString *)replacement NS_AVAILABLE(10_5, 2_0);

6.衍生字符串（Deriving）

// 当前字符串追加aString生成返回一个新的NSString对象。  
- (NSString *)stringByAppendingString:(NSString *)aString;  
- (NSString *)stringByAppendingFormat:(NSString *)format, ... NS_FORMAT_FUNCTION(1,2);

7.分解字符串（Separate & Join Components）

7.1 componentsSeparatedByString/componentsJointedByString

// NSString::componentsSeparatedByString接口按照分割字符（子串）来切分字符串：NSString->NSArray  
- (NSArray *)componentsSeparatedByString:(NSString *)separator;  
- (NSArray *)componentsSeparatedByCharactersInSet:(NSCharacterSet *)separator NS_AVAILABLE(10_5, 2_0);  
  
// NSArray::componentsJoinedByString接口将多个字符串（字符串数组）以衔接字符连接：NSArray->NSString  
- (NSString *)componentsJoinedByString:(NSString *)separator;

以下代码示例对典型的本地文件路径字符串按分隔符进行拆分分解，然后再按分隔符衔接复原：

NSString* originString = @"do-do-sol-sol-la-la-sol";  
NSArray* separatedComponents = [originString componentsSeparatedByString:@"-"]; // 拆分  
NSLog(@"separatedComponents = %@", separatedComponents);  
NSString* jointedString = [separatedComponents componentsJoinedByString:@"-"]; //复原  
NSLog(@"jointedString = %@", jointedString);

7.2 NSPathUtilities

NSPathUtilities中NSString (NSStringPathExtensions)扩展提供了pathComponents、lastPathComponent、pathExtension属性用于分析文件目录/路径字符串（Path String）。

// Mac文件系统目录路径  
NSString* pdWin8VMPath = @"/Users/faner/VM/Windows 8.1.pvm";  
// 拆分文件路径字符串  
NSArray* pathComponents = [pdWin8VMPath componentsSeparatedByString:@"/"];  
NSLog(@"separatedComponents = %@", pathComponents); // {@"",@"Users",@"faner",@"VM",@"Windows 8.1.pvm"}  
// 分析文件路径字符串  
NSString* lastPathComponent = pathComponents.lastObject; // full file name  
NSArray* lastPathSeparatedComponents = [lastPathComponent componentsSeparatedByString:@"."];  
NSString* fileName = lastPathSeparatedComponents.firstObject; // file name  
NSString* pathExtension = lastPathSeparatedComponents.lastObject; // file extension  
NSLog(@"pdWin8VMPath.fileName/pathExtension = {%@, %@}", fileName, pathExtension); // {Windows 8, pvm}  
// 复原文件路径字符串  
NSString* jointedString = [pathComponents componentsJoinedByString:@"/"];  
NSLog(@"jointedString = %@", jointedString);  
  
// NSPathUtilities.h  
NSLog(@"pdWin8VMPath.absolutePath = %d", pdWin8VMPath.absolutePath); // 1（是绝对文件路径）  
NSLog(@"pdWin8VMPath.pathComponents = %@", pdWin8VMPath.pathComponents); // {@"/",@"Users",@"faner",@"VM",@"Windows 8.1.pvm"}  
NSLog(@"pdWin8VMPath.lastPathComponents = %@", pdWin8VMPath.lastPathComponent); // Windows 8.1.pvm  
NSLog(@"pdWin8VMPath.pathExtension = %@", pdWin8VMPath.pathExtension); // pvm

注意：不同于普通的字符串，Mac绝对文件路径往往以‘/’开头，代表文件系统根节点（File System Root），故文件路径字符串中的pathComponents不能简单等同于[path componentsSeparatedByString:@"/"]！

字符串处理经常涉及到的资源路径和超链接路径的处理，它们和文件系统目录路径的构成稍有不同，一般由scheme(protocol)+resourceSpecifier构成。

本地文件系统的scheme是“file”，以下基于文件路径构建resource URL（追加"file://"前缀）：

// 基于文件路径字符串构建资源URL，例如用于共享的URL Scheme  
NSURL* pdWin8VMURL = [NSURL fileURLWithPath:pdWin8VMPath];  
NSLog(@"pdWin8VMURL = %@", pdWin8VMURL); // pdWin8VMURL.absoluteString = file:///Users/faner/VM/Windows%208.1.pvm/

构造NSURL

使用NSString相关格式化接口可以拼接构造URL，NSPathUtilities中NSString (NSStringPathExtensions)扩展提供的接口（stringByAppendingPathComponent/stringByDeletingLastPathComponent、stringByAppendingPathExtension/stringByDeletingPathExtension）也可辅助拼接构造URL。

当然，NSURL类提供了更直接的便利初始化函数（Convenience initializers），支持直接基于NSString初始化创建NSURL。

7.3 NSURLPathUtilities

字符串处理经常涉及到的资源路径和超链接路径的处理，它们和文件系统目录路径的构成稍有不同，一般由scheme(protocol)+resourceSpecifier构成。WWW网站地址HTTP URL的scheme为“http”、“https”，host后的第一个‘/’代表网站根目录（WebRoot）。

NSURL.h中的NSURL (NSURLPathUtilities)扩展也提供了pathComponents、lastPathComponent、pathExtension属性用于URL路径字符串。

沿袭上面的例子，下面以两个小例程来分别阐述本地资源路径（asset URL）和超链接路径（HTTP URL）的典型处理。

NSURLPathUtilities Demo1：iOS相册文件asset URL

// demo1：iOS相册文件assetURL  
NSString* iOSDCIMAssetPath = @"file:///var/mobile/Media/DCIM/101APPLE/IMG_1354.JPG";  
NSURL* iOSDCIMAssetURL = [NSURL URLWithString:iOSDCIMAssetPath];  
// absoluteString&relativeString  
NSLog(@"iOSDCIMAssetURL.absoluteString = %@", iOSDCIMAssetURL.absoluteString);  
NSLog(@"iOSDCIMAssetURL.relativeString = %@", iOSDCIMAssetURL.relativeString);  
// host&port = {(null), (null)}  
NSLog(@"iOSDCIMAssetURL.host/port = {%@, %@}", iOSDCIMAssetURL.host, iOSDCIMAssetURL.port);  
// scheme&resourceSpecifier  
NSLog(@"iOSDCIMAssetURL.scheme = %@", iOSDCIMAssetURL.scheme); // file  
NSLog(@"iOSDCIMAssetURL.resourceSpecifier = %@", iOSDCIMAssetURL.resourceSpecifier); // /var/mobile/Media/DCIM/101APPLE/IMG_1354.JPG  
// path&relativePath  
NSLog(@"iOSDCIMAssetURL.path/relativePath = {%@, %@}", iOSDCIMAssetURL.path, iOSDCIMAssetURL.relativePath); // 同resourceSpecifier  
// pathComponents = {"/", "var", "mobile", "Media", "DCIM", "101APPLE", "IMG_1354.JPG"}  
NSLog(@"iOSDCIMAssetURL.pathComponents = %@", iOSDCIMAssetURL.pathComponents);  
// lastPathComponent = pathComponents.lastObject; lastPathComponent/pathExtension = {IMG_1354.JPG, JPG}  
NSLog(@"iOSDCIMAssetURL.lastPathComponent/pathExtension = {%@, %@}", iOSDCIMAssetURL.lastPathComponent, iOSDCIMAssetURL.pathExtension);

说明：relativePath是相对baseURL而言，一般baseURL为nil，则relativePath等于path。

NSURLPathUtilities Demo2：知乎搜索“大圣归来”的HTTP URL

// demo2：知乎搜索“大圣归来”的HTTP URL  
NSString* zhihuSearchHttpURLPath = @"http://www.zhihu.com:80/search?q=%E5%A4%A7%E5%9C%A3%E5%BD%92%E6%9D%A5&type=question"; // 汉字部分已经使用CFURLCreateStringByAddingPercentEscapes进行URLEncode。  
NSURL* zhihuSearchHttpURL = [NSURL URLWithString:zhihuSearchHttpURLPath];  
// absoluteString&relativeString  
NSLog(@"zhihuSearchHttpURL.absoluteString = %@", zhihuSearchHttpURL.absoluteString);  
NSLog(@"zhihuSearchHttpURL.relativeString = %@", zhihuSearchHttpURL.relativeString);  
// host&port = {www.zhihu.com, 80}  
NSLog(@"zhihuSearchHttpURL.host/port = {%@, %@}", zhihuSearchHttpURL.host, zhihuSearchHttpURL.port);  
// scheme&resourceSpecifier = [url componentsSeparatedByString:@":"]  
NSLog(@"zhihuSearchHttpURL.scheme = %@", zhihuSearchHttpURL.scheme); // http  
NSLog(@"zhihuSearchHttpURL.resourceSpecifier = %@", zhihuSearchHttpURL.resourceSpecifier); // 协议冒号（不包括）之后双斜杠开始至末尾  
// path&relativePath = {/search, /search}  
NSLog(@"zhihuSearchHttpURL.path/relativePath = {%@, %@}", zhihuSearchHttpURL.path, zhihuSearchHttpURL.relativePath);  
// pathComponents = {"/","search"}  
NSLog(@"zhihuSearchHttpURL.pathComponents = %@", zhihuSearchHttpURL.pathComponents);  
// lastPathComponent = pathComponents.lastObject  
NSLog(@"zhihuSearchHttpURL.lastPathComponent = %@", zhihuSearchHttpURL.lastPathComponent);  
// fragment/query，其中query为问号之后的部分  
NSLog(@"zhihuSearchHttpURL.fragment/query = {%@, %@}", zhihuSearchHttpURL.fragment, zhihuSearchHttpURL.query); // {(null), q=%E5%A4%A7%E5%9C%A3%E5%BD%92%E6%9D%A5&type=question}  
// parameterString(RFC 1808)  
NSLog(@"zhihuSearchHttpURL.parameterString = %@", zhihuSearchHttpURL.parameterString); // (null)

在处理URL时，经常要解析提取query items。主要有两种途径析取：

（1）基于分隔符‘&’和‘=’对字符串进行分解提取。

// 解析查询字符串的key-value（parseComponentsFromQueryString）  
NSString* queryString = zhihuSearchHttpURL.query;  
NSMutableDictionary *queryDict = [NSMutableDictionary dictionary];  
if (queryString.length) {  
    NSArray *kvComponents = [queryString componentsSeparatedByString:@"&"];  
    for (NSString *kvComponent in kvComponents) {  
        NSString *key, *value;  
          
        if ([kvComponent hasContainStr:@"="]) {  
            NSArray* KV = [kvComponent componentsSeparatedByString:@"="];  
            key = KV.firstObject;  
            value = KV.lastObject;  
        } else {  
            key = kvComponent;  
            value = @"";  
        }  
          
        // 暂未考虑重名参数：k={v1,v2,...}  
        [queryDict setObject:value forKey:key];  
    }  
}  
// {q = "%E5%A4%A7%E5%9C%A3%E5%BD%92%E6%9D%A5"; type = question;}  
NSLog(@"parseComponentsFromQueryString = %@", queryDict);

（2）基于iOS7提供的新接口，基于URLString构造NSURLComponents，自动提取queryItems（NSURLQueryItem[]）。

// NS_CLASS_AVAILABLE(10_9, 7_0)提供了NSURLComponents；NS_CLASS_AVAILABLE(10_10, 8_0)进一步提供了NSURLQueryItem  
NSURLComponents* zhihuSearchHttpURLComponents =  [NSURLComponents componentsWithString:zhihuSearchHttpURLPath];  
NSLog(@"zhihuSearchHttpURLComponents.queryItems = %@", zhihuSearchHttpURLComponents.queryItems); // array of NSURLQueryItem

8.可变字符串（NSMutableString）

8.1 Initializing an String(NS_DESIGNATED_INITIALIZER)

/* In addition to these two, NSMutableString responds properly to allNSString creation methods.*/  
- (NSMutableString *)initWithCapacity:(NSUInteger)capacity;  
+ (NSMutableString *)stringWithCapacity:(NSUInteger)capacity;

8.2 set/append aString

// Replaces the characters of the receiver with those in a given string.aString must not be nil.  
- (void)setString:(NSString *)aString;

setString类似于对retain propery的赋值（setter）。

字符串置为空串：=@””或setString:@””;

// Adds to the end of the receiver the characters of a given string.aString must not be nil  
- (void)appendString:(NSString *)aString;  
- (void)appendFormat:(NSString *)format, ... NS_FORMAT_FUNCTION(1,2);

8.3 insert/replace

- (void)insertString:(NSString *)aString atIndex:(NSUInteger)loc;  
   
// 替换  
- (void)replaceCharactersInRange:(NSRange)range withString:(NSString *)aString;  
- (NSUInteger)replaceOccurrencesOfString:(NSString *)target withString:(NSString *)replacement options:(NSStringCompareOptions)options range:(NSRange)searchRange;

8.4 delete

// 删除指定位置、指定长度的子串  
- (void)deleteCharactersInRange:(NSRange)range;

9.文件、URL与字符串

9.1 从指定文件读取内容到字符串

- (instancetype)initWithContentsOfFile:(NSString *)path encoding:(NSStringEncoding)enc error:(NSError **)error;  
+ (instancetype)stringWithContentsOfFile:(NSString *)path encoding:(NSStringEncoding)enc error:(NSError **)error;

9.2 从指定url读取（下载）内容到字符串

- (instancetype)initWithContentsOfURL:(NSURL *)url encoding:(NSStringEncoding)enc error:(NSError **)error;  
+ (instancetype)stringWithContentsOfURL:(NSURL *)url encoding:(NSStringEncoding)enc error:(NSError **)error;

9.3 将字符串内容写到指定url/path

/* Write to specified url or path using the specified encoding.*/  
- (BOOL)writeToURL:(NSURL *)url atomically:(BOOL)useAuxiliaryFileencoding:(NSStringEncoding)encerror:(NSError **)error;  
- (BOOL)writeToFile:(NSString *)path atomically:(BOOL)useAuxiliaryFileencoding:(NSStringEncoding)encerror:(NSError **)error;

10.对象字符串化

10.1 NSString From CGPoint/CGSize/CGRect

NSStringFromCGPoint/CGPointFromString、NSStringFromCGSize/CGSizeFromString、 NSStringFromCGRect/CGRectFromString。

可用于对C复合类型（struct）进行OC描述（description），生成对应的字符串，方便NSLog调试输出。

10.2 NSString From Class

// 获取某个对象的具体类名，传入[obj class]，相当于[obj className]  
FOUNDATION_EXPORT NSString *NSStringFromClass(Class aClass);  
   
// 基于类名获取类  
FOUNDATION_EXPORT Class NSClassFromString(NSString *aClassName);

示例：

NSLog(@"[constString2 className] = %@", [constString2 className]); // __NSCFConstantString  
NSLog(@"NSStringFromClass([constString2 class]) = %@", NSStringFromClass([constString2class])); //__NSCFConstantString  
NSLog(@"object_getClassName(constString2) = %s", object_getClassName(constString2));// __NSCFConstantString  
// 返回C字符串格式的类名  
const char* className = class_getName([obj class]);

类似的还有NSStringFromSelector/NSSelectorFromString，可以基于方法名称来创建SEL进而实现调用。

10.3 Class & SEL from NSString

以下代码片段中，hideAlertView接口试图dismiss当前顶层UIAlertView/UIAlertController，在iOS 7.x系统中调用半公开的_UIAlertManager::topMostAlert获取顶层UIAlertView。

- (void)hideAlertView  
{  
    if (SYSTEM_VERSION < 7.0) { // iOS 7.0之前采用UIAlertView  
        for (UIWindow* w in [UIApplication sharedApplication].windows) {  
            for (NSObject* o in w.subviews) {  
                if ([o isKindOfClass:[UIAlertView class]]) {  
                    UIAlertView *alert = (UIAlertView*)o;  
                    [alert dismissWithClickedButtonIndex:alert.cancelButtonIndex animated:NO];  
                }  
            }  
        }  
    } else if (SYSTEM_VERSION < 8.0) { // iOS 8.0之前采用UIAlertView  
        Class UIAlertManager = NSClassFromString(@"_UIAlertManager");  
        SEL selTopMostAlert = NSSelectorFromString(@"topMostAlert");  
        if ([UIAlertManager respondsToSelector:selTopMostAlert]) {  
             UIAlertView *topMostAlert = [UIAlertManager performSelector:selTopMostAlert];  
             [topMostAlert dismissWithClickedButtonIndex:topMostAlert.cancelButtonIndex animated:NO];  
        }  
    } else { // iOS 8.0之后采用UIAlertController  
        UINavigationController *navCtrl = self.navigationController;  
        if(navCtrl.presentedViewController && [navCtrl.presentedViewController isKindOfClass:[UIAlertController class]]) {  
            [navCtrl.presentedViewController dismissViewControllerAnimated:NO completion:nil];  
        }  
    }  
}