版本记录
| 版本号 | 时间 |
|---|---|
| V1.0 | 2017.05.05 |
前言
前面我简单的写了些NSString的初始化，写了几篇，都不难，但是可以对新手有一定的小帮助，对于大神级人物可以略过这几篇，NSString本来就没有难的，都是细枝末节，忘记了查一下就会了，没有技术难点，下面我们继续~~~
1. NSString简单细说（一）—— NSString整体架构
2. NSString简单细说（二）—— NSString的初始化
3. NSString简单细说（三）—— NSString初始化
4. NSString简单细说（四）—— 从URL初始化
5. NSString简单细说（五）—— 向文件或者URL写入
详述
求字符串的长度
一、length
先看代码吧。
NSString *str1 = @"uidgiugeo1e2eyy";
NSLog(@"length--%ld",str1.length);
NSString *str2 = @"uid giug eo1e2eyy";//中间加载一起是4个空格
NSLog(@"length--%ld",str2.length);
NSString *str3 = @"uid 我的eo1e2eyy";//中间是一个空格
NSLog(@"length--%ld",str3.length);
NSString *str4 = @"我的我的";
NSLog(@"length--%ld",str4.length);
```
然后看输出结果。
```
2017-05-06 18:21:28.747 NSString你会用吗？[6591:221295] length--15
2017-05-06 18:21:28.748 NSString你会用吗？[6591:221295] length--19
2017-05-06 18:21:28.748 NSString你会用吗？[6591:221295] length--14
2017-05-06 18:21:28.748 NSString你会用吗？[6591:221295] length--4
```
**结论**：求长度时我特意加了汉字和空格字符，可以看见他们和英文字符在计算长度时是一样看待的。
----------------
### 二、- (NSUInteger)lengthOfBytesUsingEncoding:(NSStringEncoding)enc;
我们先看代码。
```
/**
*2.- (NSUInteger)lengthOfBytesUsingEncoding:(NSStringEncoding)enc;
*
* @param enc:The encoding for which to determine the receiver's length.
*
* @return :The number of bytes required to store the receiver in the encoding enc in a non-external representation. The length does not include space for a terminating NULL character. Returns 0 if the specified encoding cannot be used to convert the receiver or if the amount of memory required for storing the results of the encoding conversion would exceed NSIntegerMax.
*/
NSString *str1 = @"AABBCC";
NSInteger utf8Legth = [str1 lengthOfBytesUsingEncoding:NSUTF8StringEncoding];
NSInteger utf16Length = [str1 lengthOfBytesUsingEncoding:NSUTF16StringEncoding];
NSLog(@"utf8Legth-%ld---utf16Length--%ld",utf8Legth,utf16Length);
NSString *str2 = @"A A B B C C";
NSInteger utf8Legth2 = [str2 lengthOfBytesUsingEncoding:NSUTF8StringEncoding];
NSInteger utf16Length2 = [str2 lengthOfBytesUsingEncoding:NSUTF16StringEncoding];
NSLog(@"utf8Legth2-%ld---utf16Length2--%ld",utf8Legth2,utf16Length2);
```
看输出结果。
```
2017-05-06 18:31:20.727 NSString你会用吗？[6726:228362] utf8Legth-6---utf16Length--12
2017-05-06 18:31:20.728 NSString你会用吗？[6726:228362] utf8Legth2-11---utf16Length2--22
```
**结论**：这个求长度空格也算字符，不同的编码格式长度也是不一样的。当存储空间或者存储值大于NSIntegerMax的情况就会返回0。
------------
### 三、- (NSUInteger)maximumLengthOfBytesUsingEncoding:(NSStringEncoding)enc;
看代码。
```
/**
*3.- (NSUInteger)maximumLengthOfBytesUsingEncoding:(NSStringEncoding)enc;
*
* @param enc:The encoding for which to determine the receiver's length.
*
* @return :The maximum number of bytes needed to store the receiver in encoding in a non-external representation. The length does not include space for a terminating NULL character. Returns 0 if the amount of memory required for storing the results of the encoding conversion would exceed NSIntegerMax.
* @noti:The result is an estimate and is returned in O(1) time; the estimate may be considerably greater than the actual length needed.
*/
NSString *str1 = @"AABBCC";
NSInteger utf8Legth = [str1 maximumLengthOfBytesUsingEncoding:NSUTF8StringEncoding];
NSInteger utf16Length = [str1 maximumLengthOfBytesUsingEncoding:NSUTF16StringEncoding];
NSLog(@"utf8Legth-%ld---utf16Length--%ld",utf8Legth,utf16Length);
NSString *str2 = @"A A B B C C";
NSInteger utf8Legth2 = [str2 maximumLengthOfBytesUsingEncoding:NSUTF8StringEncoding];
NSInteger utf16Length2 = [str2 maximumLengthOfBytesUsingEncoding:NSUTF16StringEncoding];
NSLog(@"utf8Legth2-%ld---utf16Length2--%ld",utf8Legth2,utf16Length2);
```
看输出结果。
```
2017-05-06 18:42:28.014 NSString你会用吗？[6863:236467] utf8Legth-18---utf16Length--12
2017-05-06 18:42:28.014 NSString你会用吗？[6863:236467] utf8Legth2-33---utf16Length2--22
```
**结论**：这个一般不怎么用，输出的长度一般也是大于实际的长度。
## 求字符串的字节或者字符
### 一、- (unichar)characterAtIndex:(NSUInteger)index;
看代码。
```
/**
*1.- (unichar)characterAtIndex:(NSUInteger)index;
*
* @param index:The index of the character to retrieve.
Important:Raises an NSRangeException if index lies beyond the end of the receiver.
*
* @return :The character at the array position given by index.Returns the character at a given UTF-16 code unit index.
* @noti:You should always use the rangeOfComposedCharacterSequenceAtIndex: or rangeOfComposedCharacterSequencesForRange: method to determine character boundaries, so that any surrogate pairs or character clusters are handled correctly.
*/
NSString *str = @"bwgigwiw753674560FGGEQOWNVHJDYT";
NSInteger strLength = str.length;
NSLog(@"strLength--%ld",strLength);
unichar *chr = [str characterAtIndex:10];
NSLog(@"chr--%c",chr);
unichar *chr1 = [str characterAtIndex:32];
NSLog(@"chr1--%c",chr1);
```
看结果。
```
2017-05-06 19:05:21.830 NSString你会用吗？[7115:251324] strLength--31
2017-05-06 19:05:21.831 NSString你会用吗？[7115:251324] chr--3
2017-05-06 19:05:21.913 NSString你会用吗？[7115:251324] *** Terminating app due to uncaught exception 'NSRangeException', reason: '-[__NSCFConstantString characterAtIndex:]: Range or index out of bounds'
*** First throw call stack:
(
0 CoreFoundation 0x000000010ff18d4b __exceptionPreprocess + 171
1 libobjc.A.dylib 0x000000010f97a21e objc_exception_throw + 48
2 CoreFoundation 0x000000010ff822b5 +[NSException raise:format:] + 197
```
**结论**：这里要注意长度，超过字符串的长度会crash并throw exception。
---------------
### 二、- (void)getCharacters:(unichar *)buffer range:(NSRange)range;
看代码。
```
/**
*2.- (void)getCharacters:(unichar *)buffer range:(NSRange)range;
*
* @desc :Copies characters from a given range in the receiver into a given buffer.
*
* @param buffer:Upon return, contains the characters from the receiver. buffer must be large enough to contain the characters in the range aRange (aRange.length*sizeof(unichar)).
* @param range: The range of characters to retrieve. The range must not exceed the bounds of the receiver.Important:Raises an NSRangeException if any part of aRange lies beyond the bounds of the receiver.
*/
NSString *str = @"bwgigwiw753674560FGGEQOWNVHJDYT";
NSInteger strLength = str.length;
NSRange range = NSMakeRange(0, 7);
unichar buffer[10];
[str getCharacters: buffer range:range];
for (NSInteger i = 0; i < 10; i++) {
NSLog(@"%c",buffer[i]);
}
```
看输出。
```
2017-05-06 19:37:47.824 NSString你会用吗？[7600:273930] b
2017-05-06 19:37:47.825 NSString你会用吗？[7600:273930] w
2017-05-06 19:37:47.825 NSString你会用吗？[7600:273930] g
2017-05-06 19:37:47.825 NSString你会用吗？[7600:273930] i
2017-05-06 19:37:47.826 NSString你会用吗？[7600:273930] g
2017-05-06 19:37:47.826 NSString你会用吗？[7600:273930] w
2017-05-06 19:37:47.826 NSString你会用吗？[7600:273930] i
2017-05-06 19:37:47.826 NSString你会用吗？[7600:273930] �
```
**结论**：这里注意range的length不能超过字符串的长度，要拷贝的地址buffer的长度也不能小于range的长度，必须确保buffer足够大。
---------------
### 三、- (BOOL)getBytes:(void ***)buffer maxLength:(NSUInteger)maxBufferCount usedLength:(NSUInteger )usedBufferCount encoding:(NSStringEncoding)encoding options:(NSStringEncodingConversionOptions)options range:(NSRange)range remainingRange:(NSRangePointer)leftover;
这个方法的参数有点多，我们先看一下。
这里有一个options枚举类型，如下：
```
typedef NS_OPTIONS(NSUInteger, NSStringEncodingConversionOptions) {
//允许文件丢失
NSStringEncodingConversionAllowLossy = 1,
//不允许文件丢失
NSStringEncodingConversionExternalRepresentation = 2
};
```
接着看代码。
```
/**
*3.- (BOOL)getBytes:(void *)buffer maxLength:(NSUInteger)maxBufferCount usedLength:(NSUInteger *)usedBufferCount encoding:(NSStringEncoding)encoding options:(NSStringEncodingConversionOptions)options range:(NSRange)range remainingRange:(NSRangePointer)leftover;
*
* @param buffer:A buffer into which to store the bytes from the receiver. The returned bytes are not NULL-terminated.
* @param maxBufferCount: The maximum number of bytes to write to buffer.
* @param usedBufferCount:The number of bytes used from buffer. Pass NULL if you do not need this value.
* @param encoding: The encoding to use for the returned bytes. For possible values, see NSStringEncoding.
* @param options: A mask to specify options to use for converting the receiver’s contents to encoding (if conversion is necessary).
* @param range:The range of characters in the receiver to get. (aRange.length*sizeof(unichar)).
* @param leftover: The remaining range. Pass NULL If you do not need this value.
* @return : YES if some characters were converted, otherwise NO.
*/
NSString *str = @"A";
unichar buffer[10];
NSUInteger maxBufferCount = 9;
NSRange range = NSMakeRange(0, 1);
BOOL isSuccess = [str getBytes:buffer maxLength:maxBufferCount usedLength:NULL encoding:NSUTF8StringEncoding options:NSStringEncodingConversionExternalRepresentation range:range remainingRange:NULL];
NSLog(@"isSuccess--%d",isSuccess);
for (NSInteger i = 0; i < 10; i++) {
NSLog(@"%c",buffer[i]);
}
```
看输出结果。
```
2017-05-06 22:25:26.491 NSString你会用吗？[8138:301537] isSuccess--1
2017-05-06 22:25:26.492 NSString你会用吗？[8138:301537] A
2017-05-06 22:25:26.492 NSString你会用吗？[8138:301537] �
2017-05-06 22:25:26.493 NSString你会用吗？[8138:301537] �
```
**结论**：这个方法的参数比较长，我工作这么久基本没用过这个方法，今天也是看文档，才看见这个方法，大家看看就可以了。
# 后记
> 今天是周六先写这么多吧，我要休息一下了，哈哈，未完，待续，欢迎留言和批评指正~~~
