前面我简单的写了写NSString的初始化，写了三篇，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，忘记了查一下就回了，没有技术难点。
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

下面接着说NSString的初始化，这里说的是从URL进行初始化。

NSString从URL初始化

一、+ (instancetype)stringWithContentsOfURL:(NSURL ***)url encoding:(NSStringEncoding)enc error:(NSError * _Nullable *)error;

*1. + (instancetype)stringWithContentsOfURL:(NSURL *)url encoding:(NSStringEncoding)enc error:(NSError * _Nullable *)error; * @param url :The URL to read. * @param enc :he encoding of the data at url. * @param error :If an error occurs, upon returns contains an NSError object that describes the problem. If you are not interested in possible errors, pass in NULL. * @return :A string created by reading data from the file named by path using the encoding, enc. If the file can’t be opened or there is an encoding error, returns nil.. NSString *filePath = [[NSBundle mainBundle] pathForResource:@"test.txt" ofType:nil]; NSURL *URL = [NSURL fileURLWithPath:filePath]; NSError *error; NSString *str = [NSString stringWithContentsOfURL:URL encoding:NSUTF8StringEncoding error:&error]; NSLog(@"str---%@---%p---%@",str,str,error);

2017-04-21 00:21:35.966 NSString你会用吗？[1351:29759] str---AASsswj1wjihoi`w`9897298`7298`2@&#&%#*(&)!&)!---0x60800004bc70---(null)
结论：简单不多说了。
二、- (instancetype)initWithContentsOfURL:(NSURL ***)url encoding:(NSStringEncoding)enc error:(NSError * _Nullable *)error;
//    *2.- (instancetype)initWithContentsOfURL:(NSURL *)url encoding:(NSStringEncoding)enc error:(NSError * _Nullable *)error;
//    *
//    *  @param url :The URL to read.
//    *  @param enc :he encoding of the data at url.
//    *  @param error :If an error occurs, upon returns contains an NSError object that describes the problem. If you are not interested in possible errors, pass in NULL.
//    *
//    *  @return :A string created by reading data from the file named by path using the encoding, enc. If the file can’t be opened or there is an encoding error, returns nil..
//    */
    NSString *filePath = [[NSBundle mainBundle] pathForResource:@"test.txt" ofType:nil];
    NSURL *URL = [NSURL fileURLWithPath:filePath];
    NSError *error;
    NSString *str = [[NSString alloc] initWithContentsOfURL:URL encoding:NSUTF8StringEncoding error:&error];
    NSLog(@"str---%@---%p---%@",str,str,error);
2017-04-21 00:37:50.460 NSString你会用吗？[1535:36973] str---AASsswj1wjihoi`w`9897298`7298`2@&#&%#*(&)!&)!---0x600000054670---(null)
结果：这里很简单就不多说了。
三、+ (instancetype)stringWithContentsOfURL:(NSURL ****)url usedEncoding:(NSStringEncoding ***)enc error:(NSError * _Nullable *)error;
    //    *3.+ (instancetype)stringWithContentsOfURL:(NSURL *)url usedEncoding:(NSStringEncoding *)enc error:(NSError * _Nullable *)error;
    //    *
    //    *  @param url :The URL to read.
    //    *  @param enc :Upon return, if url is read successfully, contains the encoding used to interpret the data
    //    *  @param error :If an error occurs, upon returns contains an NSError object that describes the problem. If you are not interested in possible errors, pass in NULL.
    //    *
    //    *  @return :A string created by reading data from url. If the URL can’t be opened or there is an encoding error, returns nil.
    //    */
    NSString *filePath = [[NSBundle mainBundle] pathForResource:@"test.txt" ofType:nil];
    NSURL *URL = [NSURL fileURLWithPath:filePath];
    NSStringEncoding encode;
    NSError *error;
    NSString *str = [NSString stringWithContentsOfURL:URL usedEncoding:&encode error:&error];
    NSLog(@"str---%@---%p---%@",str,str,error);
    if (!error) {
        NSLog(@"encoding---%ld",encode);
2017-04-21 00:53:09.080 NSString你会用吗？[1869:49496] str---AASsswj1wjihoi`w`9897298`7298`2@&#&%#*(&)!&)!---0x600000056c20---(null)
2017-04-21 00:53:09.081 NSString你会用吗？[1869:49496] encoding---4
结论：这里和我第三篇写的类似了，这里就是自动编码方式读取字符串，如果没有错误就会返回所用的编码方式。4对应的就是UTF-8编码。
四、- (instancetype)initWithContentsOfURL:(NSURL ****)url usedEncoding:(NSStringEncoding ***)enc error:(NSError * _Nullable *)error;
    //    *4.- (instancetype)initWithContentsOfURL:(NSURL *)url usedEncoding:(NSStringEncoding *)enc error:(NSError * _Nullable *)error;
    //    *
    //    *  @param url :The URL to read.
    //    *  @param enc :Upon return, if url is read successfully, contains the encoding used to interpret the data
    //    *  @param error :If an error occurs, upon returns contains an NSError object that describes the problem. If you are not interested in possible errors, pass in NULL.
    //    *
    //    *  @return :A string created by reading data from url. If the URL can’t be opened or there is an encoding error, returns nil.
    //    */
    NSString *filePath = [[NSBundle mainBundle] pathForResource:@"test.txt" ofType:nil];
    NSURL *URL = [NSURL fileURLWithPath:filePath];
    NSStringEncoding encode;
    NSError *error;
    NSString *str = [[NSString alloc] initWithContentsOfURL:URL usedEncoding:&encode error:&error]; ;
    NSLog(@"str---%@---%p---%@",str,str,error);
    if (!error) {
        NSLog(@"encoding---%ld",encode);
2017-04-21 00:51:24.220 NSString你会用吗？[1835:48052] str---AASsswj1wjihoi`w`9897298`7298`2@&#&%#*(&)!&)!---0x608000246ae0---(null)
2017-04-21 00:51:24.221 NSString你会用吗？[1835:48052] encoding---4