版本記錄

前言

前面我寫了簡單細(xì)說(一)和簡單細(xì)說(二)，一中主要是介紹了NSString的主要API架構(gòu)，二中主要是對(duì)21種初始化方法進(jìn)行了介紹了，都是入門級(jí)的，相信大家都可以看得懂，好幾天沒寫這方面的東西了，今天有點(diǎn)時(shí)間，所以我就接著寫了。感興趣的可以看我上面幾篇。
1. NSString簡單細(xì)說（一）—— NSString整體架構(gòu)
2. NSString簡單細(xì)說（二）—— NSString的初始化

NSString初始化

前面兩篇也講到了NSString的初始化，這里接著講初始化，但是這里的初始化是從文件中加載。

一、+ (instancetype)stringWithContentsOfFile:(NSString *)path encoding:(NSStringEncoding)enc error:(NSError * _Nullable *)error;

我們直接看代碼

    /**
     *1. + (instancetype)stringWithContentsOfFile:(NSString *)path encoding:(NSStringEncoding)enc error:(NSError * _Nullable *)error;
     *
     *  @param path :A path to a file..
     *  @param enc :The encoding of the file at path.
     *  @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];
    NSError *error;
    NSString *str = [NSString stringWithContentsOfFile:filePath encoding:NSUTF8StringEncoding error:&error];
    NSLog(@"str---%@---%p---%@",str,str,error);

我們運(yùn)行下看結(jié)果。

2017-04-19 00:38:22.309 NSString你會(huì)用嗎？[2166:58609] str---AASsswj1wjihoi`w`9897298`7298`2@&#&%#*(&)!&)!---0x600000243420---(null)

結(jié)論：簡單不說。

二、- (instancetype)initWithContentsOfFile:(NSString *)path encoding:(NSStringEncoding)enc error:(NSError * _Nullable *)error;

看代碼

    /**
     *2. - (instancetype)initWithContentsOfFile:(NSString *)path encoding:(NSStringEncoding)enc error:(NSError * _Nullable *)error;;
     *
     *  @param path :A path to a file..
     *  @param enc :The encoding of the file at path.
     *  @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 :An NSString object initialized by reading data from the file named by path using the encoding, enc. The returned object may be different from the original receiver. If the file can’t be opened or there is an encoding error, returns nil.
     */
    
    NSString *filePath = [[NSBundle mainBundle] pathForResource:@"test.txt" ofType:nil];
    NSError *error;
    NSString *str = [[NSString alloc] initWithContentsOfFile:filePath encoding:NSUTF8StringEncoding error:&error];
    NSLog(@"str---%@---%p---%@",str,str,error);

看結(jié)果

2017-04-19 00:50:20.511 NSString你會(huì)用嗎？[2355:67660] str---AASsswj1wjihoi`w`9897298`7298`2@&#&%#*(&)!&)!---0x6000000520c0---(null)

結(jié)論：簡單不說。

三、+ (instancetype)stringWithContentsOfFile:(NSString *)path usedEncoding:(NSStringEncoding *)enc error:(NSError * _Nullable *)error;

看代碼

    /**
     *3. + (instancetype)stringWithContentsOfFile:(NSString *)path usedEncoding:(NSStringEncoding *)enc error:(NSError * _Nullable *)error;
     *
     *  @param path :A path to a file..
     *  @param enc :The encoding of the file at path.
     *  @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. If the file can’t be opened or there is an encoding error, returns nil.
     */
    
    //該方法就是自動(dòng)判斷encode，如果打開成功，就把encode放在enc里面，返回給調(diào)用者，可以先聲明一個(gè)NSStringEncoding類型，其實(shí)就是NSUInteger,然后送指針給方法。
    NSString *filePath = [[NSBundle mainBundle] pathForResource:@"test.txt" ofType:nil];
    NSError *error;
    NSStringEncoding encoding;
    NSString *str = [NSString stringWithContentsOfFile:filePath usedEncoding:&encoding error:&error];
    NSLog(@"str---%@---%p---%@",str,str,error);
    if (!error) {
        NSLog(@"encoding---%ld",encoding);
    }

看結(jié)果

2017-04-20 23:50:47.771 NSString你會(huì)用嗎？[836:14392] str---AASsswj1wjihoi`w`9897298`7298`2@&#&%#*(&)!&)!---0x600000053ce0---(null)
2017-04-20 23:50:53.045 NSString你會(huì)用嗎？[836:14392] encoding---4

結(jié)論：看見返回的是4，4對(duì)應(yīng)的編碼類型就是NSUTF8StringEncoding。
大家可能疑惑為什么返回的是0，其實(shí)這是個(gè)枚舉值，讓我們好好看看編碼方式的大家庭。

typedef NSUInteger NSStringEncoding;
NS_ENUM(NSStringEncoding) {
    NSASCIIStringEncoding = 1,      /* 0..127 only */
    NSNEXTSTEPStringEncoding = 2,
    NSJapaneseEUCStringEncoding = 3,
    NSUTF8StringEncoding = 4,
    NSISOLatin1StringEncoding = 5,
    NSSymbolStringEncoding = 6,
    NSNonLossyASCIIStringEncoding = 7,
    NSShiftJISStringEncoding = 8,          /* kCFStringEncodingDOSJapanese */
    NSISOLatin2StringEncoding = 9,
    NSUnicodeStringEncoding = 10,
    NSWindowsCP1251StringEncoding = 11,    /* Cyrillic; same as AdobeStandardCyrillic */
    NSWindowsCP1252StringEncoding = 12,    /* WinLatin1 */
    NSWindowsCP1253StringEncoding = 13,    /* Greek */
    NSWindowsCP1254StringEncoding = 14,    /* Turkish */
    NSWindowsCP1250StringEncoding = 15,    /* WinLatin2 */
    NSISO2022JPStringEncoding = 21,        /* ISO 2022 Japanese encoding for e-mail */
    NSMacOSRomanStringEncoding = 30,

    NSUTF16StringEncoding = NSUnicodeStringEncoding,      /* An alias for NSUnicodeStringEncoding */

    NSUTF16BigEndianStringEncoding = 0x90000100,          /* NSUTF16StringEncoding encoding with explicit endianness specified */
    NSUTF16LittleEndianStringEncoding = 0x94000100,       /* NSUTF16StringEncoding encoding with explicit endianness specified */

    NSUTF32StringEncoding = 0x8c000100,                   
    NSUTF32BigEndianStringEncoding = 0x98000100,          /* NSUTF32StringEncoding encoding with explicit endianness specified */
    NSUTF32LittleEndianStringEncoding = 0x9c000100        /* NSUTF32StringEncoding encoding with explicit endianness specified */
};

四、- (instancetype)initWithContentsOfFile:(NSString *)path usedEncoding:(NSStringEncoding *)enc error:(NSError * _Nullable *)error;

看代碼

    /**
     *4. - (instancetype)initWithContentsOfFile:(NSString *)path usedEncoding:(NSStringEncoding *)enc error:(NSError * _Nullable *)error;
     *
     *  @param path :A path to a file..
     *  @param enc :The encoding of the file at path.
     *  @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 An NSString object initialized by reading data from the file named by path. The returned object may be different from the original receiver. If the file can’t be opened or there is an encoding error, returns nil..
     */
    
    NSString *filePath = [[NSBundle mainBundle] pathForResource:@"test.txt" ofType:nil];
    NSError *error;
    NSStringEncoding encoding;
    NSString *str = [[NSString alloc] initWithContentsOfFile:filePath usedEncoding:&encoding error:&error];
    NSLog(@"str---%@---%p---%@",str,str,error);
    if (!error) {
        NSLog(@"encoding---%ld",encoding);
    }

看結(jié)果

2017-04-20 23:54:06.808 NSString你會(huì)用嗎？[907:17025] str---AASsswj1wjihoi`w`9897298`7298`2@&#&%#*(&)!&)!---0x60000004d350---(null)
2017-04-20 23:54:06.809 NSString你會(huì)用嗎？[907:17025] encoding---4

結(jié)論：和三中說的是一種原理，就不多說了。

后記

我們上面介紹了四種從文件加載字符串的方法，其實(shí)過程都差不多，就是打開文件，并根據(jù)不同的編碼方式，初始化字符串，這些并不難，但是我還是總結(jié)出來了，可以給初學(xué)者看看。謝謝大家對(duì)我的支持。