基礎(chǔ)知識(shí)

• 位 & 字節(jié)

  計(jì)算機(jī)存儲(chǔ)的是二進(jìn)制（比如101010）。每一個(gè)數(shù)字就是一位 (bit), 每 8 位是一個(gè)字節(jié) (Byte), 位是計(jì)算機(jī)傳輸?shù)淖钚挝唬止?jié)是編碼的最小單位。

• 字符

  每個(gè)文字都是一個(gè)字符，不同的編碼對(duì)應(yīng)同樣字符所需的字節(jié)數(shù)也是不同的。（英文占一個(gè)字節(jié)，中文占兩個(gè)字節(jié)）

• 編碼

  編碼就是字節(jié)到字符的規(guī)則。（比如 0001 代表 1，也可以用 01 代表 1 這個(gè)字符）

• 解碼

  有編碼就有解碼，數(shù)據(jù)在網(wǎng)絡(luò)中是二進(jìn)制形式傳輸，加密與解密配套時(shí)可以正確解碼二進(jìn)制流信息。

• 總結(jié)

  位和字節(jié)都是單位，字符是看到的結(jié)果，解碼編碼則是固定的規(guī)則

NSString & NSData & char

Definition

• NSData

  • 遵循 NSCopying NSCoding 協(xié)議, 它提供面向?qū)ο蟮臄?shù)組存儲(chǔ)為字節(jié)，即二進(jìn)制數(shù)據(jù)流類(lèi)型。
  • 讀寫(xiě)文件需要一個(gè)緩沖區(qū)，而 NSData 就提供緩存區(qū)。

Coding

• char <-> NSString

  逐 char 打印時(shí)，中文亂碼是因?yàn)闊o(wú)法一次打印兩個(gè), 后面 unicode 部分有解決方法。

    const char *chars = "this is a string, and contain 中文";
  
    // char to NSString
    NSString *string = [[NSString alloc] initWithCString:chars encoding:NSUTF8StringEncoding];
    NSLog(@"%lu, %@", string.length, string); // 32, this is a string, and contain 中文
  
    // NSString to char
    chars = [string cStringUsingEncoding:NSUTF8StringEncoding];
    NSLog(@"%lu, %s", strlen(chars), chars); // 36, this is a string, and contain ‰∏≠ê?á
  

• char -> NSData

    NSData *dataWithBytes = [NSData dataWithBytes:chars length:strlen(chars)];
  
    NSLog(@"%@", dataWithBytes);
    // <74686973 20697320 61204320 73747269 6e672c20 616e6420 636f6e74 61696e20 e4b8ade6 9687>
  

• NString <-> NSData

    NSString *string = @"this is a string, and contain 中文";
  
    // NSString to data
    NSData *data = [string dataUsingEncoding:NSUTF8StringEncoding];
    NSLog(@"%@", data); // <74686973 20697320 61207374 72696e67 2c20616e 6420636f 6e746169 6e20e4b8 ade69687>
  
    // NSData to NSString
    string = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
    NSLog(@"%@", string); // this is a string, and contain 中文
  

• NSString <-> ASCII

    // NSString to int ASCII
    NSString *charString = @"A";
    int asciiCode = [charString characterAtIndex:0];
    NSLog(@"%i", asciiCode); // 65
  
    // int to NSString
    asciiCode = 66;
    charString = [NSString stringWithFormat:@"%c", asciiCode];
    NSLog(@"%@", charString); // B
  

• char <-> ASCII

    // char to int ASCII
    char character = 'A';
    int asciiCode = (int)character;
    NSLog(@"%i", asciiCode); // 65
  
    // int to char
    asciiCode = 66;
    character = (char)asciiCode;
    NSLog(@"%c", character); // B
  

unicode

• %c

  8-bit unsigned character (unsigned char), printed by NSLog() as an ASCII character, or, if not an ASCII character, in the octal format \ddd or the Unicode hexadecimal format \udddd, where d is a digit.

• %C

  16-bit Unicode character (unichar), printed by NSLog() as an ASCII character, or, if not an ASCII character, in the octal format \ddd or the Unicode hexadecimal format \udddd, where d is a digit.

    NSString *theString = @"g";
    unichar theChar = [theString characterAtIndex:0];
    NSString *theString1 = [NSString stringWithFormat:@"%c", theChar];
    NSString *theString2 = [NSString stringWithFormat:@"%C", theChar];
    NSLog(@"%@, %d, %@, %@",theString, theChar, theString1, theString2);
    // g, 103, g, g
  
    theString = @"家";
    theChar = [theString characterAtIndex:0];
    theString1 = [NSString stringWithFormat:@"%c", theChar];
    theString2 = [NSString stringWithFormat:@"%C", theChar];
    NSLog(@"%@, %d, %@, %@",theString, theChar, theString1, theString2);
    // 家, 23478, ?, 家
  

參考

• 字節(jié) & 字符 & 二進(jìn)制
• How to convert ASCII value to a character in Objective-C?
• 中文字符 ASCII 碼和 NSString 相互轉(zhuǎn)換