简单实例NSFileHandle的文件读写

//
//  main.m

/* 演示文件句柄的使用:读文件和写文件操作 */

#import <Foundation/Foundation.h>

#define PATH @"/Users/apple/Documents/ios_dev/test_case/TestFileHandle"

int main(int argc, const char * argv[]) {
@autoreleasepool {

/* 新建文件和初始化文本内容 */
NSString *path = [NSString stringWithFormat:@"%@/hello.txt", PATH];
NSData *content = [@"123456789" dataUsingEncoding:NSUTF8StringEncoding];
NSFileManager *fm = [NSFileManager defaultManager];
BOOL ret = [fm createFileAtPath:path contents:content attributes:nil];
if (ret == 0)
{
NSLog(@"Create file fail.");
return 0;
}

/*
The NSFileHandle class is an object-oriented wrapper for a file descriptor. You use file handle
objects to access data associated with files, sockets, pipes, and devices. For files, you can
read, write, and seek within the file.

fileHandleForReadingAtPath:Returns a file handle initialized for reading the file, device,
or named socket at the specified path.
*/
NSString *str1 = [NSString stringWithFormat:@"%@/hello.txt",PATH];
NSFileHandle *rfh = [NSFileHandle fileHandleForReadingAtPath:str1];

/* readDataOfLength:Synchronously reads data up to the specified number of bytes. */
NSData *da1 = [rfh readDataOfLength:2];  /* 句柄位置变动 */

/* readDataToEndOfFile:Synchronously reads the available data up to the end of file or maximum
number of bytes. */
NSData *da2 = [rfh readDataToEndOfFile]; /* 从上次句柄处开始读取 */

NSString *str2 = [[NSString alloc] initWithData:da1 encoding:NSUTF8StringEncoding];
NSLog(@"first read contents:%@", str2);
NSString *str3 = [[NSString alloc] initWithData:da2 encoding:NSUTF8StringEncoding];
NSLog(@"seconde read contents:%@", str3);

/* fileHandleForWritingAtPath:Returns a file handle initialized for writing to the file,
device, or named socket at the specified path. */
NSFileHandle *wfh = [NSFileHandle fileHandleForWritingAtPath:path];
NSData *da3 = [@"BEST&" dataUsingEncoding:NSUTF8StringEncoding];

/* writeData:If the receiver is a file, writing takes place at the file pointer’s current position.
After it writes the data, the method advances the file pointer by the number of bytes written.*/
[wfh writeData:da3];

/* seekToEndOfFile:Moves the file pointer to the specified offset within the file represented
by the receiver. */
[rfh seekToFileOffset:0];

NSData *da4 = [rfh readDataToEndOfFile];
NSString *str4 = [[NSString alloc] initWithData:da4 encoding:NSUTF8StringEncoding];
NSLog(@"read contents after first write:%@", str4);

/* 注意此时wfh的句柄不在文件首位置，而在执行了写入da3后的那个位置，因此不是从文件首或文件末尾写入新的字符串 */
NSData *da5 = [@"##" dataUsingEncoding:NSUTF8StringEncoding];
[wfh writeData:da5];

[rfh seekToFileOffset:0];
da4 = [rfh readDataToEndOfFile];
str4 = [[NSString alloc] initWithData:da4 encoding:NSUTF8StringEncoding];
NSLog(@"read contents after second write:%@", str4);

/* 从文件末尾处新增内容 */
[wfh seekToEndOfFile];
da5 = [@"hero" dataUsingEncoding:NSUTF8StringEncoding];
[wfh writeData:da5];

[rfh seekToFileOffset:0];
da4 = [rfh readDataToEndOfFile];
str4 = [[NSString alloc] initWithData:da4 encoding:NSUTF8StringEncoding];
NSLog(@"read contents after third write:%@", str4);
}
return 0;
}

2015-12-12 18:41:46.570 TestFileHandle[836:39034] first read contents:12
2015-12-12 18:41:46.571 TestFileHandle[836:39034] seconde read contents:3456789
2015-12-12 18:41:46.571 TestFileHandle[836:39034] read contents after first write:BEST&6789
2015-12-12 18:41:46.572 TestFileHandle[836:39034] read contents after second write:BEST&##89
2015-12-12 18:41:46.572 TestFileHandle[836:39034] read contents after third write:BEST&##89hero