将 nsdata 写入文件的特定位置

Question

我有以下代码用于在文件中写入数据：

NSData *chunk=...; //some data
 NSArray *docDirectories = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *docDirectory = [docDirectories objectAtIndex:0];
    NSString *fileName   = [docDirectory stringByAppendingPathComponent:@"TestFile.txt"];
 [chunk writeToFile:fileName atomically:NO];

如果我知道文件的大小（比如说 10*chunk），并且如果我还收到每个块在文件总长度中的位置，我如何在该特定位置将写入数据添加到文件?

Answer 1

要解决您的问题，最好的办法是使用NSOutputStream，它使此类操作更容易处理。

话虽如此，您可以像这样附加到文件的末尾：

NSOutputStream *stream = [[NSOutputStream alloc] initToFileAtPath:filePath append:YES];
[stream open];
NSData *chunk = ...; // some data
[stream write:(uint8_t *)[chunk bytes] maxLength:[chunk length]];
[stream close];
// remember to always handle memory (if not using ARC) //

在文件中间插入一大块数据有点复杂：

NSData *chunk = ...; // some data
NSString *filePath = ... ; // get the file //
NSUInteger insertionPoint = ...; // get the insertion point //
// make sure the file exists, if it does, do the following //
NSData *oldData = [NSData dataWithContentsOfFile:filePath];
// error checking would be nice... if (oldData) ... blah //
NSOutputStream *stream = [[NSOutputStream alloc] initToFileAtPath:filePath append:NO];
[stream open];
[stream write:(uint8_t *)[oldData bytes] maxLength:insertionPoint]; // write the old data up to the insertion point //
[stream write:(uint8_t *)[chunk bytes] maxLength:[chunk length]]; // write the new data //
[stream write:(uint8_t *)&[oldData bytes][insertionPoint] maxLength:[oldData length] - insertionPoint]; // write the rest of old data at the end of the file //
[stream close];
// remember to always handle memory (if not using ARC) //

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