Creare ed usare files persistenti in un app iPhone

In un applicativo iPhone può essere utile creare dei file, ad esempio di testo, ma non solo e magari poterli inviare via posta. Per questo scopo ci vengono in aiuto due classi di Cocoa (sia desktop che mobile) NSFileManager ed NSFileHandle.

Per usare filemanager basta scrivere:

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NSFileManager *filemgr;
filemgr = [NSFileManager defaultManager];

Da quì le possibilità sono varie:

Verificare l’esistenza di un file

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NSFileManager *filemgr;
filemgr = [NSFileManager defaultManager];
if ([filemgr fileExistsAtPath: @"/tmp/myfile.txt" ] == YES)
 NSLog (@"File exists");
else
 NSLog (@"File not found");

Comparare il contenuto di due file

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NSFileManager *filemgr;
filemgr = [NSFileManager defaultManager];
if ([filemgr contentsEqualAtPath: @"/tmp/myfile.txt" andPath: @"/tmp/sales.txt"] == YES)
 NSLog (@"File contents match");
else
 NSLog (@"File contents do not match");

Verificare i permessi di scrittura e lettura di un file

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NSFileManager *filemgr;
filemgr = [NSFileManager defaultManager];
if ([filemgr isWritableFileAtPath: @"/tmp/myfile.txt"] == YES)
 NSLog (@"File is writable");
else
 NSLog (@"File is read only");

Muovere/rinominare un file

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NSFileManager *filemgr;
filemgr = [NSFileManager defaultManager];
if ([filemgr moveItemAtPath: @"/tmp/myfile.txt" toPath: @"/tmp/newfile.txt" error: NULL] == YES)
 NSLog (@"Move successful");
else
 NSLog (@"Move failed");

Creare una copia di un file

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if ([filemgr copyItemAtPath: @"/tmp/myfile.txt" toPath: @"/Users/demo/newfile.txt" error: NULL] == YES)
 NSLog (@"Copy successful");
else
 NSLog (@"Copy failed");

Eliminare un file

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NSFileManager *filemgr;
filemgr = [NSFileManager defaultManager];
if ([filemgr removeItemAtPath: @"/tmp/myfile.txt" error: NULL] == YES)
 NSLog (@"Remove successful");
else
 NSLog (@"Remove failed")

Trovare l’offset

NSFileHandle serve per gestire il contenuto dei file.

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file = [NSFileHandle fileHandleForUpdatingAtPath: @"/tmp/myfile.txt"];
if (file == nil)
 NSLog(@"Failed to open file");
NSLog (@"Offset = %llu", [file offsetInFile]);
[file seekToEndOfFile];
NSLog (@"Offset = %llu", [file offsetInFile]);
[file seekToFileOffset: 30];
NSLog (@"Offset = %llu", [file offsetInFile]);
[file closeFile];

Leggere i dati da un file

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NSData *databuffer;
file = [NSFileHandle fileHandleForReadingAtPath: @"/tmp/myfile.txt"];
if (file == nil)
 NSLog(@"Failed to open file");
[file seekToFileOffset: 10]; //10 byte alla volta
databuffer = [file readDataOfLength: 5];
[file closeFile];

Scrivede dati in un file

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#import <Foundation/Foundation.h>
int main (int argc, const char * argv[])
{
 NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
 NSFileHandle *file;
 NSMutableData *data;
 const char *bytestring = "black dog";
 data = [NSMutableData dataWithBytes:bytestring length:strlen(bytestring)];
 file = [NSFileHandle fileHandleForUpdatingAtPath: @"/tmp/quickfox.txt"];
 if (file == nil)
 NSLog(@"Failed to open file");
 [file seekToFileOffset: 10];
 [file writeData: data];
 [file closeFile];
 [pool drain];
 return 0;
}

Fonte: http://www.techotopia.com/index.php/Working_with_Files_in_Objective