UIImagePNGRepresentation的问题

 [UIImagePNGRepresentation(image, 1.0) writeToFile:pngPath atomically:YES];

stringByAppendingPathComponent

myPlistPath = [documentsDirectory stringByAppendingPathComponent: 
 [NSString stringWithFormat: @"%@.plist", plistName] ];
[myPlistPath retain]

NSString *file = @"myfile.png";
NSString *directory = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,NSUserDomainMask,YES) objectAtIndex:0];
NSString *path = [directory stringByAppendingPathComponent: file];

[UIImagePNGRepresentation(image) writeToFile:path atomically:YES];