NSFileManager says a file not writable Mac - macos

Question

I have been trying to use AVFoundation to record screen outputs. For reasons unknown it stopped working after I moved to the latest version of Mac (Mountain Lion). I have been trying to getting it work but is not fruitful so far. I know that the AVFoundation method startRecordingToOutputFileURL will not work if the output file already exists. So, I tried using NSFileManager to see if my destination file exists and if it is writable. My Filemanager always returns the values corresponding to non-existence of the destination file and not writable. I tried to set file permissions to no avail, could anyone throw some light on my possible mistake:
dest = [[NSURL alloc] initFileURLWithPath:#"~/Desktop/myMovie.mov"];
NSFileManager *fileManager = [NSFileManager defaultManager];
NSMutableDictionary *attributes = [[NSMutableDictionary alloc] init];
[attributes setObject:[NSNumber numberWithInt:777] forKey:NSFilePosixPermissions]; //I tried 511 too, no avail
[fileManager setAttributes:attributes ofItemAtPath:[dest path] error:nil];
if (![fileManager fileExistsAtPath:[dest path]]) {
if ([fileManager isWritableFileAtPath:[dest path]]) {
/* Starts recording to a given URL. */
[captureMovieFileOutput startRecordingToOutputFileURL:dest recordingDelegate:self];
}
else{
NSLog(#"File doesnot exist but is not writable"); //This is the message I get as result
}
}
else
{
NSLog(#"File Exists...");
}

Answer 1

Unexpanded tildes are not valid paths in Cocoa. You must use -stringByExpandingTildeInPath or better, -stringByStandardizingPath on the string passed into NSURL's -initFileURLWithPath:.
Because of this, NSFileManager will return NO for isWritableFileAtPath because it's an invalid path (so it's not writable). This leads you to your NSLog() being fired.
Update based on comments:
You may still find NSURL is returning nil upon creation (so calling -path will return nil) because the path is still invalid. Also worth noting, the documentation says for -isWritableFileAtPath:, "It's far better to attempt an operation (such as loading a file or creating a directory), check for errors, and handle those errors gracefully than it is to try to figure out ahead of time whether the operation will succeed."
Take Peter Hosey's suggestion and make use of the NSError if the call fails as you attempt to write to the file and don't try to figure it out ahead of time.