Detecting when a User takes a Screenshot (Implementation File)

- (void)checkForScreenshotsAtPath:(NSString *)dirpath {
    NSDictionary *files;
    NSArray *paths;

    // find new screenshots
    if (!(files = [self findUnprocessedScreenshotsOnDesktop]))
        return;

    // sort on key (path)
    paths = [files keysSortedByValueUsingComparator:^(id a, id b) { return [b compare:a]; }];

    // process each file
    for (NSString *path in paths) {
        NSLog(@"Proccessing file %@", path);
    }
}

- (NSDictionary *)findUnprocessedScreenshotsOnDesktop {
    NSDictionary *currentFiles;
    NSMutableDictionary *files;
    NSMutableSet *newFilenames;

    currentFiles = [self screenshotsOnDesktop];
    files = nil;

    if ([currentFiles count]) {
        newFilenames = [NSMutableSet setWithArray:[currentFiles allKeys]];
        // filter: remove allready processed screenshots
        [newFilenames minusSet:[NSSet setWithArray:[knownScreenshotsOnDesktop allKeys]]];
        if ([newFilenames count]) {
            files = [NSMutableDictionary dictionaryWithCapacity:1];
            for (NSString *path in newFilenames) {
                [files setObject:[currentFiles objectForKey:path] forKey:path];
            }
        }
    }

    knownScreenshotsOnDesktop = currentFiles;
    return files;
}

- (NSDictionary *)screenshotsOnDesktop {
    NSDate *lmod = [NSDate dateWithTimeIntervalSinceNow:-5]; // max 5 sec old
    return [self screenshotsAtPath:screenshotLocation modifiedAfterDate:lmod];
}

- (NSDictionary *)screenshotsAtPath:(NSString *)dirpath modifiedAfterDate:(NSDate *)lmod {
    NSFileManager *fm = [NSFileManager defaultManager];
    NSArray *direntries;
    NSMutableDictionary *files = [NSMutableDictionary dictionary];
    NSString *path;
    NSDate *mod;
    NSError *error;
    NSDictionary *attrs;

    dirpath = [dirpath stringByExpandingTildeInPath];

    direntries = [fm contentsOfDirectoryAtPath:dirpath error:&error];
    if (!direntries) {
        return nil;
    }

    for (NSString *fn in direntries) {

        //[log debug:@"%s testing %@", _cmd, fn];

        // always skip dotfiles
        if ([fn hasPrefix:@"."]) {
            //[log debug:@"%s skipping: filename begins with a dot", _cmd];
            continue;
        }

        // skip any file not ending in screenshotFilenameSuffix (".png" by default)
        if (([fn length] < 10) ||
            // ".png" suffix is expected
            (![fn compare:screenshotFilenameSuffix options:NSCaseInsensitiveSearch range:NSMakeRange([fn length]-5, 4)] != NSOrderedSame)
            )
        {
            continue;
        }

        // build path
        path = [dirpath stringByAppendingPathComponent:fn];

        // Skip any file which name does not contain a space.
        // You want to avoid matching the filename against
        // all possible screenshot file name schemas (must be hundreds), we make the
        // assumption that all language formats have this in common: it contains at least one space.
        if ([fn rangeOfString:@" "].location == NSNotFound) {
            continue;
        }

        // query file attributes (rich stat)
        attrs = [fm attributesOfItemAtPath:path error:&error];
        if (!attrs) {
            continue;
        }

        // must be a regular file
        if ([attrs objectForKey:NSFileType] != NSFileTypeRegular) {
            continue;
        }

        // check last modified date
        mod = [attrs objectForKey:NSFileModificationDate];
        if (lmod && (!mod || [mod compare:lmod] == NSOrderedAscending)) {
            // file is too old
            continue;
        }

        // find key for NSFileExtendedAttributes
        NSString *xattrsKey = nil;
        for (NSString *k in [attrs keyEnumerator]) {
            if ([k isEqualToString:@"NSFileExtendedAttributes"]) {
                xattrsKey = k;
                break;
            }
        }
        if (!xattrsKey) {
            // no xattrs
            continue;
        }
        NSDictionary *xattrs = [attrs objectForKey:xattrsKey];
        if (!xattrs || ![xattrs objectForKey:@"com.apple.metadata:kMDItemIsScreenCapture"]) {
            continue;
        }

        // ok, let's use this file
        [files setObject:mod forKey:path];
    }

    return files;
}