Detect when a User takes a Screenshot

Question

I am looking for way for my app to receive a notification when the user takes a screenshot either with Command-Shift-3 or Command-Shift-4.

An example of this are apps like Droplr and Cloud App which automatically upload the screenshot taken.

I have been searching around and found out that it may have something to do with Darwin Notifications but I am unsure as to where to start.

Answer 1

This was mentioned in one of the earlier comments, but you can use an NSMetadataQuery searching for files where kMDItemIsScreenCapture = 1. This is a special attribute that gets added to screenshot files.

I just whipped up a little demo showing how to do this and posted it on github:

https://github.com/davedelong/Demos/blob/master/ScreenShot%20Detector

Answer 2

This is how I have done it, it is a bit complicated but I will try and take you through it step by step:

---

Before we start, in your header file declare the following variables and methods :

BOOL shouldObserveDesktop;
NSDictionary *knownScreenshotsOnDesktop;
NSString *screenshotLocation;
NSString *screenshotFilenameSuffix;

- (void)startObservingDesktop;
- (void)stopObservingDesktop;
- (NSDictionary *)screenshotsOnDesktop;
- (NSDictionary *)screenshotsAtPath:(NSString *)dirpath modifiedAfterDate:(NSDate *)lmod;
- (void)checkForScreenshotsAtPath:(NSString *)dirpath;
- (NSDictionary *)findUnprocessedScreenshotsOnDesktop;

Now in your implementation file, firstly add this code:

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
    screenshotLocation = [[NSString stringWithString:@"~/Desktop"] retain];
    screenshotFilenameSuffix = [[NSString stringWithString:@".png"] retain];
    knownScreenshotsOnDesktop = [[self screenshotsOnDesktop] retain];
    [self startObservingDesktop];
}

This sets up the variables up for when all the methods are called. Next add:

- (void)onDirectoryNotification:(NSNotification *)n {
    id obj = [n object];
    if (obj && [obj isKindOfClass:[NSString class]]) {
        [self checkForScreenshotsAtPath:screenshotLocation];
    }
}

- (void)startObservingDesktop {
    if (shouldObserveDesktop)
        return;
    NSDistributedNotificationCenter *dnc = [NSDistributedNotificationCenter defaultCenter];
    [dnc addObserver:self selector:@selector(onDirectoryNotification:) name:@"com.apple.carbon.core.DirectoryNotification" object:nil suspensionBehavior:NSNotificationSuspensionBehaviorDeliverImmediately];
    shouldObserveDesktop = YES;
}

- (void)stopObservingDesktop {
    if (!shouldObserveDesktop)
        return;
    NSDistributedNotificationCenter *dnc = [NSDistributedNotificationCenter defaultCenter];
    [dnc removeObserver:self name:@"com.apple.carbon.core.DirectoryNotification" object:nil];
    shouldObserveDesktop = NO;
}

Here we observe the notification that will be called when a screenshot is taken and pass it the method to call (in this case onDirectoryNotification:). There is also the method to stop observing the desktop/notification. The notification calls checkForScreenshotsAtPath: which will check for screenshots on the desktop. The following is the code for that method and the other methods that it calls:

-(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) {
        // Process the file at the 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];
}

They were the first 3 methods that the notification in turn calls and the following code is the final method screenshotsAtPath:modifiedAfterDate: which I will warn you is extremely long as it has to confirm that the file is definitely a screenshot:

-(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) {

        // 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;
}

---

Well, there you have it. That's how I was able to detect when the user takes a screenshot, it probably has a few bugs but it seems to work fine at the moment. If you want all the code in one here are the links for it at pastebin.com:

Header - http://pastebin.com/gBAbCBJB

Implementation - http://pastebin.com/VjQ6P3zQ