capture screen activity to a movie file using AV Foundation on OS X 10.7 Lion and later

Technical Q&A QA1740

How to capture screen activity to a movie file using AV Foundation on OS X 10.7 Lion and later

Q:  How do I capture screen activity to a QuickTime movie on OS X 10.7 Lion and later?

A: On OS X 10.7 Lion and later, the way to make a movie of screen activity is to use AV Foundation.

Important: AV Foundation grabs the screen repeatedly. If you want to capture a static image instead of a real-time movie, a more efficient way is to call CGDisplayCreateImage orCGDisplayCreateImageForRect, for example:

// Create an image from the entire main display

CGImageRef image = CGDisplayCreateImage(kCGDirectMainDisplay);

See How to take an image snapshot of the screen on Mac OS X Lion for more information.

To perform a real-time screen capture and save it to a QuickTime movie file, you need minimally three AV objects:

• An AVCaptureSession object, which coordinates the flow of data from AV input sources to outputs.

• An AVCaptureScreenInput object, which is the input data source that provides video data from the given display.

• An AVCaptureMovieFileOutput object, which is the output destination for you to write captured media data to a QuickTime movie file.

Note: By default, AVCaptureScreenInput captures the entire screen. You may set its cropRect property to limit the capture rectangle to a subsection of the screen.

In the following example, the code creates a capture session, adds a screen input to provide video frames, adds an output destination to save captured frames, starts the flow of data from the input to the output, and stops the flow in 5 seconds. By letting the class conform to the AVCaptureFileOutputRecordingDelegate protocol and setting the recording delegate properly, you can monitor when the recording is finished via the delegate method.

Note: To capture both a main and secondary display at the same time, use two active capture sessions, one for each display. On OS X, AVCaptureMovieFileOutput only supports writing to a single video track.

Listing 1  Conforming to the AVCaptureFileOutputRecordingDelegate protocol

#import <Foundation/Foundation.h>

#import <AVFoundation/AVFoundation.h>

@interface Recorder : NSObject <AVCaptureFileOutputRecordingDelegate> {

@private

AVCaptureSession *mSession;

AVCaptureMovieFileOutput *mMovieFileOutput;

NSTimer *mTimer;

}

-(void)screenRecording:(NSURL *)destPath;

@end

Listing 2  Screen recording example

-(void)screenRecording:(NSURL *)destPath

{

// Create a capture session

mSession = [[AVCaptureSession alloc] init];

// Set the session preset as you wish

mSession.sessionPreset = AVCaptureSessionPresetMedium;

// If you're on a multi-display system and you want to capture a secondary display,

// you can call CGGetActiveDisplayList() to get the list of all active displays.

// For this example, we just specify the main display.

// To capture both a main and secondary display at the same time, use two active

// capture sessions, one for each display. On Mac OS X, AVCaptureMovieFileOutput

// only supports writing to a single video track.

CGDirectDisplayID displayId = kCGDirectMainDisplay;

// Create a ScreenInput with the display and add it to the session

AVCaptureScreenInput *input = [[[AVCaptureScreenInput alloc] initWithDisplayID:displayId] autorelease];

if (!input) {

[mSession release];

mSession = nil;

return;

}

if ([mSession canAddInput:input])

[mSession addInput:input];

// Create a MovieFileOutput and add it to the session

mMovieFileOutput = [[[AVCaptureMovieFileOutput alloc] init] autorelease];

if ([mSession canAddOutput:mMovieFileOutput])

[mSession addOutput:mMovieFileOutput];

// Start running the session

[mSession startRunning];

// Delete any existing movie file first

if ([[NSFileManager defaultManager] fileExistsAtPath:[destPath path]])

{

NSError *err;

if (![[NSFileManager defaultManager] removeItemAtPath:[destPath path] error:&err])

{

NSLog(@"Error deleting existing movie %@",[err localizedDescription]);

}

}

// Start recording to the destination movie file

// The destination path is assumed to end with ".mov", for example, @"/users/master/desktop/capture.mov"

// Set the recording delegate to self

[mMovieFileOutput startRecordingToOutputFileURL:destPath recordingDelegate:self];

// Fire a timer in 5 seconds

mTimer = [[NSTimer scheduledTimerWithTimeInterval:5 target:self selector:@selector(finishRecord:) userInfo:nil repeats:NO] retain];

}

-(void)finishRecord:(NSTimer *)timer

{

// Stop recording to the destination movie file

[mMovieFileOutput stopRecording];

[mTimer release];

mTimer = nil;

}

// AVCaptureFileOutputRecordingDelegate methods

- (void)captureOutput:(AVCaptureFileOutput *)captureOutput didFinishRecordingToOutputFileAtURL:(NSURL *)outputFileURL fromConnections:(NSArray *)connections error:(NSError *)error

{

NSLog(@"Did finish recording to %@ due to error %@", [outputFileURL description], [error description]);

// Stop running the session

[mSession stopRunning];

// Release the session

[mSession release];

mSession = nil;

}

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Document Revision History

Date	Notes
2015-05-20	Added additional notes about capturing the main and secondary displays at the same time. Other miscellaneous changes.
2011-08-10	Corrected a link.
2011-05-13	New document that shows how to capture screen activity to a movie file using AV Foundation on OS X 10.7 Lion and later.