AVCaptureSession 照相时获取 AVCaptureVideoPreviewLayer尺寸

http://*.com/questions/14153878/avcapturesession-preset-photo-and-avcapturevideopreviewlayer-size

I initialize an AVCaptureSession and I preset it like this :

AVCaptureSession *newCaptureSession = [[AVCaptureSession alloc] init];
if (YES==[newCaptureSession canSetSessionPreset:AVCaptureSessionPresetPhoto]) {
    newCaptureSession.sessionPreset = AVCaptureSessionPresetPhoto;
} else {
    // Error management
}

Then I setup an AVCaptureVideoPreviewLayer :

self.preview = [[UIView alloc] initWithFrame:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height/*426*/)];
CALayer *previewLayer = preview.layer;
AVCaptureVideoPreviewLayer *captureVideoPreviewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:self.session];
captureVideoPreviewLayer.frame = previewLayer.frame;
[previewLayer addSublayer:captureVideoPreviewLayer];
captureVideoPreviewLayer.videoGravity = AVLayerVideoGravityResizeAspect;

My question is:
How can I get the exact CGSize needed to display all the captureVideoPreviewLayer layer on screen ? More precisely I need the height as AVLayerVideoGravityResizeAspect make the AVCaptureVideoPreviewLayer fits the preview.size ?
I try to get AVCaptureVideoPreviewLayer size that fit right.

Very thank you for your help
objective-c ios camera
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After some research with AVCaptureSessionPresetPhoto the AVCaptureVideoPreviewLayer respect the 3/4 ration of iPhone camera. So it's easy to have the right height with simple calculus.
As an instance if the width is 320 the adequate height is:
320*4/3=426.6
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// Get your AVCaptureSession somehow. I'm getting mine out of self.videoCamera, which is a GPUImageVideoCamera
    // Get the appropriate AVCaptureVideoDataOutput out of the capture session. I only have one session, so it's easy.

AVCaptureVideoDataOutput *output = [[[self.videoCamera captureSession] outputs] lastObject];
    NSDictionary* outputSettings = [output videoSettings];

// AVVideoWidthKey and AVVideoHeightKey did not work. I had to use these literal keys.
    long width  = [[outputSettings objectForKey:@"Width"]  longValue];
    long height = [[outputSettings objectForKey:@"Height"] longValue];

// video camera output dimensions are always for landscape mode. Transpose if your camera is in portrait mode.
    if (UIInterfaceOrientationIsPortrait([self.videoCamera outputImageOrientation])) {
        long buf = width;
        width = height;
        height = buf;
    }

CGSize outputSize = CGSizeMake(width, height);