How can i check if there is an active connection to webcam ?

I'm making a GUI with a webcam and i'm taking a snapshot every time i click a push button, but first i have to create the webcam object in order to take the photo, so i'm wondering if there is a way to check if the GUI already make connection with the webcam and skip the creation of this object because it takes a lot of time in the creation. This is the code im trying but seems not working on the GUI:

exst = exist(vid)
if  exst==0
    clear('vid');
    maxAttempts = 30;
    attempt = 1;
    hasProperty = false;
    while attempt < maxAttempts
        vid = webcam('USB Camera');
        hasProperty = max(ismember(properties(vid), 'Exposure'));
        if hasProperty
    % We got all the properties so we can quit trying.
    fprintf('We have all camera properties after %d attempts.  Now continuing with program.\n', attempt);
    break;
        end
        fprintf('We do not have all camera properties after %d attempts.  Trying again.\n', attempt);
        attempt = attempt + 1;
        clear('vid');
    end
    %CAMERA CONFIGURATION 
    vid.Resolution='2048x1536';
    vid.WhiteBalanceMode='manual';
    vid.ExposureMode='manual';
    vid.Exposure=-5.7;
    vid.Gain=0;
    vid.Hue=0; %-40 40
    vid.Gamma=72; %72-500
    vid.Contrast=64;
    vid.Saturation=35;
    vid.Sharpness=1;
    vid.Brightness=15; %-64 64
    vid.WhiteBalance=6500; %2800-6500
    vid.BacklightCompensation=2; %0-2
end

img=snapshot(vid);

Answer 1

[Matthew Gingerich]

This should open the camera unless it is already open:

if (exist('vid') == 0)
    vid = webcam('USB Camera');
end