Apply CGAffineTransform prior to layer being displ

问题:

I'm trying to scale an AVCaptureVideoPreviewLayer so it displays a "zoomed in" preview from the camera; I can make this work using setAffineTransform:CGAffineTransformMakeScale(2.0f, 2.0f), but only once the layer is visible on screen. That is, the scale transform doesn't seem to take effect unless I call it (from within a CATransaction) during a callback like viewDidAppear.

What I'd really like to do is have the scale transform be applied prior to the layer becoming visible--e.g. at the time that I initialize it and add it to the UIView--but all my attempts to do so have proven futile. If I set the transform at this point, and then attempt to read the value back once it is visible, it does appear to have the desired scale transform set -- yet it does not have any effect on the view. I'm new to iOS programming, so I'm sure there's something basic I'm missing here; any help is appreciated!

回答1:

Two simple alternative approaches, one (better) for iOS 7.x and one for earlier versions of iOS:

float zoomLevel = 2.0f;
if ([device respondsToSelector:@selector(setVideoZoomFactor:)]
    && device.activeFormat.videoMaxZoomFactor >= zoomLevel) {
  // iOS 7.x with compatible hardware
  if ([device lockForConfiguration:nil]) {
    [device setVideoZoomFactor:zoomLevel];
    [device unlockForConfiguration];
  }
} else {
  // Lesser cases
  CGRect frame = captureVideoPreviewLayer.frame;
  float width = frame.size.width * zoomLevel;
  float height = frame.size.height * zoomLevel;
  float x = (frame.size.width - width)/2;
  float y = (frame.size.height - height)/2;
  captureVideoPreviewLayer.bounds = CGRectMake(x, y, width, height);
}