iPhone 相机对焦

我使用下面的代码来对焦 iPhone 相机。但它不起作用。我从Apple的AVCam示例代码中获取了这段代码。我做错了什么吗？有什么方法可以 检测iPhone是否对焦？

-(void) focusAtPoint:(CGPoint)point   
{
    AVCaptureDevice *device =  [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];;

    if (device != nil) {            
        if ([device isFocusPointOfInterestSupported] && [device isFocusModeSupported:AVCaptureFocusModeAutoFocus]) {

            NSError *error;

            if ([device lockForConfiguration:&error]) {             
                [device setFocusPointOfInterest:point];             
                [device setFocusMode:AVCaptureFocusModeAutoFocus];              
                [device unlockForConfiguration];                
            } else {                
                NSLog(@"Error in Focus Mode");
            }        

        }
    }
}

要点为[AVCaptureDevice setFocusPointOfInterest:]与视图的触摸点不同，而是 0,0 和 1,1 之间的点（请参阅文档）。 所以你必须这样做：

CGPoint point = [touch locationInView:self.cameraView];
point.x /= self.cameraView.frame.size.width;
point.y /= self.cameraView.frame.size.height;
[self focusAtPoint:point];