//实例化

AVCaptureDevice  *captureDevice = [[AVCaptureDevice alloc] init];

//先进行判断是否支持控制对焦

if (captureDevice.isFocusPointOfInterestSupported &&[captureDevice.isFocusModeSupported:AVCaptureFocusModeAutoFocus]) {

        NSError *error = nil;

//对cameraDevice进行操作前，需要先锁定，防止其他线程访问，

        [self.recorder.cameraDevice lockForConfiguration:&error];

        [self.recorder.cameraDevice setFocusMode:AVCaptureFocusModeAutoFocus];

        [self.recorder.cameraDevice setFocusPointOfInterest:CGPointMake(pointX,pointY)];

//操作完成后，记得进行unlock。

        [self.recorder.cameraDevice unlockForConfiguration];

    }