AVCaptureMetadataOutput 的setRectOfInterest属性来控制
CGRectMake(y的起点/屏幕的高,x的起点/屏幕的宽,扫描的区域的高/屏幕的高,扫描的区域的宽/屏幕的宽)
[_output setRectOfInterest:CGRectMake((124)/SCREEN_HEIGHT,0,SCREEN_WIDTH/SCREEN_HEIGHT,1)];
The default value of this property is the value CGRectMake(0, 0, 1, 1).