一、介绍
首先要知道 AVCaptureMetadataOutput 能做什么？ 看官方文档的介绍:
A capture output for processing timed metadata produced by a capture session.
翻译过来就是： 用于处理AVCaptureSession产生的定时元数据的捕获输出
类的声明：
@interface AVCaptureMetadataOutput : AVCaptureOutput
可以看到它继承自 AVCaptureOutput
An AVCaptureMetadataOutput object intercepts metadata objects emitted by its associated capture connection and forwards them to a delegate object for processing. You can use instances of this class to process specific types of metadata included with the input data. You use this class the way you do other o