MIDIConnectionEvent

The MIDIConnectionEvent interface of the Web MIDI API (en-US) is the event passed to the onstatechange (en-US) event of the MIDIAccess interface and the onstatechange event of the MIDIPorts interface. This occurs any time a new port becomes available, or when a previously available port becomes unavailable. 例如，这个事件无论是在一个 MIDI 设备插入或拔出设备时都会触发。