Add MIDIUSB.h compatibility for Teensy 4.0

teensy4/MIDIUSB.h

+#ifndef MIDIUSB_h
+#define MIDIUSB_h
+
+// For compatibility with Arduino's MIDIUSB library
+
+#include "usb_midi.h"
+
+#ifdef __cplusplus
+#if !defined(USB_MIDI) && !defined(USB_MIDI4) && !defined(USB_MIDI16) && !defined(USB_MIDI_SERIAL) && !defined(USB_MIDI4_SERIAL) && !defined(USB_MIDI16_SERIAL) && !defined(USB_MIDI_AUDIO_SERIAL) && !defined(USB_MIDI16_AUDIO_SERIAL) && !defined(USB_EVERYTHING)
+#error "Please select MIDI in Tools > USB Type to use MIDIUSB.h"
+#endif
+
+typedef struct {
+ union {
+  struct {
+	uint8_t header;
+	uint8_t byte1;
+	uint8_t byte2;
+	uint8_t byte3;
+  };
+	uint32_t word;
+ };
+} midiEventPacket_t;
+
+
+class MIDI_
+{
+public:
+	constexpr MIDI_(void) { }
+	uint32_t available(void) {
+		return usb_midi_available();
+	}
+	midiEventPacket_t read(void) {
+		midiEventPacket_t event;
+		event.word = usb_midi_read_message();
+		return event;
+	}
+	void flush(void) {
+		usb_midi_flush_output();
+	}
+	void sendMIDI(midiEventPacket_t event) {
+		usb_midi_write_packed(event.word);
+	}
+	size_t write(const uint8_t *buffer, size_t size) {
+		// TODO - is this really needed?
+		return 0;
+	}
+	operator bool() {
+		// TODO - is this really needed?
+		return true;
+	}
+};
+extern MIDI_ MidiUSB;
+
+#endif // __cplusplus
+#endif // MIDIUSB_h