c++ - 如何读取 MIDI 脚踏 Controller 的初始状态？

Question

我知道 MIDI 允许我通过捕捉指示控制变化的 MIDI 消息来读取 MIDI 脚踏 Controller 的状态。但是，如果用户还没有触摸/更改控件呢？我还能读取状态/值吗？有什么方法可以做到这一点？

这是我使用 OSX CoreMIDI 捕捉 Midi 消息的代码

void initMidi()
{
    MIDIClientRef   midiClient;
    MIDIPortRef     inputPort;
    OSStatus        status;
    MIDIEndpointRef src;

    status = MIDIClientCreate(CFSTR("testing"), NULL, NULL, &midiClient);
    if (status != noErr)
        NSLog(@"Error creating MIDI client: %d", status);

    status = MIDIInputPortCreate(midiClient, CFSTR("Input"), midiInputCallback, NULL, &inputPort);
    if (status != noErr)
        NSLog(@"Error creating MIDI input port: %d", status);

    ItemCount numOfDevices = MIDIGetNumberOfDevices();

    // just try to connect to every device
    for (ItemCount i = 0; i < numOfDevices; i++) {
        src = MIDIGetSource(i);
        status = MIDIPortConnectSource(inputPort, src, NULL);
    }
}

void midiInputCallback(const MIDIPacketList *list,
    void *procRef,
    void *srcRef)
{
    for (UInt32 i = 0; i < list->numPackets; i++) {
        const MIDIPacket *packet = &list->packet[i];

        for (UInt16 j = 0, size = 0; j < packet->length; j += size) {
            UInt8 status = packet->data[j];

            if (status <  0xC0)  size = 3;
            else if (status <  0xE0)  size = 2;
            else if (status <  0xF0)  size = 3;
            else if (status <  0xF3)  size = 3;
            else if (status == 0xF3)  size = 2;
            else                      size = 1;

            switch (status & 0xF0) {
            case 0xb0:
                NSLog(@"MIDI Control Changed: %d %d", packet->data[j + 1], packet->data[j + 2]);
                break;
            }
        }
    }
}

Answer 1

如果您没有重置设备，也没有更改控件，那么您的程序在收到消息之前不会知道控件的状态。

某些设备可能具有特定于供应商的命令来读取控件的当前状态，或转储整个状态。