MIDISend:在 iPhone 上播放音符

Question

我正在尝试生成一个音符，该音符将使用Objective-C和MIDI通过iPhone扬声器播放。我有下面的代码，但它没有做任何事情。我做错了什么？

MIDIPacketList packetList;
packetList.numPackets = 1;
MIDIPacket* firstPacket = &packetList.packet[0];
firstPacket->timeStamp = 0; // send immediately
firstPacket->length = 3;
firstPacket->data[0] = 0x90;
firstPacket->data[1] = 80;
firstPacket->data[2] = 120;
MIDIPacketList pklt=packetList;
MIDISend(MIDIGetSource(0), MIDIGetDestination(0), &pklt);

Answer 1

你有三个问题：

1. 声明MIDIPacketList不会分配内存或初始化结构
2. 您将 MIDIGetSource 的结果（返回一个 MIDIEndpointRef ）作为第一个参数传递给它期望MIDIPortRef的位置MIDISend。（您可能忽略了有关此的编译器警告。切勿忽略编译器警告。
3. 在 iOS 中发送 MIDI 音符不会发出任何声音。如果您没有将外部 MIDI 设备连接到 iOS 设备，则需要使用 CoreAudio 设置一些可以生成声音的内容。这超出了这个答案的范围。

所以这段代码会运行，但除非你有外部硬件，否则它不会发出任何声音：

//Look to see if there's anything that will actually play MIDI notes
NSLog(@"There are %lu destinations", MIDIGetNumberOfDestinations());
// Prepare MIDI Interface Client/Port for writing MIDI data:
MIDIClientRef midiclient = 0;
MIDIPortRef midiout = 0;
OSStatus status;
status = MIDIClientCreate(CFSTR("Test client"), NULL, NULL, &midiclient);
if (status) {
    NSLog(@"Error trying to create MIDI Client structure: %d", (int)status);
}
status = MIDIOutputPortCreate(midiclient, CFSTR("Test port"), &midiout);
if (status) {
    NSLog(@"Error trying to create MIDI output port: %d", (int)status);
}
Byte buffer[128];
MIDIPacketList *packetlist = (MIDIPacketList *)buffer;
MIDIPacket *currentpacket = MIDIPacketListInit(packetlist);
NSInteger messageSize = 3; //Note On is a three-byte message
Byte msg[3] = {0x90, 80, 120};
MIDITimeStamp timestamp = 0;
currentpacket = MIDIPacketListAdd(packetlist, sizeof(buffer), currentpacket, timestamp, messageSize, msg);
MIDISend(midiout, MIDIGetDestination(0), packetlist);