Android MIDI User Guide
This document describes how to use the Android MIDI API in Java.
The Android MIDI package allows users to:
The API is “transport agnostic”. But there are several transports currently supported:
A Device is a MIDI capable object that has zero or more InputPorts and OutputPorts.
An InputPort has 16 channels and can receive MIDI messages from an OutputPort or an app.
An OutputPort has 16 channels and can send MIDI messages to an InputPort or an app.
MidiService is a centralized process that keeps track of all devices and brokers communication between them.
MidiManager is a class that the application or a device manager calls to communicate with the MidiService.
An app that requires the MIDI API should declare that in the AndroidManifest.xml file. Then the app will not appear in the Play Store for old devices that do not support the MIDI API.
<uses-feature android:name="android.software.midi" android:required="true"/>
An app can also check at run-time whether the MIDI feature is supported on a platform. This is particularly useful during development when you install apps directly on a device.
if (context.getPackageManager().hasSystemFeature(PackageManager.FEATURE_MIDI)) { // do MIDI stuff }
The primary class for accessing the MIDI package is through the MidiManager.
MidiManager m = (MidiManager)context.getSystemService(Context.MIDI_SERVICE);
When an app starts, it can get a list of all the available MIDI devices. This information can be presented to a user, allowing them to choose a device.
MidiDeviceInfo[] infos = m.getDevices();
The application can request notification when, for example, keyboards are plugged in or unplugged.
m.registerDeviceCallback(new MidiManager.DeviceCallback() { public void onDeviceAdded( MidiDeviceInfo info ) { ... } public void onDeviceRemoved( MidiDeviceInfo info ) { ... } });
You can query the number of input and output ports.
int numInputs = info.getInputPortCount(); int numOutputs = info.getOutputPortCount();
Note that “input” and “output” are from the standpoint of the device. So a synthesizer will have an “input” port that receives messages. A keyboard will have an “output” port that sends messages.
The MidiDeviceInfo has a bundle of properties.
Bundle properties = info.getProperties(); String manufacturer = properties .getString(MidiDeviceInfo.PROPERTY_MANUFACTURER);
Other properties include PROPERTY_PRODUCT, PROPERTY_NAME, PROPERTY_SERIAL_NUMBER
You can get the names and types of the ports from a PortInfo object. The type will be either TYPE_INPUT or TYPE_OUTPUT.
MidiDeviceInfo.PortInfo[] portInfos = info.getPorts(); String portName = portInfos[0].getName(); if (portInfos[0].getType() == MidiDeviceInfo.PortInfo.TYPE_INPUT) { ... }
To access a MIDI device you need to open it first. The open is asynchronous so you need to provide a callback for completion. You can specify an optional Handler if you want the callback to occur on a specific Thread.
m.openDevice(info, new MidiManager.OnDeviceOpenedListener() { @Override public void onDeviceOpened(MidiDevice device) { if (device == null) { Log.e(TAG, "could not open device " + info); } else { ... }, new Handler(Looper.getMainLooper()) );
If you want to send a message to a MIDI Device then you need to open an “input” port with exclusive access.
MidiInputPort inputPort = device.openInputPort(index);
MIDI messages are sent as byte arrays. Here we encode a NoteOn message.
byte[] buffer = new buffer[64]; int numBytes = 0; buffer[numBytes++] = 0x90 + channel; // note on buffer[numBytes++] = pitch; buffer[numBytes++] = velocity; int offset = 0; // post is non-blocking inputPort.send(buffer, offset, numBytes);
Sometimes it is convenient to send MIDI messages with a timestamp. By scheduling events in the future we can mask scheduling jitter. Android MIDI timestamps are based on the monotonic nanosecond system timer. This is consistent with the other audio and input timers.
Here we send a message with a timestamp 2 seconds in the future.
final long NANOS_PER_SECOND = 1000000000L; long now = System.nanoTime(); long future = now + (2 * NANOS_PER_SECOND); inputPort.send(buffer, offset, numBytes, future);
If you want to cancel events that you have scheduled in the future then call flush().
inputPort.flush(); // discard events
If there were any MIDI NoteOff message left in the buffer then they will be discarded and you may get stuck notes. So we recommend sending “all notes off” after doing a flush.
To receive MIDI data from a device you need to extend MidiReceiver. Then connect your receiver to an output port of the device.
class MyReceiver extends MidiReceiver { public void onSend(byte[] data, int offset, int count, long timestamp) throws IOException { // parse MIDI or whatever } } MidiOutputPort outputPort = device.openOutputPort(index); outputPort.connect(new MyReceiver());
The data that arrives is not validated or aligned in any particular way. It is raw MIDI data and can contain multiple messages or partial messages. It might contain System Real-Time messages, which can be interleaved inside other messages.
An app can provide a MIDI Service that can be used by other apps. For example, an app can provide a custom synthesizer that other apps can send messages to. The service must be guarded with permission "android.permission.BIND_MIDI_DEVICE_SERVICE".
An app declares that it will function as a MIDI server in the AndroidManifest.xml file.
<service android:name="MySynthDeviceService" android:permission="android.permission.BIND_MIDI_DEVICE_SERVICE"> <intent-filter> <action android:name="android.media.midi.MidiDeviceService" /> </intent-filter> <meta-data android:name="android.media.midi.MidiDeviceService" android:resource="@xml/synth_device_info" /> </service>
The details of the resource in this example is stored in “res/xml/synth_device_info.xml”. The port names that you declare in this file will be available from PortInfo.getName().
<devices> <device manufacturer="MyCompany" product="MidiSynthBasic"> <input-port name="input" /> </device> </devices>
You then define your server by extending android.media.midi.MidiDeviceService. Let‘s assume you have a MySynthEngine class that extends MidiReceiver.
import android.media.midi.MidiDeviceService; import android.media.midi.MidiDeviceStatus; import android.media.midi.MidiReceiver; public class MidiSynthDeviceService extends MidiDeviceService { private static final String TAG = "MidiSynthDeviceService"; private MySynthEngine mSynthEngine = new MySynthEngine(); private boolean synthStarted = false; @Override public void onCreate() { super.onCreate(); } @Override public void onDestroy() { mSynthEngine.stop(); super.onDestroy(); } @Override // Declare the receivers associated with your input ports. public MidiReceiver[] onGetInputPortReceivers() { return new MidiReceiver[] { mSynthEngine }; } /** * This will get called when clients connect or disconnect. * You can use it to turn on your synth only when needed. */ @Override public void onDeviceStatusChanged(MidiDeviceStatus status) { if (status.isInputPortOpen(0) && !synthStarted) { mSynthEngine.start(); synthStarted = true; } else if (!status.isInputPortOpen(0) && synthStarted){ mSynthEngine.stop(); synthStarted = false; } } }
MIDI devices can be connected to Android using Bluetooth LE.
Before using the device, the app must scan for available BTLE devices and then allow the user to connect. An example program will be provided so look for it on the Android developer website.
Applications that scan for Bluetooth devices must request permission in the manifest file. This LOCATION permission is required because it may be possible to guess the location of an Android device by seeing which BTLE devices are nearby.
<uses-permission android:name="android.permission.BLUETOOTH"/> <uses-permission android:name="android.permission.BLUETOOTH_ADMIN"/> <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION"/>
Apps must also request location permission from the user at run-time.
See the documentation for Activity.requestPermissions()
for details and an example.
The app will only want to see MIDI devices and not mice or other non-MIDI devices. So construct a ScanFilter using the UUID for standard MIDI over BTLE.
MIDI over BTLE UUID = "03B80E5A-EDE8-4B33-A751-6CE34EC4C700"
See the documentation for android.bluetooth.le.BluetoothLeScanner.startScan()
method for details. When the user selects a MIDI/BTLE device then you can open it
using the MidiManager.
m.openBluetoothDevice(bluetoothDevice, callback, handler);
Once the MIDI/BTLE device has been opened by one app then it will also become available to other apps using the MIDI device discovery calls described above.
MidiManager.OnDeviceOpenedListener |
Listener class used for receiving the results of openDevice(MidiDeviceInfo, MidiManager.OnDeviceOpenedListener, Handler) and
openBluetoothDevice(BluetoothDevice, MidiManager.OnDeviceOpenedListener, Handler)
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MidiDevice |
This class is used for sending and receiving data to and from a MIDI device
Instances of this class are created by openDevice(MidiDeviceInfo, MidiManager.OnDeviceOpenedListener, Handler) .
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MidiDevice.MidiConnection | This class represents a connection between the output port of one device and the input port of another. |
MidiDeviceInfo | This class contains information to describe a MIDI device. |
MidiDeviceInfo.PortInfo | Contains information about an input or output port. |
MidiDeviceService | A service that implements a virtual MIDI device. |
MidiDeviceStatus | This is an immutable class that describes the current status of a MIDI device's ports. |
MidiInputPort | This class is used for sending data to a port on a MIDI device |
MidiManager | This class is the public application interface to the MIDI service. |
MidiManager.DeviceCallback | Callback class used for clients to receive MIDI device added and removed notifications |
MidiOutputPort | This class is used for receiving data from a port on a MIDI device |
MidiReceiver | Interface for sending and receiving data to and from a MIDI device. |
MidiSender | Interface provided by a device to allow attaching MidiReceivers to a MIDI device. |