Getting Started

The easiest way to start using Oboe is to build it from source by adding a few steps to an existing Android Studio project.

Creating an Android app with native support

  • Create a new project: File > New > New Project
  • When selecting the project type, select Native C++
  • Finish configuring project

Adding Oboe to your project

1. Clone the github repository

Start by cloning the latest stable release of the Oboe repository, for example:

git clone -b 1.3-stable https://github.com/google/oboe

Make a note of the path which you cloned oboe into - you will need it shortly

If you use git as your version control system, consider adding Oboe as a submodule (underneath your cpp directory)

git submodule add https://github.com/google/oboe

This makes it easier to integrate updates to Oboe into your app, as well as contribute to the Oboe project.

2. Update CMakeLists.txt

Open your app's CMakeLists.txt. This can be found under External Build Files in the Android project view. If you don't have a CMakeLists.txt you will need to add C++ support to your project.

CMakeLists.txt location in Android Studio

Now add the following commands to the end of CMakeLists.txt. Remember to update **PATH TO OBOE** with your local Oboe path from the previous step:

# Set the path to the Oboe directory.
set (OBOE_DIR ***PATH TO OBOE***)

# Add the Oboe library as a subdirectory in your project.
# add_subdirectory tells CMake to look in this directory to
# compile oboe source files using oboe's CMake file.
# ./oboe specifies where the compiled binaries will be stored
add_subdirectory (${OBOE_DIR} ./oboe)

# Specify the path to the Oboe header files.
# This allows targets compiled with this CMake (application code)
# to see public Oboe headers, in order to access its API.
include_directories (${OBOE_DIR}/include)

In the same file find the target_link_libraries command. Add oboe to the list of libraries which your app's library depends on. For example:

target_link_libraries(native-lib oboe)

Here's a complete example CMakeLists.txt file:

cmake_minimum_required(VERSION 3.4.1)

# Build our own native library
add_library (native-lib SHARED native-lib.cpp )

# Build the Oboe library
set (OBOE_DIR ./oboe)
add_subdirectory (${OBOE_DIR} ./oboe)

# Make the Oboe public headers available to our app
include_directories (${OBOE_DIR}/include)

# Specify the libraries which our native library is dependent on, including Oboe
target_link_libraries (native-lib log oboe)

Now go to Build->Refresh Linked C++ Projects to have Android Studio index the Oboe library.

Verify that your project builds correctly. If you have any issues building please report them here.

Using Oboe

Once you've added Oboe to your project you can start using Oboe's features. The simplest, and probably most common thing you'll do in Oboe is to create an audio stream.

Creating an audio stream

Include the Oboe header:

#include <oboe/Oboe.h>

Streams are built using an AudioStreamBuilder. Create one like this:

oboe::AudioStreamBuilder builder;

Use the builder's set methods to set properties on the stream (you can read more about these properties in the full guide):

builder.setDirection(oboe::Direction::Output);
builder.setPerformanceMode(oboe::PerformanceMode::LowLatency);
builder.setSharingMode(oboe::SharingMode::Exclusive);
builder.setFormat(oboe::AudioFormat::Float);
builder.setChannelCount(oboe::ChannelCount::Mono);

The builder's set methods can be easily chained:

oboe::AudioStreamBuilder builder;
builder.setPerformanceMode(oboe::PerformanceMode::LowLatency)
  ->setSharingMode(oboe::SharingMode::Exclusive)
  ->setCallback(myCallback)
  ->setFormat(oboe::AudioFormat::Float);

Define an AudioStreamCallback class to receive callbacks whenever the stream requires new data.

class MyCallback : public oboe::AudioStreamCallback {
public:
    oboe::DataCallbackResult
    onAudioReady(oboe::AudioStream *audioStream, void *audioData, int32_t numFrames) {
        
        // We requested AudioFormat::Float so we assume we got it. For production code always check what format
    // the stream has and cast to the appropriate type.
        auto *outputData = static_cast<float *>(audioData);
    
        // Generate random numbers centered around zero.
        const float amplitude = 0.2f;
        for (int i = 0; i < numFrames; ++i){
            outputData[i] = ((float)drand48() - 0.5f) * 2 * amplitude;
        }
    
        return oboe::DataCallbackResult::Continue;
    }
};

You can find examples of how to play sound using digital synthesis and pre-recorded audio in the code samples.

Supply this callback class to the builder:

MyCallback myCallback;
builder.setCallback(&myCallback);

Declare a ManagedStream. Make sure it is declared in an appropriate scope (e.g.the member of a managing class). Avoid declaring it as a global.

oboe::ManagedStream managedStream;

Open the stream:

oboe::Result result = builder.openManagedStream(managedStream);

Check the result to make sure the stream was opened successfully. Oboe has a convenience method for converting its types into human-readable strings called oboe::convertToText:

if (result != oboe::Result::OK) {
    LOGE("Failed to create stream. Error: %s", oboe::convertToText(result));
}

Note that this sample code uses the logging macros from here.

Playing audio

Check the properties of the created stream. The format is one property which you should check. This will dictate the audioData type in the AudioStreamCallback::onAudioReady callback.

oboe::AudioFormat format = stream->getFormat();
LOGI("AudioStream format is %s", oboe::convertToText(format));

Now start the stream.

managedStream->requestStart();

At this point you should start receiving callbacks.

To stop receiving callbacks call

    managedStream->requestStop();

Closing the stream

It is important to close your stream when you're not using it to avoid hogging audio resources which other apps could use. This is particularly true when using SharingMode::Exclusive because you might prevent other apps from obtaining a low latency audio stream.

Streams can be explicitly closed:

stream->close();

close() is a blocking call which also stops the stream.

Streams can also be automatically closed when going out of scope:

{
	ManagedStream mStream;
	AudioStreamBuilder().build(mStream);
	mStream->requestStart();
} // Out of this scope the mStream has been automatically closed

It is preferable to let the ManagedStream object go out of scope (or be explicitly deleted) when the app is no longer playing audio. For apps which only play or record audio when they are in the foreground this is usually done when Activity.onPause() is called.

Reconfiguring streams

In order to change the configuration of the stream, simply call openManagedStream again. The existing stream is closed, destroyed and a new stream is built and populates the managedStream.

// Modify the builder with some additional properties at runtime.
builder.setDeviceId(MY_DEVICE_ID);
// Re-open the stream with some additional config
// The old ManagedStream is automatically closed and deleted
builder.openManagedStream(managedStream);

The ManagedStream takes care of its own closure and destruction. If used in an automatic allocation context (such as a member of a class), the stream does not need to be closed or deleted manually. Make sure that the object which is responsible for the ManagedStream (its enclosing class) goes out of scope whenever the app is no longer playing or recording audio, such as when Activity.onPause() is called.

Example

The following class is a complete implementation of a ManagedStream, which renders a sine wave. Creating the class (e.g. through the JNI bridge) creates and opens an Oboe stream which renders audio, and its destruction stops and closes the stream.

#include <oboe/Oboe.h>
#include <math.h>

class OboeSinePlayer: public oboe::AudioStreamCallback {
public:


    OboeSinePlayer() {
        oboe::AudioStreamBuilder builder;
        // The builder set methods can be chained for convenience.
        builder.setSharingMode(oboe::SharingMode::Exclusive)
          ->setPerformanceMode(oboe::PerformanceMode::LowLatency)
          ->setChannelCount(kChannelCount)
          ->setSampleRate(kSampleRate)
          ->setFormat(oboe::AudioFormat::Float)
          ->setCallback(this)
          ->openManagedStream(outStream);
        // Typically, start the stream after querying some stream information, as well as some input from the user
        outStream->requestStart();
    }

    oboe::DataCallbackResult onAudioReady(oboe::AudioStream *oboeStream, void *audioData, int32_t numFrames) override {
        float *floatData = (float *) audioData;
        for (int i = 0; i < numFrames; ++i) {
            float sampleValue = kAmplitude * sinf(mPhase);
            for (int j = 0; j < kChannelCount; j++) {
                floatData[i * kChannelCount + j] = sampleValue;
            }
            mPhase += mPhaseIncrement;
            if (mPhase >= kTwoPi) mPhase -= kTwoPi;
        }
        return oboe::DataCallbackResult::Continue;
    }

private:
    oboe::ManagedStream outStream;
    // Stream params
    static int constexpr kChannelCount = 2;
    static int constexpr kSampleRate = 48000;
    // Wave params, these could be instance variables in order to modify at runtime
    static float constexpr kAmplitude = 0.5f;
    static float constexpr kFrequency = 440;
    static float constexpr kPI = M_PI;
    static float constexpr kTwoPi = kPI * 2;
    static double constexpr mPhaseIncrement = kFrequency * kTwoPi / (double) kSampleRate;
    // Keeps track of where the wave is
    float mPhase = 0.0;
};

Note that this implementation computes sine values at run-time for simplicity, rather than pre-computing them. Additionally, best practice is to implement a separate callback class, rather than managing the stream and defining its callback in the same class. This class also automatically starts the stream upon construction. Typically, the stream is queried for information prior to being started (e.g. burst size), and started upon user input. For more examples on how to use ManagedStream look in the samples folder.

Obtaining optimal latency

One of the goals of the Oboe library is to provide low latency audio streams on the widest range of hardware configurations. On some devices (namely those which can only use OpenSL ES) the "native" sample rate and buffer size of the audio device must be supplied when the stream is opened.

Oboe provides a convenient way of setting global default values so that the sample rate and buffer size do not have to be set each time an audio stream is created.

Here's a code sample showing how the default values for built-in devices can be passed to Oboe:

MainActivity.java

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1){
    AudioManager myAudioMgr = (AudioManager) context.getSystemService(Context.AUDIO_SERVICE);
    String sampleRateStr = myAudioMgr.getProperty(AudioManager.PROPERTY_OUTPUT_SAMPLE_RATE);
    int defaultSampleRate = Integer.parseInt(sampleRateStr);
    String framesPerBurstStr = myAudioMgr.getProperty(AudioManager.PROPERTY_OUTPUT_FRAMES_PER_BUFFER);
    int defaultFramesPerBurst = Integer.parseInt(framesPerBurstStr);

    native_setDefaultStreamValues(defaultSampleRate, defaultFramesPerBurst);
}

jni-bridge.cpp

JNIEXPORT void JNICALL
Java_com_google_sample_oboe_hellooboe_MainActivity_native_1setDefaultStreamValues(JNIEnv *env,
                                                                                  jclass type,
                                                                                  jint sampleRate,
                                                                                  jint framesPerBurst) {
    oboe::DefaultStreamValues::SampleRate = (int32_t) sampleRate;
    oboe::DefaultStreamValues::FramesPerBurst = (int32_t) framesPerBurst;
}

Further information