Gyroscope sensor

This time I encountered a request from a client that they want to get the gyro info in a cocos2d-x game. For this scenario we can use iphone's built-in sensor: gyroscope to measures the rate of rotation around the three axes.

Accelerometer in cocos2d-x

In cocos2d-x there is public access to the accelerometer sensor, and we can find the example in cpp-test/Class/PhysicsTest/PhysicsTest.cpp, class name PhysicsDemoClickAdd:

Device::setAccelerometerEnabled(true);
auto accListener = EventListenerAcceleration::create(CC_CALLBACK_2(PhysicsDemoClickAdd::onAcceleration, this));
_eventDispatcher->addEventListenerWithSceneGraphPriority(accListener, this);

However, this is dealing with the accelerometer, the acceleration of three axes, which is not I want. I want the access of gyroscope, measuring the attitudes of Yaw, Roll and Pitch, which is not provided in cocos2d-x's library.

The CMDeviceMotion class in iOS Core Motion framework encapsulates measurements of the attitude, and I decide to use directly in my cocos2d code.

Euler Angles and Yaw, Roll, Pitch

These three terms come from flight dynamics, server as critical parameters of angles of rotation in three dimensions.

Put them into cocos2d-x C++ code

Just take a sample cocos2d-x project for example, I am using version 3.8 and generated a project using:

cocos new my-gyro -l cpp

I am going to use CMMotionManager in the class of HelloWorldScene.cpp. The reference example of Apple's Motion Event is written is objective-c, how can it be put into our cocos2d-x C++ source?

Actually Apple provides Objective-C++ as a convenient mechanism for mixing Objective-C code with C++ code! But wait, what I have is a C++ code HelloWorldScene.cpp, not compatible with objective-c... Solution is simple, just change the file name to HelloWorldScene.mm to make it become an objective-C++. Now it becomes an objective-C++ file only contains C++ code, and I can insert any objective-c logic into it.

Here is my HelloWorldScene.mm

# include "HelloWorldScene.h"
# import <CoreMotion/CoreMotion.h>

USING_NS_CC;

Scene* HelloWorld::createScene()
{
    // 'scene' is an autorelease object
    auto scene = Scene::create();
    
    // 'layer' is an autorelease object
    auto layer = HelloWorld::create();

    // add layer as a child to scene
    scene->addChild(layer);

    // return the scene
    return scene;
}

// on "init" you need to initialize your instance
bool HelloWorld::init()
{
    //////////////////////////////
    // 1. super init first
    if ( !Layer::init() )
    {
        return false;
    }
    initDeviceMotion();
    schedule(CC_SCHEDULE_SELECTOR(HelloWorld::doStep));
    
    return true;
}

void HelloWorld::initDeviceMotion() {
    //////////////////////////////
    // Here is the usage of CMMotionManager in objective-c
    motionManager = [[CMMotionManager alloc] init];
    referenceAttitude = nil;
    
    CMDeviceMotion *deviceMotion = ((CMMotionManager*)motionManager).deviceMotion;
    CMAttitude *attitude = deviceMotion.attitude;
    referenceAttitude = [attitude retain];
    [(CMMotionManager*)motionManager startDeviceMotionUpdates];
}

void HelloWorld::doStep(float delta) {
    _count++;
    if (_count % 10 == 0) {
        CMDeviceMotion *deviceMotion = ((CMMotionManager*)motionManager).deviceMotion;
        CMAttitude *attitude = deviceMotion.attitude;
        cocos2d::log("gyro yaw: %f ", attitude.yaw);
    }
}

Notice in HelloWorld::initDeviceMotion I created a CMMotionManager to get the device attitude, which is used to get yaw value during frame update HelloWorld::doStep.