Acceptance Tests

Extreme programming uses testing two ways: Customers develop acceptance tests that determine the overall behavior of the system, and Programmers create unit tests while programming.

As a Customer, you'll create tests for each story you request. You'll measure the progress in developing the system by running these tests.

目標: 偷竊物理模擬小遊戲.

物理模擬部分，已經抽出「合適」的 interfaces，implementor 採用 Box2D，為了「證明」Box2D 滿足遊戲 (customer) 需求，自然要有幾個簡單的小測試。這種測試就稱為 acceptance tests。

第一個測試就是自由落體 (freefall):

TEST(Box2d, Freefall) {
  Service* service = new Box2dService();

  // Construct a world, which will hold and simulate the rigid bodies.
  WorldConfig world_config;
  world_config.gravity.set(0.0f, -10.0f);
  World* world = service->createWorld(world_config);

  // Config ground body.
  BodyConfig ground_config;
  ground_config.position.set(0.0f, -10.0f);
  Body* ground = world->createBody(ground_config);

  // Config ground body fixture.
  PolygonShapeConfig ground_shape_config;
  ground_shape_config.setAsBox(50.0f, 10.0f);

  FixtureConfig ground_fixture_config;
  ground_fixture_config.shape_config = &ground_shape_config;
  ground_fixture_config.density = 0.0f;
  ground->createFixture(ground_fixture_config);

  // Config dynamic body.
  BodyConfig dynamic_config;
  dynamic_config.type = BodyConfig::Dynamic;
  dynamic_config.position.set(0.0f, 4.0f);
  Body* dynamic = world->createBody(dynamic_config);

  // Config dynamic body fixture.
  PolygonShapeConfig dynamic_shape_config;
  dynamic_shape_config.setAsBox(1.0f, 1.0f);

  FixtureConfig dynamic_fixture_config;
  dynamic_fixture_config.shape_config = &dynamic_shape_config;
  dynamic_fixture_config.density = 1.0f;
  dynamic_fixture_config.friction = 0.3f;
  dynamic->createFixture(dynamic_fixture_config);

  // Simulate.
  const float time_step = 1.0f/60.0f;
  const int steps = 60;
  const float expected[] = {
      3.997222f, 3.991667f, 3.983333f, 3.972222f, 3.958333f, 3.941667f, 
      3.922222f, 3.900000f, 3.875000f, 3.847222f, 3.816667f, 3.783333f, 
      3.747222f, 3.708333f, 3.666667f, 3.622222f, 3.575000f, 3.525000f, 
      3.472222f, 3.416667f, 3.358333f, 3.297222f, 3.233333f, 3.166667f, 
      3.097222f, 3.025000f, 2.950000f, 2.872222f, 2.791667f, 2.708333f, 
      2.622222f, 2.533333f, 2.441667f, 2.347222f, 2.250000f, 2.150000f, 
      2.047222f, 1.941667f, 1.833333f, 1.722222f, 1.608334f, 1.491667f, 
      1.372223f, 1.250000f, 1.125000f, 1.014582f, 1.014651f, 1.014708f, 
      1.014756f, 1.014796f, 1.014830f, 1.014858f, 1.014881f, 1.014900f, 
      1.014917f, 1.014930f, 1.014942f, 1.014951f, 1.014959f, 1.014966f
  };
  EXPECT_EQ(steps, arraysize(expected));

  for (int i = 0; i < steps; ++i) {
    world->simulate(time_step);
    EXPECT_TRUE(fabs(dynamic->position().x()) < 0.001f);
    EXPECT_FLOAT_EQ(expected[i], dynamic->position().y());
    EXPECT_TRUE(fabs(dynamic->angle()) < 0.001f);
  }

  // Clean up.
  delete world;
  delete service;
}

程式碼有點長，偷自 Box2d's HelloWorld，哈哈，天下文章一般抄。若要比較 float/double 大小，EXPECT_FLOAT_EQ() macro 是一個很好的選擇。總之，此 acceptance test 沒過，我們必須讓他過。(只貼有意思的部分)

// box2d_world.cc
Body* Box2dWorld::createBody(const BodyConfig& config) { 
  b2BodyDef config_impl;
 
  config_impl.position.Set(config.position.x(), config.position.y());
  config_impl.angle = config.angle;
  config_impl.linearVelocity.Set(config.linear_velocity.x(),
                                 config.linear_velocity.y());
  config_impl.angularVelocity = config.angular_velocity;
  config_impl.linearDamping = config.linear_damping;
  config_impl.angularDamping = config.angular_damping;
  if (config.type == BodyConfig::Static) {
    config_impl.type = b2_staticBody;
  } else if (config.type == BodyConfig::Dynamic) {
    config_impl.type = b2_dynamicBody;
  } else if (config.type == BodyConfig::Kinematic) {
    config_impl.type = b2_kinematicBody;
  }
  config_impl.active = config.is_active;

  return new Box2dBody(itself_->CreateBody(&config_impl));
}

---

// box2d_body.cc
void Box2dBody::createFixture(const FixtureConfig& config) {
  if (config.shape_config->type == ShapeConfig::Polygon) {
    PolygonShapeConfig* shape 
        = dynamic_cast< PolygonShapeConfig* >(config.shape_config);

    b2PolygonShape shape_impl;
    shape_impl.m_vertexCount = static_cast(shape->vertices.size());
    for (int i = 0; i != shape_impl.m_vertexCount; ++i) {
      shape_impl.m_vertices[i].Set(shape->vertices[i].x(), 
                                   shape->vertices[i].y());
      shape_impl.m_normals[i].Set(shape->normals[i].x(), 
                                  shape->normals[i].y());
    }
    shape_impl.m_centroid.Set(shape->centroid.x(), shape->centroid.y());

    b2FixtureDef config_impl;
    config_impl.shape = &shape_impl;
    config_impl.friction = config.friction;
    config_impl.restitution = config.restitution;
    config_impl.density = config.density;

    itself_->CreateFixture(&config_impl);
  }
}

---

偷加一個 method 給 PolygonShapeConfig:

void PolygonShapeConfig::setAsBox(float half_x, float half_y) {
  vertices.push_back(math::Vector2(-half_x, -half_y));
  vertices.push_back(math::Vector2( half_x, -half_y));
  vertices.push_back(math::Vector2( half_x,  half_y));
  vertices.push_back(math::Vector2(-half_x,  half_y));

  normals.push_back(math::Vector2( 0.0f, -1.0f));
  normals.push_back(math::Vector2( 1.0f,  0.0f));
  normals.push_back(math::Vector2( 0.0f,  1.0f));
  normals.push_back(math::Vector2(-1.0f,  0.0f));

  centroid.setZero();
}

跑一跑居然過了，太神奇了傑克。這就是一個簡單的 acceptance test，謝謝大家收看。