On the Line – Rocket Through - Cocos2d-x by Example Beginner's Guide - page 170

Time for action - handling touches
We need to implement onTouchBegan , onTouchMoved , and onTouchEnded .
1. Now in GameLayer.cpp , inside onTouchBegan , add the following lines:
if (!_running) return true;
Point tap = touch->getLocation();
float dx = _rocket->getPositionX() - tap.x;
float dy = _rocket->getPositionY() - tap.y;
if (dx * dx + dy * dy <= pow(_rocket->getRadius(), 2)
) {
_lineContainer->setLineType ( LINE_NONE );
_rocket->setRotationOrientation ( ROTATE_NONE );
_drawing = true;
}
return true;
When a touch begins, we only need to determine whether it's touching the ship. If
it is, we set our _drawing property to true . This will indicate we have a valid
point (one that began by touching the _rocket sprite).
2. We clear any lines we may be currently drawing in _lineContainer by calling
setLineType( LINE_NONE ) , and we make sure _rocket will not rotate
until we have a pivot point by releasing _rocket (setRotationOrienta-
tion ( ROTATE_NONE )) , so it will continue to move on its current linear
trajectory (_vector ).
3. From here, we begin drawing a new line with the next onTouchMoved method.
Inside that method, we add the following lines:
if (!_running) return;
if (_drawing) {
Point tap = touch->getLocation();
float dx = _rocket->getPositionX() - tap.x;
float dy = _rocket->getPositionY() - tap.y;
if (dx * dx + dy * dy > pow (_minLineLength, 2)) {
_rocket->select(true);
_lineContainer->setPivot ( tap );