missile code with impulse

week2/exercise1_cannon.md

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+## Exercise 1: Projectiles
+
+There are several ways we can create projectiles in Godot. To choose the right one we need to think about how the projectiles should behave. Consider the following:
+
+* Should there be a limited number of projectiles?
+* What happens to projectiles that leave the viewing area?
+* Should shots ricochet?
+* Do projectiles need momentum, acceleration, deceleration?
+
+Depending on what we need the projectiles to do, we could choose a body type for them.
+
+### Area Node Bullet
+
+The area node is probably the simplest way to achieve a projectile. Areas can receive signals when a child collision shape intersects another collision shape (so they know when they have hit something).
+
+An area node also inherits a Transform, with a position property that can be written to.
+
+```
+extends Area2D
+
+var speed = 750
+
+func setSpeed(speedVal):
+	speed = speedVal
+	
+func _physics_process(delta):
+	position += transform.x * speed * delta
+
+func _on_body_entered(body):
+	# blast the body?
+	if body.is_in_group("boxes"):
+		body.queue_free()
+		queue_free()
+```
+
+Now to use our bullet we need to be able to create the bullet and position it with code. This is called instantiation.
+
+To be able to instantiate a scene in Godot we must pre-load it.
+
+`var bullet = preload("res://scenes/bullet.tscn")`
+
+We can now create as many isntances of the bullet as we wish!
+
+But be careful! When we add our bullet to the world, we want to add it at the global scope - not at the scope of the character. Otherwise, when the character moves the bullet will move with them.
+
+```
+if Input.is_action_just_pressed("blast"):
+		var mybullet = bullet.instantiate()
+		mybullet.setSpeed(-BLAST_SPEED if faceLeft else BLAST_SPEED)
+		owner.add_child(mybullet)
+		mybullet.transform = markerLeft.global_transform if faceLeft else markerRight.global_transform
+```
+
+### RigidBody Missile
+
+For our next example, we will create our projectile from a RigidBody so that it can react via physics to the environment. 
+
+One thing to note about RigidBodies is that they do not by default hear collision signals. This is a performance consideration, and usually this is a good thing, but in the case of our missile we want the RigidBody to know about the collision.
+
+For this reason in the inspector we will need to turn on collision_monitor and set a value in max_collision_objects. After this, we will be able to detect collisions.
+
+```
+extends RigidBody2D
+
+func _on_body_entered(body):
+	# check here to see if the thing should explode
+	print("missile hit something")
+	
+	if body.is_in_group("boxes"):
+		body.queue_free()
+	if not body.is_in_group("player"):
+		queue_free()
+```
+
+As before, we need to think about instantiation of the missile. Depending on the direction the player is facing, we could either apply a negative or a positive impulse to the RigidBody.
+
+```
+if Input.is_action_just_pressed("blast"):
+		var myMissile = missile.instantiate()
+		owner.add_child(myMissile)
+		myMissile.transform = markerLeft.global_transform if faceLeft else markerRight.global_transform
+		var missileForce = -600 if faceLeft else 600
+		myMissile.apply_impulse(Vector2(missileForce,10), Vector2(0, 0))
+```