Platformer

A coin disappears, the score climbs, the door opens, and a stomp turns a walking enemy into a defeated one. The Platformer sample gives you a complete minute of play, then lets you inspect each decision as ordinary C# and as a Round Trip graph.

The Platformer level in Play Mode: coins, a patrolling enemy, and the goal door at the end of the run.

Play it in one minute

1 minute

Scene

Assets/Platformer/Scenes/Platformer.unity

Select

Player in the Hierarchy

Controls

Arrows or A/D move, Space jumps

Goal

Collect coins, stomp enemies, reach the goal

Teaches

• Guard clauses
• Call a neighbor
• Switch on a state
• Count and threshold

What this game teaches

• Guard clauses stop duplicate collection and missing collaborators before they change state. They also prevent repeat stomps.
• Calls make collaborator boundaries visible: the player calls the coin and score counter, then the HUD, motor and audio service.
• Loops scan collision contacts to decide whether the player is standing on the ground.
• Branches choose score and presentation paths when references are present.
• Switch maps EnemyState to patrol and collider behavior, plus the matching visual behavior.
• GET and SET show where tunables and current state enter each decision.

Figure 1. The playable level has coins and patrolling enemies, together with a HUD and the goal door.

Open it in Round Trip

1. Open unity/RoundTrip_Platformer in Unity 6000.5.x, then open Assets/Platformer/Scenes/Platformer.unity.
2. In the Project window, right-click Assets/Platformer/Runtime.
3. Choose Assets > Round Trip > Add Folder to Round Trip.
4. Open Tools > Round Trip > Graph View.
5. Pick PlayerController, then choose CollectCoin(Coin) from the method list.

You should see

The Explorer lists the Platformer runtime classes. CollectCoin opens as a graph with two guards followed by calls into the coin and score counter, then the HUD and optional audio service.

Graph versus code

Method to open	What the graph makes easy to see	What to notice in C#
PlayerController.CollectCoin(Coin)	Guard exits followed by four collaborator calls.	The coin comes from the trigger. Score and HUD are scene references; audio is optional.
PlayerController.StompEnemy(Enemy)	A guarded state change, bounce, then nested score and HUD branches.	LevelRules.stompScoreValue supplies the score amount.
PlayerMotor.UpdateGrounded(Collision2D)	A for loop checks each contact and returns after the first upward-facing match.	The grounding threshold separates floor contacts from walls.
ScoreCounter.OnCoinCountChanged()	Missing-reference guard, threshold decision, and call to Door.Unlock.	The threshold comes from LevelRules; the pure CoinUnlockRule owns the decision.
EnemyStateMachine.ApplyState()	One switch arm per enemy state.	Each arm calls small helpers for patrol, collision, and visuals.

Trace one coin

CollectCoin starts with the collision-provided Coin. The first guard rejects null or already collected coins. The second guard checks the scene-bound score counter and HUD. Only then does the method call coin.Collect(), add one coin, refresh the labels, and ask optional audio to play.

That order is the lesson. Each card names one neighbor, and the wires show which values cross the boundary. The graph does not invent a second behavior model; it is the method body you can open in your editor.

Exercises

Raise the stomp score

Select Assets/Platformer/Config/LevelRules.asset and change stompScoreValue from 25 to 50. Enter Play Mode, stomp one enemy, and read the score. StompEnemy passes that field directly to AddScore.

Read the stomp before playing it

Open StompEnemy, predict which branch runs when _hud is missing, then read the C# beside the graph. The enemy still changes state and score is still awarded when the score counter exists; the missing HUD produces a warning instead of cancelling the stomp.

Follow one state

Open EnemyStateMachine.ApplyState and trace the Defeated arm. It stops patrol, disables the body collider, and applies the defeated visual in that order.

The same game, rebuilt without writing code

The repository also holds unity/Platformer_Tutorial: the same platformer being rebuilt on camera, one episode at a time, with the methods built on the Round Trip canvas instead of typed. It contains only what the published episodes have built so far, starting with LevelRules, PlayerMotor and the composition root, plus script-free prefabs for the tiles, player, coin, enemy and goal door to build from. Use the finished RoundTrip_Platformer when you want to read complete methods; use the tutorial project when you want to follow the build.

Next

• Platformer graph tour
• Project layout
• Read a class you did not write
• Common shapes