flauwekeul / workshop-rxjs-breakout

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Create Breakout! with RxJS

A workshop in vanilla TypeScript.

Getting started

git clone git@github.com:flauwekeul/workshop-rxjs-breakout.git
cd workshop-rxjs-breakout
npm install
  1. Clone the project from GitHub: https://github.com/flauwekeul/workshop-rxjs-breakout
  2. cd into the folder
  3. Install dependencies


This project uses Snowpack. Run npm start to start a dev server and http://localhost:8080/ will be opened automatically. You can use types, settings and utils from the shared folder. Take care not to import from the finished folder as that would make this workshop too easy 😉.

Theory and exercises

Slides with theory are made using slidev. Show the slides with npm run slides or see them online.

After the slides, some exercises can be done by writing code in the exercises folder. To execute an exercise, run npx ts-node exercises/01.ts (replace 01.ts with whatever file you want executed). A message shows whether you wrote the expected code.

Finished game

With the dev server running, go to http://localhost:8080/finished/ (including the trailing slash unfortunately) to see what you'll be making. Or see it online.


Creating the game

9 steps in (more or less) increasing difficulty.

Step 1: Render a paddle that "follows" the mouse

  1. Change createPaddleStream() in paddle.ts so that it returns an observable that emits the mouse's x position and the (static) paddle's y position.
  2. Change renderPaddle() in paddle.ts to use the drawRectangle() util and PADDLE_WIDTH, PADDLE_HEIGHT and PADDLE_COLOR settings (from shared/settings.ts).
  3. Subscribe to the observable returned from createPaddleStream() in index.ts. Also import renderPaddle() and use it to draw the paddle on screen.
  4. Use canvasContext.clearRect(0, 0, canvas.width, canvas.height) before rendering start with a "clean slate" on each frame.
  5. Use the clamp() util to prevent the paddle to go off screen (note that it returns a function).
  6. Optional: make the paddle start in the middle of the screen (before any mouse events have fired).

Step 2: Place a ball on the paddle

  1. Change createBallStream() in ball.ts so that it wraps the initial ball in an Observable and returns it.
  2. Subscribe to both the paddle$ and the ball$ observables in index.ts; choose a suitable creation operator.
  3. Before any rendering happens, update the ball's x position to the paddle's center top position on each emit. Do this by mutating ball in-place (I know this isn't pure, but it's very performant and simple). Use the centerTopOfPaddle() util.
  4. Change renderBall() in ball.ts to use drawCircle() (from shared/utils.ts) and BALL_COLOR (from shared/settings.ts) to render the ball. Import renderBall() in index.ts to draw the ball on screen.

Step 3: Detach the ball on click

  1. Change createBallStream() so that it starts listening to a single mouse click event, that's mapped to a ball object with a speed set to BALL_INITIAL_SPEED.
  2. Notice that nothing is rendered until a click event happens. How come? Fix it.
  3. If the ball's speed > 0 (after a click), it should leave the paddle and move up. Check the ball's speed in index.ts before its position is updated. If speed > 0 use the nextBallPosition() util to update the ball's position, else: use the centerTopOfPaddle() util as you already did.
  4. Notice that the ball now only moves when the mouse moves. Why is this? Try to figure this out before moving on.
  5. Fix it by making ticks$ an observable that emits every TICK_INTERVAL (use the correct operator). Combine this observable into your "paddle and ball" stream. Also pass a scheduler to the operator that internally uses requestAnimationFrame(). Do you know why?
  6. Notice that the ball now moves faster when the mouse moves. How come? Fix it by limiting the stream's "throughput" (the amount of events per unit of time). Start by looking for the correct operator, then where to use it. Use the same scheduler as you did to create ticks$.
  7. Notice that each time you click, the ball position is reset to that of the paddle. This shouldn't happen. Fix it by only taking a single click event.
  8. Optional: add the CSS class hide-cursor to the canvas when the ball speed > 0, remove the class otherwise.

Step 4: Make the ball bounce

  1. When the ball's speed > 0 and it's touching or passed the "ceiling" (top of screen), the ball's upward motion should become a downward motion. Use the hasBallTouchedTop() util and when it returns true, this code flips the ball's vertical motion: ball.direction = ball.direction * -1 + 180. Would it better to do this before or after the ball's position is updated?
  2. Similarly, when the ball touches or passes the sides of the screen, its horizontal motion should be "flipped". Use the hasBallTouchedSide() util and simply invert the ball's direction to make it bounce off the walls (ball.direction *= -1).
  3. Then the ball needs to bounce off the paddle when it hits. Flip the ball's vertical motion when the hasBallTouchedPaddle() returns true.
  4. If may want to refactor your code in index.ts now. E.g.: move the rendering of entities to a separate function and rearrange your conditionals to reduce nesting.
  5. Optional: give the player more control over the ball by changing its direction depending on where the ball hits the paddle. If the ball hits near the left of right edge of the paddle, it should go West or East. If the ball hits the center of the paddle, it should go North. For this linear interpolation is needed and there's a util called lerp() for that. First pass it the boundaries (use FAR_LEFT_BOUNCE_DIRECTION and FAR_RIGHT_BOUNCE_DIRECTION), that returns a function that needs a value between 0 and 1 and will return the new ball direction. The value between 0 and 1 is the normalized x position where the ball hits the paddle (because that determines the new direction). To get this normalized value use (ball.x - paddle.x) / PADDLE_WIDTH.

Step 5: Add blocks

  1. Change createBricksStream() in bricks.ts so that it calls the brickBuilder() util. It returns a function that accepts a column and row which returns a brick.
  2. Create a "wall of bricks" by looping from 0 to BRICK_ROWS (rows) and inside that loop from 0 to BRICKS_PER_ROW (cols). Bonus points if you can keep it declarative (without using for loops, but using Array methods instead). The result should be a flat array of bricks wrapped in an observable.
  3. Change renderBricks() in bricks.ts to use the drawRectangle() util and BRICK_COLOR_MAP and BRICK_STROKE_COLOR settings to render each brick. Note that BRICK_COLOR_MAP is an object; to set fill use BRICK_COLOR_MAP[brick.color]. Also, you may want to set strokeWidth to 3 so that each brick has some spacing around it.
  4. Import renderBricks() in index.ts to draw the bricks on screen.

Now may be a good time to learn about Subjects!

Step 6: Break bricks when the ball collides

  1. Use the getBrickCollision() util to determine if the ball has collided with a brick. Do this where you also check for wall and paddle collisions. When there's a collision getBrickCollision() returns an object with two props:
    • brickIndex: the index of the collided brick
    • hasCollidedVertically: whether the collision took place on the vertical sides of a brick. If this is false, the collision was on the top or bottom of a brick. The ball needs to change direction differently depending on this value.
  2. When the ball hits a brick, change the ball's direction similar to when the ball hits the left or right screen sides or when the ball hits the top of the screen. Also increase the ball's speed by multiplying it with BALL_SPEED_INCREASE.
  3. When the ball hits a brick, the brick should be removed. First rename createBricksStream() to createBricksSubject() and make it return a Subject (what kind?) instead of an observable. A new array of bricks (with the collided brick removed) can now be send to bricks$.

Step 7: Miss the paddle, reset the ball

  1. Use the hasBallPassedPaddle() util in index.ts to determine if the ball moved below the paddle. When this happens, return from the function because the ball needs to be reset and all the collision detection can be skipped.
  2. To reset the ball, start by renaming createBallStream() to createBallSubject(), comment-out the existing code and make it create and return a BehaviorSubject with initialBall as initial value instead.
  3. Obviously, clicking now won't make the ball leave the paddle anymore. To fix this again, a ball with a speed > 0 should be emitted to the BehaviorSubject when the user clicks, but only if the ball's speed is currently 0. Emitting a value to a Subject is as simple as using the Subject as an observer to an observable. We already have the Subject, our commented-out code is (nearly) the observable we need. So, "un-comment" the commented-out code and subscribe the BehaviorSubject to it.
  4. To reset the ball, the BehaviorSubject needs to be send the initial ball (which has a speed of 0). Do this in index.ts where hasBallPassedPaddle() returns true. See what happens when the ball now passes the paddle. Why isn't the ball being reset? It has to do with the take() operator, if that's removed, the ball is reset, but now the ball is reset on every click. Fix this problem with an operator that doesn't complete the stream (like take() does).
  5. Do you still need the startWith() operator?

Step 8: Keeping score

  1. Change createScoreSubject() in score.ts so that it simply returns a Subject (what kind?) with the score that's passed to it.
  2. Change renderScore() in score.ts to show the score on screen. Use the drawText() util (and optionally formatNumber() to format the score). Call renderScore() in index.ts.
  3. Make sure to update the score by adding BRICK_SCORE when a brick is "popped".

Step 9: Add lives and game over

  1. Change createLivesSubject() in lives.ts so that it simply returns a Subject (what kind?) with the amount of lives that's passed to it.
  2. Change renderLives() in lives.ts to show the amount of lives on screen. Use the drawText() util. Call renderLives() in index.ts.
  3. In index.ts, when the ball passes the paddle, use the lives$ Subject to update the amount of lives.
  4. The ball is now always reset, even if there aren't any lives left. Fix this.
  5. See what happens when the final life is lost. Let's fix this by completing the main stream when lives === 0. (or put another way: keep the stream going while lives > 0).
  6. The completion of the main stream can be used to display a "Game over" message. Use the drawGameOver() util for this.

Step 10: …?

This concludes the workshop, but there's plenty more to do of course! Some suggestions:

  • Some buttons to pause/restart the game.
  • Actually keeping score (e.g. in localStorage)
  • Power-ups (that randomly fall from popped bricks)
  • Keyboard controls
  • Fancy graphics with animations
  • Sounds
  • Multiplayer


License:MIT License


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