The Trees The Fork Oak Day38 - Block Manipulation

Implemented block dragging, falling, and intentional grid advancement in a big spaghetti mess

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Today I did a bunch of shoot-from-the-hip hacking to knock out a number of block manipulation gameplay features. I did them very fast and dirty in a giant update method, but I will do my best to describe what happened and I will definitely clean it up later. In particular, I implemented block dragging and swapping, block falling, matches, and the ability to intentionally advance the grid.

Although the code is a big mess, the exercise of moving quickly and cleanup later helped me wrap my head around all of the moving parts. Now that I have a better grasp on things I am more able to cleanup my spaghetti mess in the coming dailies.

Block State Overview

To manage a little bit of all the complexity, I added another "enum" similar to the one I created for the block type to specify all of the possible block states.

export const state = {
  WAITING: "Waiting",
  SPAWNING: "Spawning",
  DRAGGING: "Dragging",
  FALLING: "Falling",
  MATCHED: "Matched",
  CLEARING: "Clearing"

I haven't implemented all of the different states yet, but these roughly match my expectations for a given block. When a block is in the spawn row and has not quite been activated, it will take on the Spawning state. After passing the bottom of the grid, it will transition into the Waiting state which means it is ready to be acted upon. From there if the player were to tap and drag the block, it will move to the Dragging state and will move with the mouse cursor. Finally if the block finds itself over an empty gap, it will move to the Falling state and slide down the grid until it finds a block to rest upon.


Dragging a block has 3 main parts. First, it has to be visibly obvious that the block has been picked up. Second, the block must return to a valid state when dropped. And third, the other blocks must swap out of the way as the dragged block is moved over them.

I solved the first requirement by adding a simple grow and shrink animation. If the block is in the Dragging state, I linearly animate the current scale toward a particular larger constant. When not in the dragging state, I linearly animate the scale back to 1. Since the animation is linear, I don't need to know how long it has been since it started and the animation can be started and stopped at any point.

animateBlockSize() {
  if (this.state === state.DRAGGING) {
    if (this.scale < pickedUpScale) {
      this.scale += scaleVelocity;
    } else {
      this.scale = pickedUpScale;
  } else {
    if (this.scale > 1) {
      this.scale -= scaleVelocity;
    } else {
      this.scale = 1;

Then to actually do something, I modified the render function to utilize the scale. At the same time I also modified the z value of the image position to draw the block above the other blocks when grabbed.

render() {
  let { center, dimensions } = this.calculateLocation();

  let opacity = this.calculateOpacity(center.y);
  let tint = new Color(1, 1, 1, opacity);

  if (this.state === state.DRAGGING) {
    center = center.withZ(10);
  dimensions = dimensions.multiply(this.scale);

  image(blockImages[this.type], center, dimensions, 0, tint,;

Grab Animation

Before actually implementing the drag code, I wrote the logic that causes blocks to return to their correct location after moving off axis. To do this I split the block position into gridPosition and gridSlot variables where the gridPosition would contain the current block animation position and the gridSlot would contain the current integer grid location in block coordinates. Animating back into the correct gridSlot was as simple as applying difference fraction trick I outlined here. The basic idea is to modify the current position by the difference between the desired position and the current position times some fraction. This results in a smooth jump to the correct location which slows down as it gets close.

if (this.state !== state.DRAGGING) {
  // Bounced Back to Grid
  this.gridPosition.x += (this.gridSlot.x - this.gridPosition.x) * settleVelocity;

The third and most complicated requirement was solved in a big nested if statement. I'm not super proud of the code at the moment and will likely try to clean it up soon, but I will attempt to describe the current state of things. The dragging functionality depends on the state of these variables: touchDown/touchStarted, heldBlock and this.state. The touch status is used to decide when to start or stop dragging, heldBlock is used to keep track of which block to move around, and the block state is used to start and stop the drag animations.

To initiate a drag, no block must be currently held, the block being touched must be in the Waiting state, and the touch must have started on the current frame.

if (touchDown) {
  if (heldBlock == null) {
    // Start Dragging
    if (this.state === state.WAITING && touchPosition.within(center, dimensions) && touchStarted) {
      this.state = state.DRAGGING;
      this.dragOffset = touchPosition.subtract(center);
      heldBlock = this;

Upon starting a drag, the state is updated to Dragging, the heldBlock is set, and the touch offset is recorded so that the block doesn't jump to the touch location.

After a block is being dragged, the x position must be updated to follow the touch location. Crucially though, the block must only move one block position at a time. At 60fps a block is very capable of being dragged across multiple locations in one frame allowing the player to skip over a location. Although potentially interesting as a way to add input difficulty, I don't want this feature for now, so I impose a 0.9 block length speed limit.

The block gridSlot.x is set to the rounded gridPosition.x value. If the gridSlot.x is updated, then any block in the new position is swapped to the previous grid position. Since the bounce animation is already setup above, this doesn't require anymore managing.

Lastly the x position is locked between 0 and 5 in order to prevent blocks from moving off of the grid.

} else if (heldBlock === this) {
  // Handle Dragged Block
  let previousSlotX = this.gridSlot.x;

  // correctedTouchX accounts for the initial touch offset
  let correctedTouchX = touchPosition.x - this.dragOffset.x - blockWidth / 2;
  let gridLeft = gridCenter.x - gridDimensions.width / 2;
  let newGridXTarget = (correctedTouchX - gridLeft) * gridBlockDimensions.width / gridDimensions.width;

  let diff = newGridXTarget - this.gridPosition.x;
  if (diff > maxDragVelocity) diff = maxDragVelocity;
  if (diff < -maxDragVelocity) diff = -maxDragVelocity;
  this.gridPosition.x += diff;

  if (this.gridPosition.x < 0) this.gridPosition.x = 0;
  if (this.gridPosition.x > 5) this.gridPosition.x = 5;

  this.gridSlot.x = Math.round(this.gridPosition.x);

  if (previousSlotX !== this.gridSlot.x) {
    let blockInTheWay = blocks[this.gridSlot.y][this.gridSlot.x];
    blocks[this.gridSlot.y][this.gridSlot.x] = this;
    blocks[this.gridSlot.y][previousSlotX] = blockInTheWay;

    if (blockInTheWay) {
      blockInTheWay.gridSlot.x = previousSlotX;

To wrap up the dragging animation, the block state is set to Waiting if the touch is no longer active. This will cause the block to stop being managed by the touch location and bounce back into the current grid slot.

} else if (this.state === state.DRAGGING) {
  // Stop Dragging
  this.state = state.WAITING;
  heldBlock = null;

The combination of the above results in this satisfying animation:



Falling blocks was somewhat frustrating to get right. My initial implementation did not manage the block state properly and allowed blocks to get caught in a half way state between grid locations. I think at this point I have ironed out the kinks, but it was frustrating to say the least.

To start out I detect if a block should be falling by looking up grid slot below the block.

handleFalling(center, dimensions) {
  if (this.state !== state.SPAWNING && !blocks[this.gridSlot.y + 1][this.gridSlot.x]) {
    this.state = state.FALLING;
    if (heldBlock === this) heldBlock = null;

Then each frame a block is falling, the position is updated by the falling speed, the new grid position is checked to see if the falling can stop, and if not, the grid structure is updated to reflect the new block position.

if (this.state === state.FALLING) {
  this.gridPosition.y += fallSpeed;
  if (this.gridPosition.y > this.gridSlot.y && blocks[this.gridSlot.y + 1][this.gridSlot.x]) {
    this.gridPosition.y = this.gridSlot.y;
    this.state = state.WAITING;
  } else {
    let previousSlot = this.gridSlot.clone();
    this.gridSlot.y = Math.ceil(this.gridPosition.y);
    if (!previousSlot.equals(this.gridSlot)) {
      blocks[previousSlot.y][previousSlot.x] = null;
      blocks[this.gridSlot.y][this.gridSlot.x] = this;

Combined, these result in this animation:


Intentional Grid Advancement

Finally to speed things along when the grid is moving too slowly, I added a feature where the user can click on a spawning block to force it to spawn immediately jumping the grid up. The action must be initiated on the block itself, but the position modification must happen in the grid module, so I created a simple function in grid.js which increments the blockAdvancement by 1,

export function intentionalAdvance() {
  blockAdvancement = Math.floor(blockAdvancement) + 1;

And then added a check in block.update to call the function when tapped and spawning.

if (this.state === state.SPAWNING && touchPosition.within(center, dimensions) && touchStarted) {

And with that, the update function is all caught up and documented:

update() {
  let { center, dimensions } = this.calculateLocation();

  this.handleDragging(center, dimensions);
  this.handleFalling(center, dimensions);

  if (this.state === state.SPAWNING && touchPosition.within(center, dimensions) && touchStarted) {

As it turns out, the act of documenting the code forced me to explain it and rewrite portions to be more understandable. I'm still not super pleased with the overall structure as it feels somewhat unprincipled, but its way better than it was. Yay rubber ducky programming!


I also spent some time working on getting a basic match recognition routine running. I haven't built the clear animations yet though, so I will save the details for a future post; this one is long enough as is. As a teaser though, here is a gif of me playing the current version of the game. Its so satisfying to reach the playable stage of a game. It motivates me to push it further!


As always, the current version of the game as it exist in real time (not post time) can be found here.

Till tomorrow!