Feature Reference - The Game Maker's Companion: Game Development - page 355

It's time to deal with death... You might be tempted to simply use a Collision event and make
the explorer die upon contact with a scorpion, but in grid-based games, player characters
typically die when there is considerable overlap between the player sprite and his enemies,
sometimes only when they both fully occupy the same square. We could do this by changing the
collision masks, but it's just as easy to build a check in the collision event.
Dealing with Collisions
1. Create an object obj_hat and set the Sprite to spr_hat . We'll drop this hat where the
explorer dies.
2. Add a Create event and include an Align to Grid action with Snap hor and Snap vert
both set to 32 . This makes sure that wherever we drop the hat once the explorer dies, it
always aligns to the grid.
3. Double-click obj_explorer and add a Collision event for obj_scorpion . Include an
Execute Code action and insert the following lines:
1: {
2: if ( abs(x-other.x) <= 16 && abs(y-other.y) <= 16 )
3: {
4: instance_create(x,y,obj_hat);
5: with ( other ) instance_destroy();
6: instance_destroy();
7: }
8: }
Line 2 checks the horizontal and vertical distance between obj_explorer and its
colliding enemy. Only if they are both smaller than 16 (half of both sprites) do we
register the kill. The lower the values you use, the closer the instances must be
together, and If you set them at 0 then they must be lined up exactly. Lines 4-6 drop
the hat and kill both instances in the collision. It doesn't matter if the explorer was
halfway two squares as the hat drops itself neatly in the nearest square.
Result: Reference/Result/grid_movement3.gmk