As mentioned in the previous game, explaining the mechanism of the gameplay loop will be better for you if you plan to work from scratch or to understand the current logic you've got with the book. With this said, let's break down the gameplay logic:
Once the game starts, all of the spawnPoint blueprints in the scene will keep spawning random enemy types as long as the player never reaches the targeted score or is still alive.
The mainChar blueprint will be able to receive inputs from the keyboard and the touch screen as long as the game is running. Once the player hits jump, this action will force the player controller to jump, while hitting fire will force him to stop and shoot some bullet blueprints.
While shooting, the player's amount of ammo keeps decreasing while the score increases if the bullets hit some enemies, and both are displayed through the uiText blueprint.
Finally, if the player falls through a gap or touches an enemy, the level will restart; whereas, if...
