Additional exercises – Sharpening the axe

1. Enemies get spawned at a slow, fixed rate. This can get a little boring because the difficulty never really increases. Make it so that enemies get spawned faster and faster after every round. For a simple way of doing this, you could follow the next steps:
   1. Add start_interval, end_interval, and time_delta as export variables to the EntitySpawner node. The start_interval variable will be the time we use between spawning entities at the start of the game, end_interval will be the final value, and time_delta is the increment at which we will go from the start_interval variable to the end_interval variable:

Figure 10.54 – The new exported variables for the EntitySpawner node

2. Now, track in a separate variable, _current_spawn_interval, the time for the next enemy to spawn. Set _current_spawn_interval equal to the start_interval variable at the start of the game. This variable replaces the old...