the boss!

systems/boss/boss_phase.gd

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+class_name BossPhase
+extends Node
+
+
+signal started()
+signal ended()
+signal damaged(damage: int)
+
+
+## Amount of HP this boss stage has. If 0, the boss is invulnerable during
+## this phase and must survive until time runs out or [method end_phase] is called.
+@export_range(0, 1, 1, "or_greater") var hp: int = 0
+## "Survival Time" of the phase. The phase will end when time is up, whether
+## HP has been depleted or not.
+@export_range(0.0, 1.0, 0.01, "or_greater", "suffix:s") var time: float = INF
+
+## The phase to automatically proceed to when this phase ends.
+@export var next_phase: BossPhase
+
+
+## [code]true[/code] if the phase is currently active.
+var active: bool = false
+
+
+var hp_left: int:
+	set(value):
+		hp_left = value
+		if active and hp_left <= 0 and hp > 0:
+			end_phase()
+
+var time_left: float = INF:
+	set(value):
+		time_left = value
+		if active and time_left <= 0.0:
+			end_phase()
+
+
+func _physics_process(delta: float) -> void:
+	if active:
+		time_left -= delta
+
+
+func start_phase() -> void:
+	if not active:
+		hp_left = hp
+		time_left = time
+		active = true
+		started.emit()
+
+
+func end_phase() -> void:
+	if active:
+		active = false
+		ended.emit()
+		if next_phase:
+			next_phase.start_phase()
+
+
+func hurt(damage: int) -> void:
+	if active:
+		hp_left = maxi(0, hp_left - damage)
+		damaged.emit(damage)