Godot 4 源码分析 - Path2D与PathFollow2D

学习演示项目dodge_the_creeps，发现里面多了一个Path2D与PathFollow2D

 研究GDScript代码发现，它主要用于随机生成Mob

	var mob_spawn_location = get_node(^"MobPath/MobSpawnLocation")
	mob_spawn_location.progress = randi()

	# Set the mob's direction perpendicular to the path direction.
	var direction = mob_spawn_location.rotation + PI / 2

	# Set the mob's position to a random location.
	mob.position = mob_spawn_location.position

	# Add some randomness to the direction.
	direction += randf_range(-PI / 4, PI / 4)
	mob.rotation = direction

	# Choose the velocity for the mob.
	var velocity = Vector2(randf_range(150.0, 250.0), 0.0)
	mob.linear_velocity = velocity.rotated(direction)

这个有这么大的作用，不明觉厉

但不知道如何下手

查看源码，有编辑器及类源码

先从应用角度，到B站上找找有没有视频，结果发现这个

Godot塔防游戏 - 01 -核心路径制作 Path2D_哔哩哔哩_bilibili

看了之后，就知道使用方法了：

• 添加Path2D
• 在编辑器中设置路径各关键点，形成路径

• 在Path2D下增加PathFollow2D

这就OK了。剩下的就是使用

所谓使用，输入为PathFollow2D的progress，输出为路径上的点信息(position, rotation...)，然后用户再根据这些信息去确定相应的属性

比如演示项目中，Path2D定制了一个外框路径(左上角 > 右上角 > 右下角 > 左下角 > 左上角)，在生成MOB时，随机指定其下的PathFollow2D的progress值为randi()，即为0 ~ 2^32 - 1的随机整数。因为路径是有长度的，本例中为2400，randi()值将按2400取模得到最终的随机值0 - 2399，当然也可以归一化，设置其progress_ratio值为0.0 - 1.0，意思一样。

查看源码，set_progress的逻辑不只是取模，还有限制范围。即PathFollow2D还有一个Loop属性，如果Loop为真，才会取模，为false时，会直接限制在路径长度范围内 progress = CLAMP(progress, 0, path_length); 之后统一更新_update_transform

void PathFollow2D::set_progress(real_t p_progress) {
	ERR_FAIL_COND(!isfinite(p_progress));
	progress = p_progress;
	if (path) {
		if (path->get_curve().is_valid()) {
			real_t path_length = path->get_curve()->get_baked_length();

			if (loop && path_length) {
				progress = Math::fposmod(progress, path_length);
				if (!Math::is_zero_approx(p_progress) && Math::is_zero_approx(progress)) {
					progress = path_length;
				}
			} else {
				progress = CLAMP(progress, 0, path_length);
			}
		}

		_update_transform();
	}
}

void PathFollow2D::_update_transform() {
	if (!path) {
		return;
	}

	Ref c = path->get_curve();
	if (!c.is_valid()) {
		return;
	}

	real_t path_length = c->get_baked_length();
	if (path_length == 0) {
		return;
	}

	if (rotates) {
		Transform2D xform = c->sample_baked_with_rotation(progress, cubic);
		xform.translate_local(v_offset, h_offset);
		set_rotation(xform[1].angle());
		set_position(xform[2]);
	} else {
		Vector2 pos = c->sample_baked(progress, cubic);
		pos.x += h_offset;
		pos.y += v_offset;
		set_position(pos);
	}
}

从PathFollow2D代码来看，它派生于Node2D，所以具备transform属性：Position、Rotation、Scale、Skew，对于路径上的点使用而言，这些信息就足够了，能够确定这些点的位置、方向，其实就是一个矢量 

Loop属性值的含义前面已明确，Rotates、Cubic、H Offsets、V Offsets都是在_update_transform中起作用，具体算法可以不深究。但lookahead没找到具体用处，感觉影响不大。

class PathFollow2D : public Node2D {
	GDCLASS(PathFollow2D, Node2D);

public:
private:
	Path2D *path = nullptr;
	real_t progress = 0.0;
	Timer *update_timer = nullptr;
	real_t h_offset = 0.0;
	real_t v_offset = 0.0;
	real_t lookahead = 4.0;
	bool cubic = true;
	bool loop = true;
	bool rotates = true;

	void _update_transform();

protected:
	void _validate_property(PropertyInfo &p_property) const;

	void _notification(int p_what);
	static void _bind_methods();

public:
	void path_changed();

	void set_progress(real_t p_progress);
	real_t get_progress() const;

	void set_h_offset(real_t p_h_offset);
	real_t get_h_offset() const;

	void set_v_offset(real_t p_v_offset);
	real_t get_v_offset() const;

	void set_progress_ratio(real_t p_ratio);
	real_t get_progress_ratio() const;

	void set_lookahead(real_t p_lookahead);
	real_t get_lookahead() const;

	void set_loop(bool p_loop);
	bool has_loop() const;

	void set_rotates(bool p_rotates);
	bool is_rotating() const;

	void set_cubic_interpolation(bool p_enable);
	bool get_cubic_interpolation() const;

	PackedStringArray get_configuration_warnings() const override;

	PathFollow2D() {}
};


void PathFollow2D::path_changed() {
	if (update_timer && !update_timer->is_stopped()) {
		update_timer->start();
	} else {
		_update_transform();
	}
}

void PathFollow2D::_update_transform() {
	if (!path) {
		return;
	}

	Ref c = path->get_curve();
	if (!c.is_valid()) {
		return;
	}

	real_t path_length = c->get_baked_length();
	if (path_length == 0) {
		return;
	}

	if (rotates) {
		Transform2D xform = c->sample_baked_with_rotation(progress, cubic);
		xform.translate_local(v_offset, h_offset);
		set_rotation(xform[1].angle());
		set_position(xform[2]);
	} else {
		Vector2 pos = c->sample_baked(progress, cubic);
		pos.x += h_offset;
		pos.y += v_offset;
		set_position(pos);
	}
}

void PathFollow2D::_notification(int p_what) {
	switch (p_what) {
		case NOTIFICATION_READY: {
			if (Engine::get_singleton()->is_editor_hint()) {
				update_timer = memnew(Timer);
				update_timer->set_wait_time(0.2);
				update_timer->set_one_shot(true);
				update_timer->connect("timeout", callable_mp(this, &PathFollow2D::_update_transform));
				add_child(update_timer, false, Node::INTERNAL_MODE_BACK);
			}
		} break;

		case NOTIFICATION_ENTER_TREE: {
			path = Object::cast_to(get_parent());
			if (path) {
				_update_transform();
			}
		} break;

		case NOTIFICATION_EXIT_TREE: {
			path = nullptr;
		} break;
	}
}

void PathFollow2D::set_cubic_interpolation(bool p_enable) {
	cubic = p_enable;
}

bool PathFollow2D::get_cubic_interpolation() const {
	return cubic;
}

void PathFollow2D::_validate_property(PropertyInfo &p_property) const {
	if (p_property.name == "offset") {
		real_t max = 10000.0;
		if (path && path->get_curve().is_valid()) {
			max = path->get_curve()->get_baked_length();
		}

		p_property.hint_string = "0," + rtos(max) + ",0.01,or_less,or_greater";
	}
}

PackedStringArray PathFollow2D::get_configuration_warnings() const {
	PackedStringArray warnings = Node::get_configuration_warnings();

	if (is_visible_in_tree() && is_inside_tree()) {
		if (!Object::cast_to(get_parent())) {
			warnings.push_back(RTR("PathFollow2D only works when set as a child of a Path2D node."));
		}
	}

	return warnings;
}

void PathFollow2D::_bind_methods() {
	ClassDB::bind_method(D_METHOD("set_progress", "progress"), &PathFollow2D::set_progress);
	ClassDB::bind_method(D_METHOD("get_progress"), &PathFollow2D::get_progress);

	ClassDB::bind_method(D_METHOD("set_h_offset", "h_offset"), &PathFollow2D::set_h_offset);
	ClassDB::bind_method(D_METHOD("get_h_offset"), &PathFollow2D::get_h_offset);

	ClassDB::bind_method(D_METHOD("set_v_offset", "v_offset"), &PathFollow2D::set_v_offset);
	ClassDB::bind_method(D_METHOD("get_v_offset"), &PathFollow2D::get_v_offset);

	ClassDB::bind_method(D_METHOD("set_progress_ratio", "ratio"), &PathFollow2D::set_progress_ratio);
	ClassDB::bind_method(D_METHOD("get_progress_ratio"), &PathFollow2D::get_progress_ratio);

	ClassDB::bind_method(D_METHOD("set_rotates", "enable"), &PathFollow2D::set_rotates);
	ClassDB::bind_method(D_METHOD("is_rotating"), &PathFollow2D::is_rotating);

	ClassDB::bind_method(D_METHOD("set_cubic_interpolation", "enable"), &PathFollow2D::set_cubic_interpolation);
	ClassDB::bind_method(D_METHOD("get_cubic_interpolation"), &PathFollow2D::get_cubic_interpolation);

	ClassDB::bind_method(D_METHOD("set_loop", "loop"), &PathFollow2D::set_loop);
	ClassDB::bind_method(D_METHOD("has_loop"), &PathFollow2D::has_loop);

	ClassDB::bind_method(D_METHOD("set_lookahead", "lookahead"), &PathFollow2D::set_lookahead);
	ClassDB::bind_method(D_METHOD("get_lookahead"), &PathFollow2D::get_lookahead);

	ADD_PROPERTY(PropertyInfo(Variant::FLOAT, "progress", PROPERTY_HINT_RANGE, "0,10000,0.01,or_less,or_greater,suffix:px"), "set_progress", "get_progress");
	ADD_PROPERTY(PropertyInfo(Variant::FLOAT, "progress_ratio", PROPERTY_HINT_RANGE, "0,1,0.0001,or_less,or_greater", PROPERTY_USAGE_EDITOR), "set_progress_ratio", "get_progress_ratio");
	ADD_PROPERTY(PropertyInfo(Variant::FLOAT, "h_offset"), "set_h_offset", "get_h_offset");
	ADD_PROPERTY(PropertyInfo(Variant::FLOAT, "v_offset"), "set_v_offset", "get_v_offset");
	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "rotates"), "set_rotates", "is_rotating");
	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "cubic_interp"), "set_cubic_interpolation", "get_cubic_interpolation");
	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "loop"), "set_loop", "has_loop");
	ADD_PROPERTY(PropertyInfo(Variant::FLOAT, "lookahead", PROPERTY_HINT_RANGE, "0.001,1024.0,0.001"), "set_lookahead", "get_lookahead");
}

void PathFollow2D::set_progress(real_t p_progress) {
	ERR_FAIL_COND(!isfinite(p_progress));
	progress = p_progress;
	if (path) {
		if (path->get_curve().is_valid()) {
			real_t path_length = path->get_curve()->get_baked_length();

			if (loop && path_length) {
				progress = Math::fposmod(progress, path_length);
				if (!Math::is_zero_approx(p_progress) && Math::is_zero_approx(progress)) {
					progress = path_length;
				}
			} else {
				progress = CLAMP(progress, 0, path_length);
			}
		}

		_update_transform();
	}
}

void PathFollow2D::set_h_offset(real_t p_h_offset) {
	h_offset = p_h_offset;
	if (path) {
		_update_transform();
	}
}

real_t PathFollow2D::get_h_offset() const {
	return h_offset;
}

void PathFollow2D::set_v_offset(real_t p_v_offset) {
	v_offset = p_v_offset;
	if (path) {
		_update_transform();
	}
}

real_t PathFollow2D::get_v_offset() const {
	return v_offset;
}

real_t PathFollow2D::get_progress() const {
	return progress;
}

void PathFollow2D::set_progress_ratio(real_t p_ratio) {
	if (path && path->get_curve().is_valid() && path->get_curve()->get_baked_length()) {
		set_progress(p_ratio * path->get_curve()->get_baked_length());
	}
}

real_t PathFollow2D::get_progress_ratio() const {
	if (path && path->get_curve().is_valid() && path->get_curve()->get_baked_length()) {
		return get_progress() / path->get_curve()->get_baked_length();
	} else {
		return 0;
	}
}

void PathFollow2D::set_lookahead(real_t p_lookahead) {
	lookahead = p_lookahead;
}

real_t PathFollow2D::get_lookahead() const {
	return lookahead;
}

void PathFollow2D::set_rotates(bool p_rotates) {
	rotates = p_rotates;
	_update_transform();
}

bool PathFollow2D::is_rotating() const {
	return rotates;
}

void PathFollow2D::set_loop(bool p_loop) {
	loop = p_loop;
}

bool PathFollow2D::has_loop() const {
	return loop;
}

从代码与用途来看，Path2D就没啥看头了，就负责提供一条曲线路径

class Path2D : public Node2D {
	GDCLASS(Path2D, Node2D);

	Ref curve;

	void _curve_changed();

protected:
	void _notification(int p_what);
	static void _bind_methods();

public:
#ifdef TOOLS_ENABLED
	virtual Rect2 _edit_get_rect() const override;
	virtual bool _edit_use_rect() const override;
	virtual bool _edit_is_selected_on_click(const Point2 &p_point, double p_tolerance) const override;
#endif

	void set_curve(const Ref &p_curve);
	Ref get_curve() const;

	Path2D() {}
};

#ifdef TOOLS_ENABLED
Rect2 Path2D::_edit_get_rect() const {
	if (!curve.is_valid() || curve->get_point_count() == 0) {
		return Rect2(0, 0, 0, 0);
	}

	Rect2 aabb = Rect2(curve->get_point_position(0), Vector2(0, 0));

	for (int i = 0; i < curve->get_point_count(); i++) {
		for (int j = 0; j <= 8; j++) {
			real_t frac = j / 8.0;
			Vector2 p = curve->sample(i, frac);
			aabb.expand_to(p);
		}
	}

	return aabb;
}

bool Path2D::_edit_use_rect() const {
	return curve.is_valid() && curve->get_point_count() != 0;
}

bool Path2D::_edit_is_selected_on_click(const Point2 &p_point, double p_tolerance) const {
	if (curve.is_null()) {
		return false;
	}

	for (int i = 0; i < curve->get_point_count(); i++) {
		Vector2 s[2];
		s[0] = curve->get_point_position(i);

		for (int j = 1; j <= 8; j++) {
			real_t frac = j / 8.0;
			s[1] = curve->sample(i, frac);

			Vector2 p = Geometry2D::get_closest_point_to_segment(p_point, s);
			if (p.distance_to(p_point) <= p_tolerance) {
				return true;
			}

			s[0] = s[1];
		}
	}

	return false;
}
#endif

void Path2D::_notification(int p_what) {
	switch (p_what) {
		// Draw the curve if path debugging is enabled.
		case NOTIFICATION_DRAW: {
			if (!curve.is_valid()) {
				break;
			}

			if (!Engine::get_singleton()->is_editor_hint() && !get_tree()->is_debugging_paths_hint()) {
				return;
			}

			if (curve->get_point_count() < 2) {
				return;
			}

#ifdef TOOLS_ENABLED
			const real_t line_width = get_tree()->get_debug_paths_width() * EDSCALE;
#else
			const real_t line_width = get_tree()->get_debug_paths_width();
#endif
			real_t interval = 10;
			const real_t length = curve->get_baked_length();

			if (length > CMP_EPSILON) {
				const int sample_count = int(length / interval) + 2;
				interval = length / (sample_count - 1); // Recalculate real interval length.

				Vector frames;
				frames.resize(sample_count);

				{
					Transform2D *w = frames.ptrw();

					for (int i = 0; i < sample_count; i++) {
						w[i] = curve->sample_baked_with_rotation(i * interval, false);
					}
				}

				const Transform2D *r = frames.ptr();
				// Draw curve segments
				{
					PackedVector2Array v2p;
					v2p.resize(sample_count);
					Vector2 *w = v2p.ptrw();

					for (int i = 0; i < sample_count; i++) {
						w[i] = r[i].get_origin();
					}
					draw_polyline(v2p, get_tree()->get_debug_paths_color(), line_width, false);
				}

				// Draw fish bones
				{
					PackedVector2Array v2p;
					v2p.resize(3);
					Vector2 *w = v2p.ptrw();

					for (int i = 0; i < sample_count; i++) {
						const Vector2 p = r[i].get_origin();
						const Vector2 side = r[i].columns[0];
						const Vector2 forward = r[i].columns[1];

						// Fish Bone.
						w[0] = p + (side - forward) * 5;
						w[1] = p;
						w[2] = p + (-side - forward) * 5;

						draw_polyline(v2p, get_tree()->get_debug_paths_color(), line_width * 0.5, false);
					}
				}
			}
		} break;
	}
}

void Path2D::_curve_changed() {
	if (!is_inside_tree()) {
		return;
	}

	if (!Engine::get_singleton()->is_editor_hint() && !get_tree()->is_debugging_paths_hint()) {
		return;
	}

	queue_redraw();
	for (int i = 0; i < get_child_count(); i++) {
		PathFollow2D *follow = Object::cast_to(get_child(i));
		if (follow) {
			follow->path_changed();
		}
	}
}

void Path2D::set_curve(const Ref &p_curve) {
	if (curve.is_valid()) {
		curve->disconnect("changed", callable_mp(this, &Path2D::_curve_changed));
	}

	curve = p_curve;

	if (curve.is_valid()) {
		curve->connect("changed", callable_mp(this, &Path2D::_curve_changed));
	}

	_curve_changed();
}

Ref Path2D::get_curve() const {
	return curve;
}

void Path2D::_bind_methods() {
	ClassDB::bind_method(D_METHOD("set_curve", "curve"), &Path2D::set_curve);
	ClassDB::bind_method(D_METHOD("get_curve"), &Path2D::get_curve);

	ADD_PROPERTY(PropertyInfo(Variant::OBJECT, "curve", PROPERTY_HINT_RESOURCE_TYPE, "Curve2D", PROPERTY_USAGE_DEFAULT | PROPERTY_USAGE_EDITOR_INSTANTIATE_OBJECT), "set_curve", "get_curve");
}

当然，也不是啥用处没有，比如动态指定路径的时候，就可以设置一条Curve2D，然后赋给Path2D，后面就照此行事。

比如，该演示项目中，

var curve = Curve2D.new()
curve.add_point(Vector2i(100, 100))
curve.add_point(Vector2i(400, 600))
$MobPath.curve = curve

然后，玩家呆在右上角，这就是那些MOB的死角，玩家可以活到把用户送走