115 lines
3.8 KiB
Markdown
115 lines
3.8 KiB
Markdown
*| [JavaECS](../../README.md) | [docs](../overview.md) | [system](./dir.md) | System[]().md*
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# System
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### In this section
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[About](#about)
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[Implementation](#implementation)
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[Examples](#examples)
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[Notes](#notes)
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## About
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>A system is a functionality that is executed regularly by the game engine.
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The [SystemManager](./SystemManager.md) ensures that the system has an up-to-date list of entities that the system would act upon. This is performed by comparing the system's declared required components with the components of a potential entity. An entity containing all required components is added to the system's list.
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A system does not explicitly require any entities to be registered; but it can make more sense to directly perform such operations in the game loop.
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`TODO`
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There is a single layer of inheritence in this design, which is the overriden `init()` and `update()` functions.
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This allows each system's `update()` to be called in a uniform way.
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## Implementation
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All systems should extend [ECSSystem](./ECSSystem.md), which implements the entities list as a simple set.
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This can be overriden if there are certain requirements for the order of the entities (e.g. a Render System might require the further-most elements to be drawn first, to ensure the correct z-ordering)
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``` java
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class ECSSystem{
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Set<Integer> entities = new HashSet<>();
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}
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```
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``` java
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class NewSystem extends ECSSystem{
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void init() {}
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void update(/* any required parameters*/){
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/* implementation */
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}
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}
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```
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## Examples
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### System that operates on entities with <span style="color:white">multiple components</span>
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``` java
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public class PhysicsSystem extends ECSSystem{
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ECS gameEngine;
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public PhysicsSystem(ECS gameEngine){
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this.gameEngine = gameEngine;
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}
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void init() {}
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void update(double dt){
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for (Integer entity : entities) {
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// Get entity components
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Vec2D pos = (Vec2D)gameEngine.getComponentData(entity, Vec2D.class.getName());
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RidgidBody ridgidBody = (RidgidBody)gameEngine.getComponentData(entity, RidgidBody.class.getName());
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Gravity gravity = (Gravity)gameEngine.getComponentData(entity, Gravity.class.getName());
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// Add the result of the accelerative forces to the ridgidbody's velocity
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ridgidBody.xdot += ridgidBody.xAcc * dt;
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ridgidBody.ydot += ridgidBody.yAcc * dt;
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// Gravity is a force of acceleration as above, but is generally considered to be constant.
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if (gravity.terminalX < 0 || Math.abs(ridgidBody.xdot) < gravity.terminalX){
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ridgidBody.xdot += gravity.x * dt;
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}
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if (gravity.terminalY < 0 || Math.abs(ridgidBody.ydot) < gravity.terminalY){
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ridgidBody.ydot += gravity.y * dt;
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}
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// Finally, move the vec2d by the new velocity
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pos.x += ridgidBody.xdot * dt;
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pos.y += ridgidBody.ydot * dt;
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}
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}
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}
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```
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### System that operates on a <span style="color:white">single component</span>
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``` java
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public class LogVec2DSystem extends ECSSystem{
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ECS gameEngine;
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public LogVec2DSystem(ECS gameEngine){
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this.gameEngine = gameEngine;
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}
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void init() {}
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void update(double dt){
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for (Integer entity : entities) {
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Vec2D pos = (Vec2D)gameEngine.getComponentData(entity, Vec2D.class.getName());
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System.out.println(String.format("X: %.6g, Y: %.6g", pos.x, pos.y));
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}
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}
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}
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```
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### System that operates on <span style="color:white">no components</span>
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Note that it does not require a reference the the `gameEngine`; no component data is accessed.
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``` java
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public class FrameRateSystem extends ECSSystem{
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void init() {}
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void update(double dt, double idleTime){
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System.out.print(String.format("dt: %.3g (%.3g idle) ", dt, idleTime));
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}
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}
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```
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## Notes
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