Add method to check if component changed since last time step

[osrf-migration]

Original report (archived issue) by Shane Loretz (Bitbucket: Shane Loretz, GitHub: sloretz).

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Currently a system can query entities that have all required components. It gets a list of EntityId that match. This proposal is to add a way to see if a component on an entity has changed since the last update. It would mean :

• Add a method Entity::IsDifferent(ComponentType) which returns 0 if the component was not modified, or a non-zero value indicating if the component was created, deleted, or just changed.
• change Entity::Component() to return a pointer to a const object
• Add Entity::ComponentMutable() to return a pointer to a non-const component for writing and set an internal flag that the component was modified
• Add convenience methods using templates for the above.
• Supporting changes in Manager, EntityComponentDatabase, and EntityQuery.

The purpose is to enable a system to update its internal state only if there have been changes. For example, a physics system would only change the shape of an object if the shape changed, or stop simulating a body if the component describing it was removed.

Example

#!c++

void MySystemUpdate(const EntityQuery &_query)
{
  for (EntityId id : _query->EntityIds())
  {
    auto e = manager->Entity(id);
    switch (e.IsDifferent<components::WorldPose>())
      {
        case WAS_CREATED:
          // add thing
          break;
        case WAS_DELETED:
          // add thing
          break;
        case WAS_MODIFIED:
          // add thing
          break;
         default: break;
      }
      // ...
      if (needsChanging)
      {
        auto p = e.ComponentMutable<components::worldPose>();
        // ...  Do changes
      }
      else
      {
        auto p = e.Component<components::WorldPose>();
        // ... read only access to component
      }
  }
}

[osrf-migration]

Original comment by Shane Loretz (Bitbucket: Shane Loretz, GitHub: sloretz).

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Created and modified are in pull request #7. I changed the name of WasModified() to IsDifferent(), and WAS_CHANGED to WAS_MODIFIED.

[osrf-migration]

Original comment by Shane Loretz (Bitbucket: Shane Loretz, GitHub: sloretz).

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• set assignee_account_id to "557058:684383ab-ac95-4859-a350-4a6f41a94a22"
• set assignee to "sloretz (Bitbucket: sloretz, GitHub: sloretz)"

[osrf-migration]

Original comment by Shane Loretz (Bitbucket: Shane Loretz, GitHub: sloretz).

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• Edited issue description Changed methods to match names used in pull request #7

[osrf-migration]

Original comment by Shane Loretz (Bitbucket: Shane Loretz, GitHub: sloretz).

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• changed state from "new" to "resolved"

Fixed in pull request #7