Abstract base class inherited by all resources.
A resource is content that is usually provided at runtime or is generated dynamically, as opposed to assets which are usually packaged with the application. Resources can therefore be slower to load and might even fail to load in some situations (for example, with corrupt data or a bad network connection).
All resources go through several states during loading. To listen to the state changes and get other data from the resource, a ResourceTracker must be used (for example, an ImageTracker is used when listening for state changes on an Image). The ResourceTracker encapsulates the asynchronous behavior of a resource by providing means to receive signals and access resource data and states.
Resources are value objects that use implicit data sharing, which allow them to be passed around as values.
Returns true if the resource is null.
true if this resource is null, false otherwise.