Expose model ready through the Entity API

Users want to know when the model is ready so they can have custom functionality when the model is loading and when the scene is ready

From #3910:

Unfortunately this approach won’t really work well for several reasons. The model can change dynamically with time or programatically, but the ready promise can only resolve a single time. So promises may not be the best context here. Additionally, the ModelGraphics object may actually be associated with several different visualizers, so there may not be a single “model ready” event.

Maybe tie it into the boolean returned from DataSourceDisplay.update? This boolean is supposed to the true when when all data sources are ready to be displayed and false otherwise.

[hpinkos]

I don’t have a good answer for what to do for things like this, other than “don’t use the entity API for models” =/ There’s a reason we didn’t add 3D Tiles to the Entity API. If you want more control, you need to work with the primitives at a lower level. These kind of use-cases are just really not what the Entity API was designed for.

[OmarShehata]

Here’s a potential use case that can’t be done in the Entity layer that I don’t see addressed here. This forum thread is trying to use viewer.trackedEntity, which will not work immediately if the model isn’t loaded. This delay could be anywhere between a few milliseconds to several seconds. The application developer just needs to know when it’s ready to display some sort of loading message/reaction to the user in the mean time. A entity.model.isLoaded boolean could potentially solve this.

Thoughts?