Field linkage is a way to give exclusive control of the value of a field to one singular source in various different ways. These ways are known as links, drives, and hooks. Linked fields grab their value from some source usually known as the driver.
Links
Target field in this image is linked, as shown by it being cyan. This is due to it being a FieldDrive<T> type.The most basic form of linkage is known as a link. Linked fields appear as cyan in the inspector. Links will continuously set the linked field with whatever value the driver provides. This type of field relation is synced, and will not differ locally between users other than the case of network delays.
Links are rarely used within the game and are only necessary for specific purposes. For example, attempting to drive a FieldDrive<T> type will result in the field becoming linked. This is due to FrooxEngine not wanting the drive state of a field to be local. Links are also used in some parts of user and avatar setup in worlds.
Drives
Rotation field on the slot is driven by the AutoLookAtUser, as shown by it being purple.Drives are an extension of links and the most common form of linkage that one will encounter in Resonite. Driven fields appear as purple in the inspector. As with links, drives will continuously set the driven field with whatever value the driver provides. However, unlike links, the value of a driven field is inherently local to each individual user. Because of this locality, drives are usually used to reduce network traffic by having each individual user calculate a value on their own machine.
Fields can become driven with either components or ProtoFlux. With components, most drivers are found under the Transform/Drivers or Relations categories. With ProtoFlux, while holding the ProtoFlux Tool, you can drag out a field from an inspector, open the context menu while still holding the field, and choose the "Drive" option. This will create a Value Field Drive or Object Field Drive node to drive the value of a field.
Another way to drive fields in ProtoFlux is via the Field Hook node. This is a lower-level interface to driving, as it allows one to start and stop drives arbitrarily on a field using impulses.
Hooks
Hooks are an optional part of drives that certain components may utilize. They are an optional function that will be run when the driven field attempts to be written to. This function is defined by the driver.
WriteBack
The most commonly-named hook that one might encounter is called WriteBack. This makes it so that when one attempts to write to the driven field, the write will go through, writing both to the source value and driven value at once.
This hook can cause an interesting effect when combined with the locality of drives. If one were to ValueCopy a field onto itself and enable writeback, the field becomes a local field with the backbone of the data model to support it, as opposed to something like a Store in ProtoFlux.