Mixin which wires up mutation observers to report any changes in a component's content (direct children, or nodes distributed to slots).
For the time being, this can only support a single level of distributed content. That is, if a component contains a slot, and the set of nodes directly assigned to that slot changes, then this mixin will detect the change. However, this mixin does not yet detect changes in reprojected nodes. If a component's host places a slot as a child of the component instance, nodes assigned to the outer host will be assigned to the host's slot, then reassigned to the slot element inside the component. Changes in such reprojected nodes will not (yet) be detected by this mixin.
For comparison, see Polymer's observeNodes API, which does solve the problem of tracking changes in reprojected content.
Note: The web platform team creating the specifications for web components plan to request that a new type of MutationObserver option be defined that lets a component monitor changes in distributed children. This mixin will be updated to take advantage of that MutationObserver option when that becomes available.
Kind: global class
This event is raised when the component's contents (including distributed children) have changed.
Kind: event emitted by ObserveContentChanges
Invoked when the contents of the component (including distributed children) have changed.
This method is also invoked when a component is first instantiated; the contents have essentially "changed" from being nothing. This allows the component to perform initial processing of its children.
Kind: instance method of ObserveContentChanges