Previously, the SourceInterceptor made the assumption that the QualitySelector button component
is a direct child of the controlBar component. That may not always be true. Video.js allows you
to specify a nested hierarchy of components, and so when plugin users choose to move the
QualitySelector button elsewhere, the plugin does not work properly.
This commit introduces a new event type called QUALITY_REQUESTED to signal when the user is
requesting a quality change. The old QUALITY_SELECTED event is now used to denote when the
plugin actually uses a new quality source. This dichotomy eliminates the need for the
SourceInterceptor to have a reference to the QualitySelector button component.
From a basic functionality standpoint, this accomplishes the same purpose as
the previous implementation using `_.omit`. However, rather than removing the
`selected` attribute from the sources, this will keep the structure the same by
only altering the value of the source.
If your usage of this plugin requires a stable source format, e.g. for
comparing sources to see if the source changed, please provide the `selected`
attribute when programmatically setting the player source. At this time, we
don't have a simple way to standardize the `selected` attribute on initial
source load. (i.e. It's up to the programmer to set the initial format on the
sources before giving them to the player)
As things turn out, the middleware constructor is called every time `setSource`
is called [1]. This was causing a new listener to get bound to
`QUALITY_SELECTED` on each change by the quality selector. :( This change makes
it so the listener is only bound on the initial creation of the player.
[1] 03529163b6/src/js/tech/middleware.js (L66)
