The SourceBuffers

Overview

Technically speaking SourceBuffers are browser objects allowing JavaScript applications to “append” media segments for them to be decoded at the right time through their attached media element (e.g. <audio> or <video> media elements).

Likewise in the RxPlayer code, the core/source-buffer directory contains the part of the code directly related to the insertion and removal of media segments.

You’ll find there:

• code that interacts with browser SourceBuffer Objects.

• custom implementations of the SourceBuffer interface for text (subtitles) or image (thumbnails) buffers.

• decoders for the custom SourceBuffer implementation (e.g. the text decoder parse subtitles and display them on screen at the right time).

• functions which help to perform memory management on custom SourceBuffer implementations as well as native ones (this is mostly needed on some peculiar low-memory target with legacy browsers, but you might want to control at best your memory footprint even on a classical web-applications).

SourceBuffersManager

The SourceBuffersManager is the main export from there. It facilitates the creation and destruction of SourceBuffers. More specifically, it keep track of every SourceBuffer created for a given content.

As a rule, only ONE SourceBuffer is allowed by type of buffer (e.g. audio, video, text (subtitles), images (thumbnails)).

QueuedSourceBuffer

A QueuedSourceBuffer is a wrapper on top of a SourceBuffer (a native one or a custom RxPlayer implementation) that allows to push and remove segments sequentially. Basically, it waits for the previous action to be finished before going on the next step.

It is the main interface the rest of the RxPlayer code has with SourceBuffers. As a wrapper it copy most of the original browser API to lower the cognitive complexity of using it.

BufferGarbageCollector

The BufferGarbageCollector is a function used by the RxPlayer to periodically perform “garbage collection” manually on a given SourceBuffer.

It is based on the following building bricks:

• A clock, which is an observable emitting the current time (in seconds) when the garbage collection task should be performed

• The QueuedSourceBuffer on which the garbage collection task should run

• The maximum time margin authorized for the buffer behind the current position

• The maximum time margin authorized for the buffer ahead of the current position

Basically, each times the given clock ticks, the BufferGarbageCollector will ensure that the volume of data before and ahead of the current position does not grow into a larger value than what is configured.

One of them is created per SourceBuffer.