SourceBuffer.updating
Limited availability
This feature is not Baseline because it does not work in some of the most widely-used browsers.
SourceBuffer 接口的只读属性 updating 表示 SourceBuffer 目前是否正在更新——即当前是否有 SourceBuffer.appendBuffer()、SourceBuffer.appendStream() 或 SourceBuffer.remove() 操作正在进行。