Collects command output without streaming to console.

Implements the Collectable protocol for use with System.cmd/3's :into option. Stores all output in memory for later retrieval.

Example

collector = ExQuality.OutputCollector.new()
{_output, _exit_code} = System.cmd("mix", ["compile"], into: collector)
output = ExQuality.OutputCollector.get_output(collector)

Watching output as it arrives

A collector can also be given an :on_line handler, called with each complete line as the command produces it:

collector = ExQuality.OutputCollector.new(on_line: &IO.puts/1)

A stage whose tool can stall for minutes needs this to report progress while it happens; the collected output is only available once the command has exited. Lines are reassembled here because a chunk arrives at whatever boundary the port gives it, which is rarely a line.