Observer Pattern

The Observer Pattern notifies many dependents when one object changes, without the source knowing who they are.

When a build finishes, four things should happen: post to Slack, update the dashboard, record metrics, and start a deployment. Written directly, the pipeline calls all four:

void finish(int buildNumber, String result) {
    slack.post(buildNumber, result);
    dashboard.update(result);
    metrics.record(buildNumber);
    deployer.start();
}

That works, and it costs more than it looks. `BuildPipeline