DOORS

Printer Middleware

doors.WithPrinter lets you observe or adjust the HTML Doors emits, at the point where render jobs are serialized.

Use it for cross-cutting output concerns: stamping elements with extra attributes, collecting render metrics, or auditing produced markup.

Option

app := doors.NewApp(page,
	doors.WithPrinter(func(next gox.Printer) gox.Printer {
		return &stamp{next: next}
	}),
)

The middleware is a func(next gox.Printer) gox.Printer. It applies to every page render and Door render cycle in the app. Passing nil disables wrapping.

Contract

Doors calls the middleware once per drain:

  • the initial page render — the whole document, including fully static pages
  • a Door render cycle — the updated Door plus all descendant Doors rendered in that cycle, delivered as one payload

The returned printer must be non-nil. It receives that drain's jobs as sequential Send calls on a single goroutine, in document order, and is never reused across drains. The middleware function itself can be called concurrently — separate pages and Door updates can print at the same time — so keep per-drain state in the returned printer, not in shared variables.

Rules:

  • forward each job to next exactly once; jobs are pooled and single-use, so never retain a job or its Attrs after Send returns — use Attrs.Clone to keep an independent copy
  • read job.Context() for scope information: each job carries the context of the render scope that produced it, so helpers like doors.SessionContext work on it
  • gox.JobOpen attrs can be inspected or changed until the job is serialized; they may be nil on container heads that emit no HTML
  • do not modify Doors-managed output: d0* elements and attributes, and rewritten resource URLs
  • do not inject head-relevant elements into a page render stream before the document head is printed
  • do not block: the middleware runs on the render hot path

Example

Count elements and stamp each with its render index:

type stamp struct {
	next  gox.Printer
	count int
}

func (s *stamp) Send(j gox.Job) error {
	if open, ok := j.(*gox.JobOpen); ok && open.Tag != "d0-r" && open.Attrs != nil {
		s.count++
		open.Attrs.Get("data-render-index").Set(strconv.Itoa(s.count))
	}
	return s.next.Send(j)
}

app := doors.NewApp(page, doors.WithPrinter(
	func(next gox.Printer) gox.Printer {
		return &stamp{next: next}
	},
))

Not Wrapped

The middleware only sees Doors render output. It does not wrap:

  • error pages rendered through doors.WithErrorPage
  • the error replacement HTML of a failed Door render cycle
  • static files and resources served through app.Use(...) middleware or resource URLs