> ## Documentation Index
> Fetch the complete documentation index at: https://print.app/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Developers

> The builder's contract with the product page: the live pushes to Print Options or a theme input, the canvas facts on the saved design, and the price readback.

The Gang Sheet Builder speaks to the product page through the same
channels every Print.App editor uses. Nothing here is required to use it;
this page is for stores with their own pricing logic or integrations.

## Live pushes while the customer works

After every re-pack the builder sends the billable value to the **Price
target** configured on the design:

| Price target          | What is sent                                                                                                                                                              |
| --------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `po:dimensions`       | Two values into the Print Options widget's dimensions field: `w` = film width, `h` = billed length, with the design's unit. The widget converts units and prices.         |
| any other `po:` field | One number into that widget field, in the billing unit.                                                                                                                   |
| a CSS selector        | One number written into the matching input, in the billing unit (inches, feet, centimeters) or the sheet count for fixed sheets, followed by `input` and `change` events. |

In Transfers by size the value is the **total copy count**, sent to
`po:quantity` or to the selector named.

## The saved design

When the customer saves, the `app:saved` message the Print.App client
receives carries a `canvas` object beside the usual project fields:

```json theme={null}
{
  "projectId": "project_…",
  "canvas": { "w": 22, "h": 13, "unit": "in", "sheets": [{ "usedLength": 13 }] }
}
```

`h` is the billed length across all sheets. With Print Options the client
hands this to the widget as the file's canvas, which is what a **Per
length** modifier prices from and what is verified on our servers at
add-to-cart.

In Transfers by size the object also carries `mode`, `copies`,
`totalArea` and an `items` array with each design's size and copy count;
the client forwards the items on the file record for the production
ticket.

## Price readback

With **Price display source** set, the builder registers for the page's
price and shows it in its summary:

* `po:price` — the Print Options widget sends its total, currency and
  availability on load and on every change.
* a CSS selector — the client watches the element's text and sends it on
  every change. The element must show the **line total** for the number to
  mean anything in the builder.

The displayed price never feeds pricing; it is a mirror of the page.

## Layout validation

Before checkout the builder re-sends the sheet's placements to the packing
service, which checks them with the same geometry it packs with: inside the
film, clear of the edge padding, no two pieces closer than the gap. A
customer's manual nudge that breaks a rule blocks Preview & Save with the
reason listed, and Auto-nest clears it.