🛒Out-of-band updates

<div class="shop">
  <div id="shelf" class="grid shelf">
  {products}
  {fragment 'item'}
    <div class="card item" id="item-{$id}">
      <div class="item-top"><span class="item-emoji">{$emoji}</span>{if $inCart}<span class="badge is-accent">{$inCart} in cart</span>{/if}</div>
      <strong>{$name}</strong>
      <div class="muted small">{$price | currency('EUR')} &middot; {if $stock}{$stock} in stock{else}sold out{/if}</div>
      <div class="item-buttons">
        <button class="btn-sm" hx-post="?examples/cart&id={$id}&op=add&padFragment=item" hx-target="#item-{$id}" hx-swap="outerHTML">+ Add</button>
        {if $inCart}<button class="btn-sm btn-ghost" hx-post="?examples/cart&id={$id}&op=remove&padFragment=item" hx-target="#item-{$id}" hx-swap="outerHTML">−</button>{/if}
      </div>
    </div>
    {if $posted}
      <span id="cart-count" hx-swap-oob="true">{$cartCount}</span>
      {cartSummary}
    {/if}
  {/fragment}
  {/products}
  </div>
  {cartSummary}
</div>

<?php

  // The cart is the session's: product id => quantity. A button posts its product - op=add
  // or op=remove - and the answer is the {fragment 'item'} of that product: the card with its
  // new count, then the parts elsewhere on the page that changed too, each marked
  // hx-swap-oob - htmx puts the card back where the button was and the rest where their ids
  // are: the count in the header, the summary beside the shelf. Emptying the cart is an
  // ordinary {form} post that ends in a redirect, the page coming back whole.

  $counts   = padSession ( 'htmxCart', [] );
  $products = array_slice ( htmxData ( 'products' ), 0, 8 );
  $posted   = padRequestIs ( 'POST' );

  if ( $posted ) {

    $id    = (int) padRequest ( 'id', 0 );
    $op    = htmxText ( 'op' );
    $stock = array_column ( $products, 'stock', 'id' );

    if ( $op == 'clear' ) {
      padSessionPut ( 'htmxCart', [] );
      padRedirect ();
    }

    if ( $op == 'add' and ( $stock [$id] ?? 0 ) > ( $counts [$id] ?? 0 ) )
      $counts [$id] = ( $counts [$id] ?? 0 ) + 1;
    elseif ( $op == 'remove' and isset ( $counts [$id] ) and --$counts [$id] <= 0 )
      unset ( $counts [$id] );

    padSessionPut ( 'htmxCart', $counts );

    $products = array_values ( array_filter ( $products, fn ( $product ) => $product ['id'] == $id ) );

    // A product the shelf does not have - a stale or made-up request - has no card to
    // answer: a 404, which htmx swaps nothing for, where the empty loop rendered no
    // {fragment 'item'} and the answer was a 500, "there is no fragment named 'item'".

    if ( ! $products )
      padAbort ( 404, 'There is no such product' );

  }

  foreach ( $products as $at => $product )
    $products [$at] ['inCart'] = $counts [ $product ['id'] ] ?? 0;

  $cart      = htmxCart ( $counts );
  $cartCount = $cart ['count'];
  $lines     = $cart ['lines'];
  $total     = $cart ['total'];

?>