site/config.php

return [
  'hooks' => [
    'ww.merx.initializePayment:before' => function ($data, $cart) {
      // your code goes here
    },
  ],
];

site/config.php

return [
  'hooks' => [
    'ww.merx.initializePayment:after' => function ($virtualOrderPage, $redirect) {
      // your code goes here
    },
  ],
];

Be careful! Updating $virtualOrderPage does not have any effect.

site/config.php

return [
  'hooks' => [
    'ww.merx.completePayment:before' => function ($virtualOrderPage, $gateway, $data) {
      // your code goes here
    },
  ],
];

site/config.php

return [
  'hooks' => [
    'ww.merx.completePayment:after' => function ($orderPage) {
      // your code goes here
    },
  ],
];

Since Merx 1.1

The cart hook is executed on Cart::_construct().
You can find an example in the Shipping Costs and Discounts cookbook.

site/config/config.php

return [
  'hooks' => [
    'ww.merx.cart' => function ($cart) {
      // manipulate cart here
    }
  ],
];

Since Merx 1.9

This hook is only triggered, when you implemented Stripe Webhooks.

Merx uses this hook to set paidDate and paymentComplete for payments with delayed notification of payment success.

site/config/config.php

return [
  'hooks' => [
    'ww.merx.stripe-hooks' => function ($stripeEvent) {
      // your code here
    }
  ],
];