=============
Patching code
=============

Sometimes, we need to customize the way the UI works.  Many common needs are
covered by some supported API. For example, all registries are good extension
points: the field registry allows adding/removing specialized field components,
or the main component registry allows adding components that should be displayed
all the time.

However, there are situations for which it is not sufficient. In those cases, we
may need to modify an object or a class in place. To achieve that, Odoo
provides the utility function `patch`. It is mostly useful to override/update
the behavior of some other component/piece of code that one does not control.

Description
===========

The patch function is located in `@web/core/utils/patch`:

.. js:function:: patch(obj, patchName, patchValue, options)

    :param Object obj: object that should be patched
    :param string patchName: unique string describing the patch
    :param Object patchValue: an object mapping each key to a patchValue
    :param Object options: option object (see below)

    The `patch` function modifies in place the `obj` object (or class) and
    applies all key/value described in the `patchValue` object.  This operation
    is registered under the `patchName` name, so it can be unpatched later if
    necessary.

    Most patch operations provide access to the parent value by using the
    `_super` property (see below in the examples). To do that, the `patch` method
    wraps each pair key/value in a getter that dynamically binds `_super`.

    The only option is `pure (boolean)`. If set to `true`, the patch operation
    does not bind the `_super` property.

Patching a simple object
========================

Here is a simple example of how an object can be patched:

.. code-block:: javascript

  import { patch } from "@web/core/utils/patch";

  const object = {
    field: "a field",
    fn() {
      // do something
    },
  };

  patch(object, "patch name", {
    fn() {
      // do things
    },
  });


When patching functions, we usually want to be able to access the ``parent``
function.  Since we are working with patch objects, not ES6 classes, we cannot
use the native ``super`` keyword. So, Odoo provides a special method to simulate
this behaviour: ``this._super``:

.. code-block:: javascript

  patch(object, "_super patch", {
    fn() {
      this._super(...arguments);
      // do other things
    },
  });

.. warning::

  ``this._super`` is reassigned after each patched function is called.
  This means that if you use an asynchronous function in the patch then you
  cannot call ``this._super`` after an ``await``, because it may or may not be
  the function that you expect.  The correct way to do that is to keep a reference
  to the initial ``_super`` method:

  .. code-block:: javascript

    patch(object, "async _super patch", {
      async myAsyncFn() {
        const _super = this._super.bind(this);
        await Promise.resolve();
        await _super(...arguments);
        // await this._super(...arguments); // this._super is undefined.
      },
    });


Getters and setters are supported too:

.. code-block:: javascript

    patch(object, "getter/setter patch", {
      get number() {
        return this._super() / 2;
      },
      set number(value) {
        this._super(value * 2);
      },
    });

Patching a javascript class
===========================

The ``patch`` function is designed to work with anything: object or ES6 class.

However, since javascript classes work with the prototypal inheritance, when
one wishes to patch a standard method from a class, then we actually need to patch
the `prototype`:

.. code-block:: javascript

  class MyClass {
    static myStaticFn() {...}
    myPrototypeFn() {...}
  }

  // this will patch static properties!!!
  patch(MyClass, "static patch", {
    myStaticFn() {...},
  });

  // this is probably the usual case: patching a class method
  patch(MyClass.prototype, "prototype patch", {
    myPrototypeFn() {...},
  });


Also, Javascript handles the constructor in a special native way which makes it
impossible to be patched. The only workaround is to call a method in the original
constructor and patch that method instead:

.. code-block:: javascript

  class MyClass {
    constructor() {
      this.setup();
    }
    setup() {
      this.number = 1;
    }
  }

  patch(MyClass.prototype, "constructor", {
    setup() {
      this._super(...arguments);
      this.doubleNumber = this.number * 2;
    },
  });

.. warning::

    It is impossible to patch directly the `constructor` of a class!

Patching a component
====================

Components are defined by javascript classes, so all the information above still
holds.  For these reasons, Owl components should use the `setup` method, so they
can easily be patched as well (see the section on :ref:`best practices<frontend/owl/best_practices>`.

.. code-block:: javascript

  patch(MyComponent.prototype, "my patch", {
    setup() {
      useMyHook();
    },
  });

Removing a patch
================

The `patch` function has a counterpart, `unpatch`, also located in `@web/core/utils/patch`.

.. js:function:: unpatch(obj, patchName)

    :param Object obj: object that should be unpatched
    :param string patchName: string describing the patch that should be removed

    Removes an existing patch from an object `obj`. This is mostly useful for
    testing purposes, when we patch something at the beginning of a test, and
    unpatch it at the end.

    .. code-block:: javascript

        patch(object, "patch name", { ... });
        // test stuff here
        unpatch(object, "patch name");