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:

patch(obj, patchName, patchValue, options)

Arguments

• obj (Object()) – object that should be patched

• patchName (string()) – unique string describing the patch

• patchValue (Object()) – an object mapping each key to a patchValue

• options (Object()) – 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:

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:

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:

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:

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:

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:

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 best practices.

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

Removing a patch

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

unpatch(obj, patchName)

Arguments

• obj (Object()) – object that should be unpatched

• patchName (string()) – 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.

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