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(objToPatch, extension)¶

Argumenten

• objToPatch (object()) – the object that should be patched

• extension (object()) – an object mapping each key to an extension

Returns

a function to remove the patch

The patch function modifies in place the objToPatch object (or class) and applies all key/value described in the extension object. An unpatch function is returned, so it can be used to remove the patch later if necessary.

Most patch operations provide access to the parent value by using the native super keyword (see below in the examples).

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, {
  fn() {
    // do things
  },
});

When patching functions, we usually want to be able to access the parent function. To do so, we can simply use the native super keyword:

patch(object, {
  fn() {
    super.fn(...arguments);
    // do other things
  },
});

const obj = {
  a: function () {
    // Throws: "Uncaught SyntaxError: 'super' keyword unexpected here"
    super.a();
  },
  b: () => {
    // Throws: "Uncaught SyntaxError: 'super' keyword unexpected here"
    super.b();
  },
};

Getters and setters are supported too:

patch(object, {
  get number() {
    return super.number / 2;
  },
  set number(value) {
    super.number = value;
  },
});

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, {
  myStaticFn() {...},
});

// this is probably the usual case: patching a class method
patch(MyClass.prototype, {
  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, {
  setup() {
    super.setup(...arguments);
    this.doubleNumber = this.number * 2;
  },
});

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, {
  setup() {
    useMyHook();
  },
});

Removing a patch¶

The patch function returns its counterpart. This is mostly useful for testing purposes, when we patch something at the beginning of a test, and unpatch it at the end.

const unpatch = patch(object, { ... });
// test stuff here
unpatch();

Applying the same patch to multiple objects¶

It could happen that one wants to apply the same patch to multiple objects but because of the way the super keyword works, the extension can only be used for patching once and cannot be copied/cloned (check the documentation of the keyword). A function returning the object used to patch can be used to make it unique.

const obj1 = {
  method() {
    doSomething();
  },
};

const obj2 = {
  method() {
    doThings();
  },
};

function createExtensionObj() {
  return {
    method() {
      super.method();
      doCommonThings();
    },
  };
}

patch(obj1, createExtensionObj());
patch(obj2, createExtensionObj());

const object = {
  method1() {
    doSomething();
  },
  method2() {
    doAnotherThing();
  },
};

const ext1 = {
  method1() {
    super.method1();
    doThings();
  },
};

const invalid_ext2 = {
  ...ext1, // this will not work: super will not refer to the correct object in methods coming from ext1
  method2() {
    super.method2();
    doOtherThings();
  },
};

patch(object, invalid_ext2);
object.method1(); // throws: Uncaught TypeError: (intermediate value).method1 is not a function

const valid_ext2 = {
  method2() {
    super.method2();
    doOtherThings();
  },
};

patch(object, ext1); // first patch base extension
patch(object, valid_ext2); // then the new one
object.method1(); // works as expected