Context API

Communicate across different boundaries with the Context API

This page is a work in progress. It will be updated as the software evolves.

The Context API enables receiving APIs. Depending on where your code is executed from, it affects which and what instances of APIs can be received.

The Context API enables an element or a controller to receive an API provided via any ascending element. In other words, it can receive APIs provided via a parent element or parent of a parent element, and so forth.

The Context API enables connections between Elements and APIs. DOM structure defines the context of which an API is exposed for. APIs are provided via an element and can then be consumed by any decending element.

Consume a Context API

There are different ways to consume a Context API. The most straightforward implementation is done on an Umbraco Element with a Context Token.

All Umbraco Context APIs have a Context Token which can be imported and used for consumption, for example:

import { UMB_NOTIFICATION_CONTEXT } from '@umbraco-cms/backoffice/notification';

...

this.consumeContext(UMB_NOTIFICATION_CONTEXT, (context) => {
    // Notice this is a subscription, as the context might change or a new one appears.
    console.log("I've got the typed context: ", context);
});

The above example takes place in an Umbraco Element or Umbraco Controller.

Alternative solutions

The above examples utilize an Umbraco Controller to hook into an element's life cycle. This Controller is named UmbContextConsumerController.

If you need to consume a Context API from a non-controller host, then look at the UmbContextConsumer.

Write your own Context Token

A Context Token is a context identifier and is generally a string matched with a type. In this way, users of the token can be sure to get the right type of context.

import { UmbContextToken } from "@umbraco-cms/backoffice/context-api";

type MyContext = {
    foo: string;
    bar: number;
};

const MY_CONTEXT = new UmbContextToken <MyContext>("My.Context.Token");

Context Token with an API Alias

For additions to Contexts, we can use the API Aliases to identify the additional API. Using the same Context Alias for additional APIs will ensure that such API must be present with the first encounter of that Context Alias. Otherwise, a request will be rejected. In other words, if the addition is not part of the nearest matching Context, the request will be rejected.

Using API Alias only provides value when two or more APIs should share the same Context. This is needed for Context Extensions that are provided along with other Contexts.

import { UmbContextToken } from "@umbraco-cms/backoffice/context-api";

type MyAdditionalContext = {
    additional: string;
};

const MY_ADDITIONAL_API_TOKEN = new UmbContextToken<MyAdditionalContext>(
    "My.ContextFrame.Alias",
    "My.API.Alias"
);

The Token declared above can be used to provide an additional Context API at the same Element as another Context API is provided at. Below is an example of how the two APIs are made available.

const contextElement = new UmbLitElement();
contextElement.provideContext(
    MY_API_TOKEN,
    new MyAPiFromSomewhereNotPartOfThisExample()
);
contextElement.provideContext(
    MY_ADDITIONAL_API_TOKEN,
    new MyAdditionalAPiFromSomewhereNotPartOfThisExample()
);

const consumerElement = new UmbLitElement();
contextElement.appendChild(consumerElement);
consumerElement.consumeContext(MY_API_TOKEN, (context) => {
    console.log("I've got the default api", context);
});
consumerElement.consumeContext(MY_ADDITIONAL_API_TOKEN, (context) => {
    console.log("I've got the additional api", context);
});

This is no different than using two different Context Aliases. But it has an important effect on what happens if one of them is not provided. This is demonstrated in the example below:

const upperContextElement = new UmbLitElement();

const contextElement = new UmbLitElement();
upperContextElement.appendChild(contextElement);
contextElement.provideContext(
    MY_API_TOKEN,
    new MyAPiFromSomewhereNotPartOfThisExample()
);

const consumerElement = new UmbLitElement();
contextElement.appendChild(consumerElement);
consumerElement.consumeContext(MY_API_TOKEN, (context) => {
    console.log("I've got the default api", context);
});
consumerElement.consumeContext(MY_ADDITIONAL_API_TOKEN, (context) => {
    // This will never happen
    console.log("I've got the additional api", context);
});

The consumption of the Additional API will never happen as the token uses the same Context Alias as MY_API_TOKEN. This means that any request containing this Context Alias will be stopped at the first API it encounters. To ensure addition to a specific context, do it locally at the nearest API that uses the same Context Alias.

Context Token with a Type Discriminator

This is only relevant if you are going to make multiple context API for the same context. Discriminator only gives value for consumption of Context APIs that have a varying interface. The backoffice uses this for the different types of Workspace Contexts.

In some cases, it is needed to have different APIs for the same context. For example, the Workspace Contexts.

If someone wants the workspace name, they might not care about the specific API of the Workspace Context. These implementations can use a standard Context Token with a type of generic Workspace Context.

The features related to Publishing in the Document Workspace Context do not require a new Context. However, we should not accidentally retrieve the workspace context of a parent workspace when in a Workspace. Therefore, we need to provide a workspace context in each workspace, and the one we retrieve is the one we will be using. Since Publishing is not part of the generic Workspace Context, check if the context is a Document Workspace Context and recast it accordingly.

To avoid each implementation taking care of this, Context Tokens can be extended with a Type Discriminator. This will discard the given API if it does not meet the necessary requirements. When it is the desired type, the API will be converted to the appropriate type.

This example shows how to create a discriminator Context Token that will discard the API if it is not a Publishable Context:

Context Token Example:

import { UmbContextToken } from "@umbraco-cms/backoffice/context-api";


interface MyBaseContext {
    foo: string;
    bar: number;
}

interface MyPublishableContext extends MyBaseContext {
    publish();
}

const MY_PUBLISHABLE_CONTEXT = new UmbContextToken<

    MyContext,
    MyPublishableContext
>("My.Context.Token", (context): context is MyPublishableContext => {
    return "publish" in context;
});

Implementation of Context Token Example:

const contextElement = new UmbLitElement();
contextElement.provideContext(
    MY_PUBLISHABLE_CONTEXT,
    new MyPublishableContext()
);

const consumerElement = new UmbLitElement();
contextElement.appendChild(contextElement);
consumerElement.consumeContext(MY_PUBLISHABLE_CONTEXT, (context) => {

    // context is of type 'MyPublishableContext'
    console.log("I've got the context of the right type", context);
});

This allows implementers to request a publishable context without needing to know the Type or how to identify the context.

In detail, the Context API will search for the first API that matches the alias My.Context.Token, and not look further. If the API meets the type discriminator, it will be returned, otherwise the consumer will never reply.

Provide a Context API

You can provide a Context API from an Umbraco Element or Umbraco Controller:

this.provideContext('myContextAlias', new MyContextApi());

Or with a Controller using a 'host' reference to Controller Host (Umbraco Element/Controller):

new UmbContextProviderController(host, 'myContextAlias', new MyContextApi());