Depending on the requirements of your project, as the tree of components start to grow deeper, you may face situations where some piece of information must be available in many different components down the tree. For instance, the user's id or the timezone. The classical solution is to pass that information as properties until it reaches its destiny. For small projects where you don't have too many child components in that situation, that's not only fine, but usually preferred. However, for larger project with many levels of components, this potentially becomes hard to maintain as every single parent component must define the very same property, forcing the user to explixitly pass that unipresent piece of information all over the system. A problem known as "Property drilling".

In order to overcome such problem, Surface provides a Context API, which allow users to store values in a shared data structure on a higher level in the tree and then access those values anywhere in the lower levels.

The easiest way to use contexts is to store values using Context.put/3 and retrieving them with the from_context option.


Storing the value in a parent Liveview:

def mount(_params, _session, socket) do
  socket = Context.put(socket, timezone: "UTC")
  {:ok, socket}

Retrieving the value from the context in any component down the tree:

data timezone, :string, from_context: :timezone

def render(assigns) do
  <h1>Timezone: {@timezone}<h1>

Note: Although storing values in contexts might be an interesting way to avoid property drilling, you must use them carefully. Overusing them will make your code less efficient and less explicit, which may lead to components that are harder to reason about. We recommend using context only for more global information like user-related info that is not frequently updated.

Namespacing context values

One important thing to keep in mind it that storing values from different components might lead to naming conflicts. To avoid that, Surface allows you to "namespace" the values stored using an extra scope key.

The key is just an atom and can usually be the component's module. For instance:

Instead of:

socket = Context.put(socket, form: form)

you can use:

socket = Context.put(socket, __MODULE__, form: form)

That would create a composite key containing both atoms, i.e. {Form, :form} for that value.

Now, whenever you need to retrieve the value, you must pass the scope too:

form = Context.get(socket_or_assigns, Form, :form)
data form, :form, from_context: {Form, :form}

Note: If you want to distribute a library that stores values into the context, it's highly recommended that you always scope those values as demonstrated. This way you make sure it can play nicely with other libraries that also use contexts.

Scope-aware context

Sometimes you need to initialize values that should be propagated only inside the component's scope via its slots. For instance, when working with forms in Phoenix, you usually need to define a form variable that has to be passed to input elements along with each field name.

Using Surface contexts, you can improve the developer experience by not forcing one to pass the form and field values multiple times. Instead, you can store those values in the context and retrieve them in the child components when needed.

You can use <#slot context_put={}/> to propagate context values directly to a slot or <Context put={}>...</Context> when you need to propagate them to all slots and children inside a block. Using <Context put={}> is less efficient diff-tracking-wise so it should be used only when <#slot context_put={}/> is not an option.

All Surface built-in form/input components provided implement this approach so you can use them as:

<Form for={:user} change="change">
  <Field name="name">
  <Field name="email">

Note: Using scope-aware context is more expensive than using regular context via Context.put/3. The reason is that the former relies on variables to be passed down to the slots and should only be used when you already need to use variables, like in the example above that, behind the scenes, use the form instance created by the underlying LV's <.form> component.