Sends the result of a method to the page as if it was passed as an argument.

Imagine having data that is used by many actions across your app, such as the current user. It can get tedious to pass that data for every action. The expose macro will make sure that whatever data you need is passed automatically.

Here's what things might look like without expose:

class BrowserAction
  def current_user
    # some way to find the current user
  end
end

Each action must pass current_user manually. Note that each action inherits from BrowserAction and therefore has access to current_user:

class Messages::Index < BrowserAction
  get "/messages" do
    html IndexPage, current_user: current_user
  end
end

class Messages::New < BrowserAction
  get "/messages/new" do
    html NewPage, current_user: current_user
  end
end

Passing current_user: current_user every time gets pretty old. Enter expose:

class BrowserAction
  expose current_user

  def current_user
    # some way to find the current user
  end
end

Now our actions are much nicer, especially when we start to have multiple arguments for each action:

class Messages::Index < BrowserAction
  get "/messages" do
    html IndexPage
  end
end

class Messages::New < BrowserAction
  get "/messages/new" do
    html NewPage
  end
end

Exposing private methods

Also useful is the ability to make a private method available:

class Messages::Show < BrowserAction
  expose message

  get "/messages/:id" do
    html ShowPage
  end

  private def message
    MessageQuery.find(id)
  end
end

Using expose here will pass message to the ShowPage, while keeping the method private. Without expose the action would look like this:

get "/messages/:id" do
  html ShowPage, message: message
end