Define state mutations

Mutations are the only way to change the state of the store. Mutations are defined among your states in the *.states files. They are defined as functions that take the current state as the this argument and the payload as the first argument. The payload can be anything, but it is usually an object with the new values for the state.

Let’s define a mutation to update a post in our blog:

"""
Update a post
"""
Post.update(title: String, content: String): Post => {
  title: title,
  content: content,
}

The Post.update mutation takes two arguments, title and content, and returns an updated Post state with the new values for the title and content properties. It's also possible to use a shorthand syntax for mutations that only overrite properties:

"""
Update a post
"""
Post.update(title: String, content: String): Post

Transitioning between states

The Post.update mutation didn't change the type of the state and therefore didn't trigger a transition. To transition between states, let's define a new mutation that will publish a draft post:

"""
Publish a draft post
"""
DraftPost.publish(): PublishedPost => {
  content: Required(value: this.content),
  category: Required(value: this.category),
  publishedAt: DateTime(),
}

The DraftPost.publish mutation returns a PublishedPost state. The Required type is a special type that makes sure that the value is not null or undefined. The DateTime type will automatically set the value to the current date and time.

Creating new entities

To create a new entity, we need to define a mutation that returns the new entity. Let's define a mutation to create a new post:

"""
Create a new draft post
"""
create(
  title: String
  content?: String
  category?: Category
): DraftPost

Notice that the create mutation doesn't take any this arguments. This is because it doesn't update an existing state, but creates a new one.

Deleting entities

To delete an entity, we need to define a mutation that returns Void. Let's define a mutation to delete a post:

"""
Delete a post
"""
Post.delete(): Void

Delete mutation don't take any arguments, because they don't need to, but it's possible to pass arguments to delete mutations as well and use them to run some validation logic before deleting the entity.