Side Effects

Actions propose values to the receive function, which in turn changes the model. Actions can also have side effects: typically, making requests to a server and receiving a response. These side effects happen asynchronously. Where should these be triggered, and how do we handle the response?

Where To Perform Side Effects

Where to perform side effects is an ongoing debate with different solutions being proposed by different frameworks and libraries. As I said initially about best practices, the answer is "it depends". Use what works best for you. Meiosis is flexible enough to let you do that.

In the todo-list with server example, the details making server requests are encapsulated in services. Then, these side effects are performed in the actions. Because of the asynchronous nature of those actions, we have the opportunity (if we wish; it is not required) to propose something right away, and then propose again when we receive the response. The view can display a message to the user ("please wait...") until we get the response.

Doing this is quite straightforward:

  loadList: () => {
    services.loadTodos.then(model => propose(Action.LoadedList(model)));

Notice how there is nothing out of the ordinary here. A first call to propose is made. Then we get the loadTodos response and make another call to propose.

Performing Initial Actions

Sometimes we need to perform a call to the server on application startup, such as to load data. The todo-list with server example does this to load the todos from the server.

Again, this can be accomplished without anything out of the ordinary. Just call the action on startup. In Meiosis, the ready function is a good place to do so:

  ready: actions => actions.loadList()

Because the ready function gets propose or actions passed in, you can use them to perform your initial actions.

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