While working on it, I came across libraries like Redux and ImmutableJS and how they help solve particular problems in React and the modern JS world. This really peaked my interest, so naturally, like all new things I learn, I tried to see how they could be applied to SharePoint. So in this post, lets have a look at what problems do Redux and ImmutableJS solve and how we can use them in a SharePoint Framework web part:
Source of this web part is available as a part of Microsoft's SharePoint Framework client-side web part samples & tutorials on GitHub: https://github.com/SharePoint/sp-dev-fx-webparts/tree/master/samples/react-redux-async-immutablejs
I am going to assume that you are familiar with React and understand the basic concepts. If not, have a look at the tutorials on the react website, they are really great to get you started.
So as you know, the way React components work is every component displays it's UI based on the component's state. When the state is updated, the UI of the component is updated as well.
Since I was working with multiple components, I quickly realised that some events in my application would mean updating the state of more than one component. How do we make this happen? The easy (but not so great) solution was to move the state in the parent component of the two child components and then pass the state as properties to the child components. If we wanted to update the state from a child component, we would update it using a function (which is also passed as a property to the child component). When the state in the parent component was updated, it would automatically update all the child components as well.
This would quickly become tedious as more components are added to the application. The state will have to be passed down deeper and deeper in the component tree which is not really ideal.
Luckily, there is a nice solution to this problem with the introduction of Redux. With Redux, what we do is maintain the entire state of our application in a single object. This state is connected to our components and is passed as properties to them. Any change to the state is defined with an Action which is dispatched when an event occurs.The Reducer receives the Action and updates the necessary elements of the state. (Which in turn updates the UI of the relevant component)
This is the general idea of Redux. If you want to have a deeper look and also check out some advanced concepts, there are some excellent tutorials here: http://redux.js.org/
Working of the SharePoint Framework webpart:
When the page loads, a redux action is fired which makes a REST API call using SPHttpClient to the current site and gets all the lists. Once the lists are fetched, another redux action is fired to instruct that all lists were successfully fetched from the server. The reducer listens for these actions and updates the state (UI) accordingly.
We can also create a new list from the webpart. When the user enters a value in the text box, an action is dispatched which updates the element in the state which represents the new list title. When the user clicks on the "Add" button, an action is dispatched which again makes an async call using SPHttpClient to create the list. Once the call is successfully returned, the UI is updated with the name of the new list.
Redux Async Actions:
Each async operation ideally has 3 actions to go along. This is not a requirement but is helpful to nicely manage our UI when the operating in being performed. We will see why we need 3 actions for one operation in the Reducers sections below. For now, all we need is a "request" action which we will fire before making the async call, a "success" action which we will fire after the aync call returns successfully and then an "error" action which will be fired if the async call returns an error. Here are the 3 actions we will use in our webpart, and then a parent action to wrap them all together:
A reducer determines how the state should change after an action occurs. For example, when a "request" async action is triggered the reducer changes the application state to show a loading icon. When a request is successfully completed and it returns data from the server, the "success" action is triggered and the reducer determines that the loading icon should disappear and the data should be shown instead. Similarly, if an error would be returned from the server, the "error" action would be triggered and the reducer should show an error message on the screen instead of the loading icon. Here is our reducer code which modifies the sate using ImmutableJS. In this code, to keep things simple, we are only updating the state when the success action is dispatched. We are not showing or hiding a loading icon here.
When a reducer updates the state, a core principle of react is that we should never mutate the state object. We should always make a copy of the state, update the necessary elements in the copy and then return the copy as the new state. More details on why immutability is important here: https://facebook.github.io/react/tutorial/tutorial.html#why-immutability-is-important
Let me begin by saying that using Immutable JS with Redux is not mandatory. We can use native ES6 features like Object.assign or the spread operator to create a new copy of the state, update the necessary elements and return the new copy. (Just remember that for supporting IE, we will need polyfills for these) If we have small to medium sized application, this is perfectly fine.
But if we are maintaining lots of elements in our state, making a new copy of state for every action can be really performance intensive. For changing only a single element, we have to copy the entire state tree in memory, and that too for every action. Fortunately, ImmutableJS comes to the rescue here.
Using ImmutableJS, we can create a new state object in memory without duplicating the elements which are unchanged. When we create a new state object using ImmutableJS, the new object still points to the previous memory locations of unchanged elements. Only the elements which are changed are allocated new memory locations. The new copy of the state points to the same memory locations as the old copy for unchanged elements and it points to the new memory locations for the changed elements.
Here is our application state which extends the Immutable.Record class:
We have used TypeScript readonly properties in the state to mandate that these properties should never be updated directly. Instead, the custom "setter" functions should be used. In the setter functions, we use ImmutableJS methods such as "set" and "update". These functions take care of returning a new copy of the state where the only the changed elements are stored in new memory locations.
Hope you have enjoyed reading this post.
Check out the full code of the web part here: https://github.com/SharePoint/sp-dev-fx-webparts/tree/master/samples/react-redux-async-immutablejs