Part 3

3.0 Part 3 (clickable)

Mount

The method componentMount is one of the biggest parts of our journey! So, the method that is interesting for us is ReactCompositeComponent.mountComponent(1).

If you remember, I mentioned that the first component that is pushed into a component’s tree is TopLevelWrapper (internal React class). Here, we are going to mount it. But… it’s basically an empty wrapper, so, it’s kind of boring to debug. It doesn’t affect flow at all, so, I suggest we skip it right now and move on to its child.

That’s how mounting of a tree actually works, you mount the parent, then its child, and a child of a child, and so on. Just believe me, after TopLevelWrapper is mounted, the child of it (ReactCompositeComponent, which manages the ExampleApplication component) will be put into the same phase.

Alright, we are back to step (1). Let’s see what’s inside. There are some key actions that are going to happen, so let’s discuss this logic with details.

Assigning instance updater

That updater (2), returned from transaction.getUpdateQueue(), is actually the ReactUpdateQueue module. So, why is it actually assigned here? Well, because ReactCompositeComponent (the class we are currently looking at) is one used in all platforms, but updaters are different, so we assign it dynamically during mounting depending on the platform.

Alright. We don’t really need this updater for now, but keep it in mind. updater is really important, it will be used soon by well-known component method setState.

Actually, not only is updater assigned to an instance during this phase, the component instance (your custom component) is also extended with props, context, and refs.

Check out the code below:

// \src\renderers\shared\stack\reconciler\ReactCompositeComponent.js#255
// These should be set up in the constructor, but as a convenience for
// simpler class abstractions, we set them up after the fact.
inst.props = publicProps;
inst.context = publicContext;
inst.refs = emptyObject;
inst.updater = updateQueue;

So, then you can access props in your code from an instance, like this.props.

Create ExampleApplication instance

By calling _constructComponent (3) and through several construction methods, finally new ExampleApplication() will be created. That’s the point when the constructor from our code will be called. So, it’s the first time our code was actually touched by React’s ecosystem. Nice.

Perform initial mount

So, we go through mount (4) and the first thing that should happen here is a call of componentWillMount (if it was specified of course). It’s the first method of life-cycle hooks we meet. Also, a bit below you can see componentDidMount, but it’s actually just pushed into the transaction queue because it shouldn’t be called directly. It happens only at the very end, when mounting operations are done. Also, you could possibly add setState calls inside componentWillMount. In that case, the state will of course be re-computed, but without calling the render method (it just doesn’t make sense to, because the component is not mounted yet).

Official documentation proves the same:

componentWillMount() is invoked immediately before mounting occurs. It is called before render(), therefore setting state in this method will not trigger a re-rendering.

Let’s check the code, just to make sure ;)

// \src\renderers\shared\stack\reconciler\ReactCompositeComponent.js#476
if (inst.componentWillMount) {
    //..
    inst.componentWillMount();

    // When mounting, calls to `setState` by `componentWillMount` will set
    // `this._pendingStateQueue` without triggering a re-render.
    if (this._pendingStateQueue) {
        inst.state = this._processPendingState(inst.props, inst.context);
    }
}

True. Well, but when state is recalculated, we call the render method. Yes, exactly the one which we specify in our components! So, one more touch of ‘our’ code.

Alright, the next thing is to create a React component instance. Erm… what, again? Seems we’ve already seen this this._instantiateReactComponent(5) call, right? That’s true, but that time we instantiated a ReactCompositeComponent for our ExampleApplication component. Now, we are going to create VDOM instances for its child based on the element we got from the render method. For our exact case, the render method returns div, so the VDOM representation for it is ReactDOMComponent. When the instance is created, we call ReactReconciler.mountComponent again, but this time as internalInstance, we pass a newly created instance of ReactDOMComponent.

And, call mountComponent on it…