unicorn-utterances/content/blog/angular-internals-zonejs/index.md

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title	description	published	authors	tags	attached	license
Angular Internals: How Reactivity Works with Zone.js		2023-03-01T13:45:00.284Z	crutchcorn	angular javascript		cc-by-nc-sa-4

This article is an advanced look at how Angular works under-the-hood. This may be confusing if you're not already fairly familiar with JavaScript. If you're wanting to learn how to use Angular, and haven't before, take a look at my book "The Framework Field Guide", which teaches React, Angular, and Vue from scratch instead.

If you've been following the JavaScript framework ecosystem, you may have heard the term "Reactivity" lately; they've been a hot commodity to talk about from SolidJS' fine-grained reactivity to Preact adding in a reactive primitive with the name of "Signals".

The concept of reactivity, at least at first glance, is a straightforward one: When you change a bit of code here, it updates a bit of code there automatically. This is commonplace within frontend frameworks, where it's imperative to re-render updated content when you update the data stored in JavaScript.

During discussions of reactivity and frontend frameworks, there's one "odd duck" that stands out as a vastly different implementation from the others: Angular.

Take the following button counter reactivity example in each framework:

Angular

import { Component } from '@angular/core';

@Component({
  selector: 'my-app',
  template: `
    <button (click)="addOne()">{{count}}</button>
  `,
})
export class AppComponent {
  count = 0;

  addOne() {
    this.count++;
  }
}

React

const App = () => {
	const [count, setCount] = useState(0);
	
	const addOne = () => setCount(count+1);
	
	return <button onClick={addOne}>{count}</button>;
}

Vue

<template>
	<button @click="addOne()">{{count}}</button>
</template>

<script setup>
import {ref} from 'vue';

const count = ref(0);

function addOne() {
	count.value += 1;
}
</script>

In this example, we can see that React uses explicit update calls (setX) to track when the state changes and Vue uses a proxy and a special property name (.value) to seemingly magically track state.

But what about Angular?

Angular just mutates the count variable and the framework seems to count the state changes. How does that work under-the-hood? What mechanism is being used to tell the template to re-render?

The short answer is that Angular uses something called "Zone.js" to track all asynchronous APIs via a series of polyfills, and uses those Zones to re-render "dirty" content in Angular's tree.

What does any of that mean? That's a lot of works that doesn't seem to mean very much if you're not already in the know.

I agree; so let's answer this better with a longer step-by-step explanation of how Angular does its rendering and reactivity using Zone.js.

This step-by-step explanation will have us explore:

• How Angular's template compiler outputs functions that render contents
• How Angular's templates are called in order to update contents on-screen
• How Angular's change detection would work without Zone.js (and why it's a DX nightmare)
• How Zone.js Monkey-patches async APIs to call change detection
• How Angular has it's own internal instance of Zone.js called NgZone
• Writing our own minimal version of Angular from scratch

How Angular's template compiler works

Earlier last year, the Angular team published a blog post titled "How the Angular Compiler Works". In it, they demonstrated how the NGC compiler takes the following code:

import {Component} from '@angular/core';

@Component({
  selector: 'app-cmp',
  template: '<span>Your name is {{name}}</span>',
})
export class AppCmp {
  name = 'Alex';
}

And outputs something like this:

import { Component } from '@angular/core';                                      
import * as i0 from "@angular/core";

export class AppCmp {
    constructor() {
        this.name = 'Alex';
    }
}                                                                               
AppCmp.ɵfac = function AppCmp_Factory(t) { return new (t || AppCmp)(); };
AppCmp.ɵcmp = i0.ɵɵdefineComponent({
  type: AppCmp,
  selectors: [["app-cmp"]],
  decls: 2,
  vars: 1,
  template: function AppCmp_Template(rf, ctx) {
    if (rf & 1) {
      i0.ɵɵelementStart(0, "span");
      i0.ɵɵtext(1);
      i0.ɵɵelementEnd();
    }
    if (rf & 2) {
      i0.ɵɵadvance(1);
      i0.ɵɵtextInterpolate1("Your name is ", ctx.name, "");
    }
  },
  encapsulation: 2
});                                                   
(function () { (typeof ngDevMode === "undefined" || ngDevMode) && i0.ɵsetClassMetadata(AppCmp, [{
        type: Component,
        args: [{
                selector: 'app-cmp',
                template: '<span>Your name is {{name}}</span>',
            }]
    }], null, null); })();

While their article goes more in-depth into how the compiler works (it's a great read!), let's keep our focus narrow for the intention of this article.

Namely, let's look at the template property function on the ɵɵdefineComponent function call.

template: function AppCmp_Template(rf, ctx) {
    if (rf & 1) {
        i0.ɵɵelementStart(0, "span");
        i0.ɵɵtext(1);
        i0.ɵɵelementEnd();
    }
    if (rf & 2) {
        i0.ɵɵadvance(1);
        i0.ɵɵtextInterpolate1("Your name is ", ctx.name, "");
    }
}

Here, we're receiving two arguments: rf (short for "render flags") and ctx (short for "context"). This function is called by Angular itself when the template is ready to either be rendered for the first time or updated afterwards.

Depending on how the template needs to be re-ran, the "render flag" (rf) will be passed differently, which allows Angular more control over how code is updated or not.

There are only two flags that are currently defined in Angular 15:

// Source code from @angular/core
// angular/packages/core/src/render3/interfaces/definition.ts

export const enum RenderFlags {
  /* Whether to run the creation block (e.g. create elements and directives) */
  Create = 0b01,

  /* Whether to run the update block (e.g. refresh bindings) */
  Update = 0b10
}

The first render flag that will be passed to the template function is the Create flag, which will call the first if statement. Let's strip away everything but the first if statement:

i0.ɵɵelementStart(0, "span");
i0.ɵɵtext(1);
i0.ɵɵelementEnd();

Here, very generally, Angular is saying: "create a span element, and mark it such that text should be placed within it".

After this is ran, Angular runs the Update render flag through the template compiler:

i0.ɵɵadvance(1);
i0.ɵɵtextInterpolate1("Your name is ", ctx.name, "");

Here, it's saying that we should interpolate the string "Your name is Alex" based on the property received from ctx.name and place it into the element's text area.

By having our template function have two distinct render phases, triggered by flags passed into the function, we're able to create the span on the first render and update the text values of the span on subsequent renders, without the need for re-initializing the span element each time we change the element's text.

Exactly how is the template compiler ran by Angular?

As mentioned previously, Angular calls this render function with two different render flags: Create and Update.

But don't take my word for it! Let's take a look at Angular's source code:

Defined in @angular/core is a function called renderComponent:

// Angular 15 source code
// angular/packages/core/src/render3/instructions/shared.ts
function renderComponent(hostLView: LView, componentHostIdx: number) {
  ngDevMode && assertEqual(isCreationMode(hostLView), true, 'Should be run in creation mode');
  const componentView = getComponentLViewByIndex(componentHostIdx, hostLView);
  const componentTView = componentView[TVIEW];
  syncViewWithBlueprint(componentTView, componentView);
  renderView(componentTView, componentView, componentView[CONTEXT]);
}

This function, very generally, accesses a component's View (a concept I've written about before, core to Angular's internal reference to HTML elements) and renders it using Angular's renderView function.

Let's look in said renderView function:

// Angular 15 source code
// angular/packages/core/src/render3/instructions/shared.ts
export function renderView<T>(tView: TView, lView: LView<T>, context: T): void {
  ngDevMode && assertEqual(isCreationMode(lView), true, 'Should be run in creation mode');
  enterView(lView);
  try {
    const viewQuery = tView.viewQuery;
    if (viewQuery !== null) {
      executeViewQueryFn<T>(RenderFlags.Create, viewQuery, context);
    }

    // Execute a template associated with this view, if it exists. A template function might not be
    // defined for the root component views.
    const templateFn = tView.template;
    if (templateFn !== null) {
      executeTemplate<T>(tView, lView, templateFn, RenderFlags.Create, context);
    }
    
    // ...
    
}

Here, we can see the executeTemplate function being called with the RenderFlags.Create flag, just like we outlined before.

There's no special magic happening inside of the executeTemplate function, either. In fact, this is the whole thing:

// Angular 15 source code
// angular/packages/core/src/render3/instructions/shared.ts

function executeTemplate<T>(
    tView: TView, lView: LView<T>, templateFn: ComponentTemplate<T>, rf: RenderFlags, context: T) {
  const prevSelectedIndex = getSelectedIndex();
  const isUpdatePhase = rf & RenderFlags.Update;
  try {
    setSelectedIndex(-1);
    if (isUpdatePhase && lView.length > HEADER_OFFSET) {
      // When we're updating, inherently select 0 so we don't
      // have to generate that instruction for most update blocks.
      selectIndexInternal(tView, lView, HEADER_OFFSET, !!ngDevMode && isInCheckNoChangesMode());
    }

    const preHookType =
        isUpdatePhase ? ProfilerEvent.TemplateUpdateStart : ProfilerEvent.TemplateCreateStart;
    profiler(preHookType, context as unknown as {});
    templateFn(rf, context);
  } finally {
    setSelectedIndex(prevSelectedIndex);

    const postHookType =
        isUpdatePhase ? ProfilerEvent.TemplateUpdateEnd : ProfilerEvent.TemplateCreateEnd;
    profiler(postHookType, context as unknown as {});
  }
}

If we simplify this function a bit to narrow our focus, we're left with:

// Simplified Angular 15 source code
// angular/packages/core/src/render3/instructions/shared.ts

function executeTemplate<T>(
    tView: TView, lView: LView<T>, templateFn: ComponentTemplate<T>, rf: RenderFlags, context: T) {

    // ...
        
        templateFn(rf, context);

    // ...
}

Here, in this narrowed focus, we can see that when we execute:

// Simplified Angular 15 source code
// angular/packages/core/src/render3/instructions/shared.ts

const templateFn = tView.template;

// ...

executeTemplate<T>(tView, lView, templateFn, RenderFlags.Create, context);

We're simply calling the component's template function with a RenderFlags.Create argument as well as the function's context.

What about when the component updates?

Just as there is a clear demonstration of when the component's template function is called with RenderFlags.Create, there's also a pretty cut-and-dry example of the template function being called with RenderFlags.Update.

This Update flag is passed by Angular's refreshView function, which is called by Angular when a component is ready to update.

// Angular 15 source code
// angular/packages/core/src/render3/instructions/shared.ts

export function refreshView<T>(
    tView: TView, lView: LView, templateFn: ComponentTemplate<{}>|null, context: T) {
  ngDevMode && assertEqual(isCreationMode(lView), false, 'Should be run in update mode');
  const flags = lView[FLAGS];
  if ((flags & LViewFlags.Destroyed) === LViewFlags.Destroyed) return;
  enterView(lView);
  // Check no changes mode is a dev only mode used to verify that bindings have not changed
  // since they were assigned. We do not want to execute lifecycle hooks in that mode.
  const isInCheckNoChangesPass = ngDevMode && isInCheckNoChangesMode();
  try {
    resetPreOrderHookFlags(lView);

    setBindingIndex(tView.bindingStartIndex);
    if (templateFn !== null) {
      executeTemplate(tView, lView, templateFn, RenderFlags.Update, context);
    }
    
    // ...
    
}

That last line should look pretty familiar; the executeTemplate shows up again and is passed RenderFlags.Update this time!

While this is pretty neat to see so plainly, it leaves an important question out in the open: How does the component know when it's ready to update?

Inside Angular's change detection; when refreshView is called

To answer the question of "how does Angular know when a component is ready to update", let's follow the stack trace of when the refreshView function is called.

If we take a step one level up, we can see that refreshView is called within a function called detectChangesInternal:

// Angular 15 source code
// angular/packages/core/src/render3/instructions/shared.ts

export function detectChangesInternal<T>(
    tView: TView, lView: LView, context: T, notifyErrorHandler = true) {
  const rendererFactory = lView[RENDERER_FACTORY];

  // Check no changes mode is a dev only mode used to verify that bindings have not changed
  // since they were assigned. We do not want to invoke renderer factory functions in that mode
  // to avoid any possible side-effects.
  const checkNoChangesMode = !!ngDevMode && isInCheckNoChangesMode();

  if (!checkNoChangesMode && rendererFactory.begin) rendererFactory.begin();
  try { 
      refreshView(tView, lView, tView.template, context);
   } catch (error) {
  	// ...
  }
}

Which is called within the exposed @angular/core detectChanges function:

// Angular 15 source code
// angular/packages/core/src/render3/view_ref.ts
detectChanges(): void {
	detectChangesInternal(this._lView[TVIEW], this._lView, this.context as unknown as {});
}

Calling Change Detection Manually

Let's use Angular's NgZone's runOutsideOfAngular to run some code outside of Angular's typical change detection:

import { ApplicationRef, Component, NgZone } from '@angular/core';

@Component({
  selector: 'my-app',
  template: `
  <h1>Hello {{name}}</h1>
  <button (click)="changeName()">Change Name</button>
  `,
})
export class AppComponent {
  constructor(private ngZone: NgZone) {}

  name = '';
  changeName() {
    this.ngZone.runOutsideAngular(() => {
      setTimeout(() => {
        this.name = 'Angular';
      });
    });
  }
}

Don't worry if you're not already familiar with NgZone, we'll explain how it works fully in this article. 😄

Here, you'll notice that when you press the <button> for the first time, it does not show Hello Angular as you might expect. It's only on the subsequent button presses that the proper greeting shows up.

This is intentional behavior - after all, we've told our code to execute outside of Angular's typical change detection. To solve this, we can manually run detectChanges ourselves:

import {
  ChangeDetectorRef,
  Component,
  NgZone,
} from '@angular/core';

@Component({
  selector: 'my-app',
  template: `
  <h1>Hello {{name}}</h1>
  <button (click)="changeName()">Change Name</button>
  `,
})
export class AppComponent {
  constructor(private ngZone: NgZone, private cd: ChangeDetectorRef) {}

  name = '';
  changeName() {
    this.ngZone.runOutsideAngular(() => {
      setTimeout(() => {
        this.name = 'Angular';
        this.cd.detectChanges();
      });
    });
  }
}	

This detectChanges then calls the refreshView call that we saw earlier. That, in turn, calls executeTemplate with RenderFlags.Update, which gets passed to the component's template function which was output by NGC.