Understanding components

Components are the building blocks of Typescene’s object oriented, event driven architecture.


Like other frameworks, Typescene doesn’t just display your application as a static document — it captures events and updates the UI when needed, which allows users to interact with your application.

This means Typescene has to work out at all times which ‘view’ should be displayed, incorporate your data into this view, update UI elements when needed, and call the correct event handlers in your application. All of this is done with the help of a single class, which is at the core of Typescene’s architecture: the Component class.

Components (instances of Component) are just like regular objects, but provide additional infrastructure to emit and handle events, observe changes, and link to other components in ways that enforce a consistent state of the application’s data and UI.

Note: this guide is a long read. It may take 15-30 minutes to work your way through, and you may need to refer back to it later.

It’s a good idea to read through this guide once, then get some practical experience by following along with Your first project, and come back to this guide afterwards for a deeper understanding.

The ManagedObject class

The ManagedObject class is the base class for many other classes in the Typescene API, including Component. We refer to all instances as ‘managed objects’, which includes components.

Components are managed objects, but not all managed objects are components: other classes that derive from ManagedObject are used to create lists and other data structures.

Unique ID

The first property that’s set on all managed objects is managedId, which is simply a read-only unique ID:

import { Component } from "typescene";
class MyClass extends Component { /* ... */ }
class SomethingElse extends Component { /* ... */ }

let a = new MyClass();
let b = new SomethingElse();
a.managedId  // number
b.managedId  // different number


A key feature that powers Typescene’s event-driven architecture is the ability to ‘emit’ events from any managed object.

  • Events can be emitted using the emit(...) method on an object.
  • Events are instances of ManagedEvent. Events have a name property (a CamelCased string, e.g. Change, Click, FocusOut), and possibly other properties to communicate information about the event. These are considered read-only, so the same instance can be emitted more than once.
import { Component, ManagedEvent, CHANGE } from "typescene";

class Foo extends Component {
  doSomething() {
    // ... do something useful here ...
    this.emit(new ManagedEvent("Bar"));
    this.emit("Bar");  // same
    this.emit(CHANGE);  // imported event

Calling the doSomething method above causes 3 events to be emitted: two events named Bar, and a Change event. The CHANGE event object is created by Typescene itself and is often reused to signal that some kind of change occurred in the object’s internal state.

The easiest (but rather limited) way to handle events is by adding an event handler to the class itself, using the static handle method:

  Bar() {
    // handle Bar events, `this` is a Foo instance
    console.log("Bar on " + this.managedId);
  Change() {
    console.log("Change event handled");

let foo = new Foo();
// > Bar on 18
// > Bar on 18
// > Change event handled

These handlers are invoked for events that are emitted from any instance of this class (or derived classes). Alternatively, events can be handled using managed child references or observers, as explained below.


Life cycle management is another core feature of ManagedObject. Components and other managed objects are in one of the following states, as reflected by the read-only managedState property.

Managed states diagram

  • Newly created objects start out in a Created state.
  • Objects can be activated using the activateManagedAsync method. What ‘active’ means could be determined by the object (class) itself, although it’s mostly used for active composition (see below). All objects emit the ACTIVE event when they enter the Active state.
  • The deactivateManagedAsync method transitions objects from Active to Inactive and emits the INACTIVE event afterwards.
  • The destroyManagedAsync method transitions objects to a Destroyed state and emits the DESTROYED event. Activated objects are always inactivated first.

All of these transitions are asynchronous: objects are in ‘Activating’ / ‘Deactivating’ / ‘Destroying’ states while async methods run to perform any actions associated with a state change.

Note: Asynchronous state transitions are implemented using standard JavaScript Promises, which were introduced in ES6. TypeScript includes support for asynchronous methods using the async and await keywords.

class Foo extends Component {
  async activateMe() {
    // transition to active state (async)
    await this.activateManagedAsync();
    console.log("Foo is now active: " + this.managedId);

  // this method is called while activating:
  async onManagedStateActivatingAsync() {
    await super.onManagedStateActivatingAsync();
    console.log("Foo is activating: " + this.managedId);

Notice how the activateMe method waits for the state change to succeed, which involves calling the async onManagedStateActivatingAsync method and waiting for it to finish. Throwing an exception from this method would cancel the state transition.

If we call activateMe normally for more than one instance, the transitions will happen at the same time, in the background:

let foo1 = new Foo();
let foo2 = new Foo();
// > Foo is activating: 16
// > Foo is activating: 17
// > Foo is now active: 16
// > Foo is now active: 17

There are a total of 5 async methods to handle state transitions:

  • onManagedStateActivatingAsync — object is activating
  • onManagedStateActiveAsync — object has been activated
  • onManagedStateDeactivatingAsync — object is deactivating
  • onManagedStateInactiveAsync — object has been deactivated
  • onManagedStateDestroyingAsync — object is being destroyed

Note that there is no ‘onManagedStateDestroyedAsync’ method, since the object is considered to be in an invalid state after being ‘destroyed’ anyway, and should not be used.

Managed references

Along with managed objects, events, and states, Typescene also manages references between components and other managed objects.

As a rule, properties on one component that refer to another component (or managed object) should be decorated with the @managed decorator — or one of the other ‘managed reference’ decorators described below.

import { Component, managed, managedChild } from "typescene";

class Customer extends Component {
  constructor (public name: string) {

  updated?: Date;  // not managed

  salesRep?: Person;  // managed reference

  primaryContact?: Person;  // managed reference

class Person extends Component {
  constructor (public name: string) {

The Customer class in this example contains two managed references — properties that are decorated with @managed*. These decorators add property setters to ‘manage’ the reference, and enforce consistency during runtime:

  • Managed reference properties can’t take any other values. Setting a managed reference property to anything other than undefined or a valid managed object reference causes a TypeError.
  • If a referenced object gets destroyed (i.e. transitions to the ‘Destroyed’ state), the referencing property is automatically reset to undefined.

This means that managed reference properties always contain a valid reference to an object that is not yet destroyed.

Managed child references

Managed child references, such as the Customer.primaryContact reference in the above example, enforce strict parent-child relationships — also known as has-a relationships, e.g. each Customer component has a primary contact person component, a UI Component object has other components. The @managedChild decorator adds the following checks:

  • Child objects can have only one parent at the same time. If another @managedChild reference property was already set to the same (child) object, that property is automatically set to undefined first, i.e. the child object moves between parent objects, because the has-a relationship cannot apply to both objects at the same time.
  • If a @managedChild reference property is set to a new value (including undefined), the previously referenced child object is automatically destroyed.
  • All child objects of a destroyed managed object are automatically destroyed, too.

A reference to the current parent object (if any) can be obtained using getManagedParent(). To get a reference to the parent component (which may be different if the component is part of a managed list, for example — refer to Collections below), use the getParentComponent method.

let cust = new Customer("Acme Corp");
cust.primaryContact = new Person("John Doe");
cust.salesRep = new Person("Jane Parker");
cust.primaryContact.getManagedParent()  // => cust
cust.salesRep.getManagedParent()  // => undefined

// moving a *child* object removes it from the parent
let other = new Customer("Conto Sol Ltd");
other.primaryContact = cust.primaryContact;
cust.primaryContact  // => undefined (moved)

// ...others can be referenced more than once
other.salesRep = cust.salesRep;
cust.salesRep  // => Jane Parker (not moved)

Event propagation

Just like in the DOM (in a Web browser), it often makes sense for events to propagate or ‘bubble up’ from a low-level component to its parents. This allows events to be handled in a broader context, where a parent component can manipulate other components as well.

In Typescene this doesn’t happen automatically, but you can handle and/or re-emit events that were emitted on child components (see @managedChild) using the propagateChildEvents method on the parent component. Calling this method without any arguments causes all events from child components to be propagated, but you can also specify a list of event classes or a function to restrict which events are propagated.

import { Component, ManagedChangeEvent, managedChild } from "typescene";

class Company extends Component {
  constructor(public name: string) {

    // use a function to control propagation:
    this.propagateChildEvents(e => {
      if (e.name === "RequestCallBack") {
        // return an event here to emit it:
        return e;
      if (e instanceof ManagedChangeEvent) {
        this.updated = new Date();
        return e;

  primaryContact?: Person;
  // ...

let cust = new Company("Foo");
cust.primaryContact = new Person("John Doe");
// => emits the same event on `cust`
// => ditto, and sets `updated` property


So far, we’ve seen two methods for handling events: the static handle method for events emitted on the same class, and the propagateChildEvents method for events emitted by managed child objects.

A more powerful way to deal with events, which also allows you to handle property changes, is to add an observer to your class:

  • Managed objects can be observed by an observer class. An instance of the observer class is created for each instance of the target class. A reference to the target instance is passed to the observer’s constructor.
import { Component } from "typescene";

class ExhibitA extends Component { }

class MyObserver {
  constructor(public readonly target: ExhibitA) {
    // new observer instance for a new target instance

// observe *all* instances of ExhibitA:

The static observe method is available on component classes, as well as any class that directly or indirectly extends ManagedObject (the target class).

On the observer class:

  • Methods such as onActive, onInactive, and onClick are called when an event with that name is emitted by the target. In addition, the onEvent method is called for all events, while the onChange method is called for all events that are instances of ManagedChangeEvent.
  • Methods such as onFooChange are called when the value of a property foo changes.
  • Asynchronous handler methods such as onFooChangeAsync, or onActiveAsync (names ending with -Async) are invoked asynchronously. For property changes, async handlers are invoked only once after each series of changes or events that occur before the handler is called. Events are handled one by one, in order, but only after all the code after the orignal emit call has run.
import { Component, ManagedChangeEvent,
  managed, managedChild, rateLimit, CHANGE } from "typescene";

class Customer extends Component {
  constructor (public name: string) {

    // propagate change events:
  @managed salesRep?: Person;
  @managedChild primaryContact?: Person;
class CustomerObserver {
  constructor (public readonly customer: Customer) { }
  onNameChange() {
    console.log("Customer is named " +
  async onNameChangeAsync() {
    console.log("Eventually, the name is " +
  onSalesRepChange() {
    if (!this.customer.salesRep) return;
    console.log("Sales rep changed to " +
  onPrimaryContactChange() {
    console.log("Primary contact changed");
  async onChangeAsync(e: ManagedChangeEvent) {
    console.log("Change (rate limited)");

Notice how some events are handled synchronously, immediately after emitting the event, while others are handled asynchronously after all code has run. Additionally, the onChangeAsync method is rate limited using @rateLimit, which means it’s only invoked at most once within a limited time frame:

let cust = new Customer("Acme Inc");
// > Customer is named Acme Inc
cust.primaryContact = new Person("John Doe");
// > Primary contact changed
cust.salesRep = new Person("Jane Parker");
// > Sales rep changed to Jane Parker
cust.name = "Acme Industries Ltd";
// > Customer is named Acme Industries Ltd
// > Eventually, the name is Acme Industries Ltd
// > Change (rate limited)

Refer to the documentation for observe for more details about observers, property changes and event handling.

Observing managed references

Managed reference properties can be observed just like other properties, using observer methods such as onPrimaryContactChange above. However, emitting a ManagedChangeEvent on a referenced object also invokes the observer method. This is useful in cases where you’re not only interested in changes to the object reference, but also in changes to the object itself.

let newContact = new Person("Denise Jones");

// change the actual property value:
cust.primaryContact = newContact;
// > Primary contact changed

// emit a ManagedChangeEvent (CHANGE):
newContact.name = "Denise Doe-Jones";
// > Primary contact changed

Other (non-change) events on referenced objects can be observed as well. Refer to the documentation for @onPropertyEvent for details.


If we need a list of components we could simply use an array:

class Department extends Component {
  people: Person[] = [];

let dep = new Department();
dep.people.push(new Person("John Doe"));
dep.people.push(new Person("Jane Parker"));

However, this way we’d have no way to handle events that might be emitted on these Person instances, or check the consistency of this data structure. Also, JavaScript arrays can have gaps, duplicate elements, and lack runtime type checks, which means we need to be really careful every time we use the objects in this array.

To solve these issues, Typescene provides the ManagedList class, which is purpose-built to contain an ordered set of components and/or other managed objects:

  • Managed lists cannot contain gaps, nor duplicate items (i.e. you can’t add the same component to the same list twice).
  • Managed lists can be restricted so that they only accept instances of a certain class.
  • Components are automatically removed from managed lists when they are destroyed.
  • Managed lists can be observed for changes: events are emitted for additions, removals, and other changes (e.g. sort).
  • When a managed list is assigned to a @managedChild property, the objects in that list automatically become (nested) children, too. To propagate events from objects in the list, use the list’s propagateEvents method.
import { Component, ManagedList, managed } from "typescene";

class Department extends Component {
  people = new ManagedList<Person>();
Department.observe(class {
  constructor(public readonly department: Department) { }
  onPeopleChange() {
    // called once when the list is assigned above
    // and 3 times after add/remove below
    console.log("Number of people: " +

let dept = new Department();
// > Number of people: 0
// > Number of people: 1
// > Number of people: 2
// > Number of people: 1
dept.people.indexOf(person2)  // => 0

Note that ManagedList is itself an instance of ManagedObject, so lists may also contain other lists for nested data structures. For more information on the properties and methods available on managed lists, refer to the documentation for ManagedList.

Managed maps

The other ‘collection’ type provided by Typescene is ManagedMap. This class encapsulates an unordered set of managed objects that are indexed by key, similar to the ES6 Map type, or a Dictionary type in other programming languages. Objects in a map behave the same way as objects in a list, and maps also emit the same events.

let map = new ManagedMap<Person>;
for (let person of dept.people) {
  // add a Person to the map, indexed by name:
  map.set(person.name, person);
map.get(person2.name)  // => person2

For more information on the properties and methods available on managed maps, refer to the documentation for ManagedMap.

Note: all of the features discussed above (events, state, managed references, observers, and collections) apply to ManagedObject, although you’ll mostly use them on your Component classes. What really sets components apart, is their support for static ‘composition’ of multi-level data structures that include bindings and event handlers — read more below.


In object oriented terminology, composition refers to the combination of different objects in order to create a single more complex object. This can be done manually, creating components one by one and linking everything up using references afterwards — but Typescene offers tools that automate this process, using static composition and active composition.

Static composition: presets

We use static composition when we want to describe a data structure, without actually creating it. The result of static composition is a class, not an instance.

As a real-life example, when asked to describe your favorite pizza, you could say ‘a thin crust pizza with mushrooms and extra cheese’. This doesn’t refer to one single pizza that ever existed, but is more like a recipe or template — one that extends the basic recipe of ‘pizza’ which probably includes tomato sauce as well, something that isn’t mentioned in your description. In fact, you could extend your description further and say ‘my favorite pizza, with olives’.

The static Component.with method does exactly this: when it is invoked on a specific component class, it returns another class (constructor). We call the result a ‘preset’ constructor, because after instantiation this constructor presets the properties that were provided to with.

// UIButton is a component
let button = new UIButton();
button.label = "Click me";

// create a preset component constructor:
let ClickMeButton = UIButton.with({
  label: "Click me"
let oneButton = new ClickMeButton();
let twoButton = new ClickMeButton();
oneButton.label  // => "Click me"
twoButton.label  // => "Click me"

This allows you to create ‘templates’ (or factories in object oriented terms) that construct complex component trees from nested preset components — using only JavaScript syntax:

import { UICell, UIRow, UILabel,
  UISeparator, UIButton } from "typescene";

let MyCell = UICell.with(
  // `.with` takes properties and child components:
  { background: "yellow" },
    UILabel.with({ text: "Hello" })
    UIButton.with({ label: "Click me" })

let cell = new MyCell();
cell.content.count  // 3

Note: The types of arguments accepted by with can be defined on a component class by overriding the static Component.preset method. Refer to the reference documentation for this method for details on how to create components with custom strongly-typed with-signatures.

Active composition: @compose

In practice, many components in a ‘composed’ UI or application don’t need to be instantiated until a certain point in time, e.g. when the user navigates to a specific application activity, or when a connection to the back end has been established.

Typescene leverages the life cycle state of a ‘composite’ parent component to control deferred composition of its child components: the @compose decorator can be used on properties of high-level components to automatically create and destroy lower level components that are described using preset constructors.

This means that the @compose decorator works much like the @managedChild decorator — but also observes the parent component’s life cycle state, by adding an observer to the parent class. As soon as a composite parent instance enters the ‘Active’ life cycle state, a child component instance is created and assigned to the decorated property. When the composite parent becomes ‘Inactive’, the property is set to undefined again.

import { Component, compose } from "typescene";

class Greeter extends Component {
  greeting?: string;
  sayHi() {
    return this.greeting + "!!!";

// create a preset constructor first:
let CasualGreeter = Greeter.with({
  greeting: "Howdy"

// define the composite parent class, which
// only needs a `greeter` while active
class DepartmentStore extends Component {
  greeter?: Greeter;

  async openDoorsAsync() {
    await this.activateManagedAsync();
    // (greeter created at this point)

  async closeDoorsAsync() {
    await this.deactivateManagedAsync();
    // (greeter destroyed at this point)

let c = new DepartmentStore();
c.greeter  // => undefined
await c.openDoorsAsync();
c.greeter!.sayHi()  // => "Howdy!!!"
await c.closeDoorsAsync();
c.greeter  // => undefined

This method for creating and destroying child components is used by the Application class to create instances of all activities passed to Application.run, but also by ViewActivity to create view components only when they need to be rendered, and destroy them when they’re no longer visible.

Note: if the child component class isn’t fixed, it can be supplied to with and handled by the presetActiveComponent method from the preset method (which is called by with(…)) instead of using the @compose decorator. This is how view constructors are preset on activity classes:

import { PageViewActivity } from "typescene";

// import the view constructor from elsewhere
import view from "./view";

// create an actively composed view activity
class MyActivity extends PageViewActivity.with(view) {
  async onManagedStateActiveAsync() {
    await super.onManagedStateActiveAsync();
    // (view now exists)

    console.log(this.view instanceof view);  // => true

  async onManagedStateInactiveAsync() {
    await super.onManagedStateInactiveAsync();
    // (view is now destroyed)


If we use the with method to describe components statically in terms of preset property values and child components, we don’t really have a way to change their behavior since we’re not writing any code.

We could add code to the composite parent class to find the right components and update their values and listen for events, but then the parent class depends on the exact composition of its child components to be able to control them — especially for complex UI component structures this is something we want to avoid.

Instead, we can use bindings to add a dependency the other way around: ‘bound’ properties on child components are set to values that are observed on the composite parent (i.e. the component that includes the @compose‘d property). You can use bindings alongside normal property presets in the call to with:

// ... in a preset component constructor
    // normal property preset:
    icon: "user",
    // bind label text to `customerName` property:
    text: bind("customerName")
    // or use a string to contain more bindings
    text: bindf("Good ${dayPart}, ${customerName}")

Bindings are created using the bind and bindf functions, both of which return a Binding object (bindf actually encapsulates a string that incorporates one or more bindings at the same time, but still returns only one Binding object).

Bindings may refer to properties (e.g. customerName) as well as sub properties (e.g. customer.name). Only the first property is actually observed, but if this property is a managed reference, a change event emitted on the referenced object will also trigger an update of the bound property. Learn more about bind, bindf and filters in the reference documentation.

  { hidden: bind("!showCell") },
      dimensions: { width: 320 },
      text: bind("service.exampleValue")
      text: bindf("Howdy, ${name|or(stranger)}!")

Remember, this is a feature of the Component class, so the use of bindings is not limited to UI components — you can also bind properties on Activities, for example.

event handlers

Since the with method returns a component class, we can actually add event handlers directly using the static handle method (see Events above).

// preset button component:
  label: "Click me"
  // handle the Click event:
  Click() { alert("You clicked!") }

While this works just fine, it makes your code much harder to read.

Instead, we can use a shortcuts that’s provided by the with method: preset handlers can be used on properties such as onClick, onChange, and onFocusOut to handle events with matching names. These properties can be set to the following values:

  • "doSomething()" — the name of a method on the composite parent, followed by (). The specified method is called when the event occurs on any instance of the preset component, with the event as the only argument.
  • "+SomeEvent" — an event name, preceded by +. An event with the specified name is emitted from the preset component instance using the propagateComponentEvent method when the event occurs.
  • a function, which is invoked with the event itself as the only parameter and this set to the component instance (not recommended, since it also makes code less readable).
  label: "Click me",

  // emit an event:
  onMouseDown: "+PrepareForClick",

  // call a composite parent method:
  onClick: "handleButtonClick()",

  // inline function:
  onRendered(e: UIComponentEvent) {

Refer to Your first project for more practical examples of how bindings and event handlers can be used.

See also

  • Refer to ManagedReference for a way to store managed references outside of class properties.
  • Refer to the @managedDependency decorator for a way to enforce a ‘reverse’ parent-child relationship, added from the child (dependent) object.
  • Refer to the @shadowObservable decorator for a way to observe a read-only property with a value that’s backed by a writable ‘shadow’ property (e.g. a private property).
  • Refer to ManagedRecord for a general-purpose component class that can be used to hold arbitrary data. This class exposes public methods to browse parent/referrer objects and previous/next sibling objects in a list, and is an excellent choice for containing data loaded from a back end service.