Extension life cycle

Extension life cycle

As described in Life cycle, a LoopBack Application has its own life cycles at runtime. Corresponding events such as start and stop are emitted upon the state change. Please note that LoopBack only support start and stop events for an application’s life cycles at this moment.

Extension modules for LoopBack often contribute artifacts such as servers, datasources, and connectors to the application. They typically provide a component to bind such artifacts to the context together. Being able to listen on life cycle events is important for extension modules to collaborate with the application.

An extension module follows the same way as applications to implement and register life cycle observers.

Implement a life cycle observer

A life cycle observer class optionally implements start and stop methods to be invoked upon start and stop events emitted by an application’s life cycle respectively.

import {LifeCycleObserver} from '@loopback/core';

export class MyLifeCycleObserver implements LifeCycleObserver {
  start() {
    // It can return `void` or `Promise<void>`
  }
  stop() {
    // It can return `void` or `Promise<void>`
  }
}

A life cycle observer can be tagged with CoreTags.LIFE_CYCLE_OBSERVER_GROUP to indicate its group to be invoked for ordering. We can decorate the observer class with @lifeCycleObserver to provide more metadata for the binding.

import {lifeCycleObserver} from '@loopback/core';

@lifeCycleObserver('g1')
export class MyLifeCycleObserver {
  // ...
}

Register a life cycle observer

A life cycle observer can be registered by calling lifeCycleObserver() of the application. It binds the observer to the application context with a special tag - CoreTags.LIFE_CYCLE_OBSERVER.

app.lifeCycleObserver(MyObserver);

Life cycle observers can be declared via a component class too. when the component is mounted to an application, the observers are automatically registered.

export class MyComponentWithObservers implements Component {
  lifeCycleObservers = [XObserver, YObserver];
}

// Mount the component
app.mount(MyComponentWithObservers);
// Now `XObserver` and `YObserver` are registered in the application.