This is the problem: I have a service that makes an HTTP request in the constructor:
constructor(public http: Http, public geolocation: Geolocation) {
this.http = http;
this.geolocation = geolocation;
//Http request... this will set variable forecast of the class when complete.
this.getForecast(16);
}
then I inject that service in a component like this:
constructor(public connector: ApiConnector) {
this.forecast = connector.forecast;
}
If I try to use the forecast member of the component class on the component decorator as I do here:
#Component({
selector: 'app',
directives: [MainForecast, DailyForecast],
template: `
<main-forecast [main-weather]="forecast"></main-forecast>
<daily-forecast [weather-list]="forecast.list"></daily-forecast>
`,
})
I get an error, "Cannot read property 'list' of undefined in..."
Is there any possibility to work with promises in the component constructor?
Angular2 advise to do heavy work inside ngOnInit NOT inside the constructor.
ngOnInit is a lifecycle hook / event.
You're trying to work around the angular lifecycle.
It seems you assume that your service is initiated during startup. This is not the case. Even if your service is provided on root level it may only be initialized just in time. You may want to call some public init method during your startup logic to ensure its initialization.
Related
I have an Angular application that interacts with several REST APIs. There is some common logic when sending requests to these APIs (for example, logging the API name and endpoint). I want to have a seperate service for each API (ApiAService, ApiBService, etc), but I don't want to repeat the common logic.
I'd like to do this by creating a core service that each service depends on, but I'm not sure how to do it because the core service would need some configuration that would be specific to each place it's used (for example, the API name).
I know how to do this using traditional composition or inheritance:
Composition
I could make a non-injectable service class that accepts some configuration and instantiate it for each individual API service:
class RequestService(apiName: string) {
public makeRequest(url: string, ...) {...}
}
#Injectable()
class ApiAService {
private readonly requestService = new RequestService("API A")
public queryEndpoint() {
return this.requestService.makeRequest(...)
}
}
Inheritance
I could make the base request service abstract and extend it for each individual API:
#Injectable()
abstract class ApiService {
abstract apiName: string;
protected makeRequest(url: string, ...) {...}
}
#Injectable()
class ApiAService extends ApiService {
private readonly apiName = "API A"
public queryEndpoint() {
return this.makeRequest(...)
}
}
However, I think the most idiomatic Angular way is through dependency injection, and it has the advantage of being able to be swapped out for testing. But if I just add apiName to the injectable core service's constructor, I have no way to actually set the property when depending on it. I don't care if each service has its own instance of RequestService - it doesn't need to be a singleton.
How can I do this?
#Injectable()
class RequestService {
constructor(private apiName: string) {}
public makeRequest(url: string, ...) {...}
}
#Injectable()
class ApiAService {
constructor(private requestService: RequestService) {} // How do I inject apiName here?
public queryEndpoint() {
return this.requestService.makeRequest(...)
}
}
I am getting the transition data in the route js file like so:
beforeModel(transition) { console.log(transition) }
And I want to use it in a function in my controller like this:
import Controller from '#ember/controller';
export default class ListingsController extends Controller {
get pageTitle() {
if (this.transition.targetName == 'foo') {
return 'page title';
}
}
}
And then I want to display the result like this:
<h1>{{this.pageTitle}}</h1>
I cannot find a way to pass the transition data from the route to the controller. Any ideas?
While you technically can leverage the beforeModel to get the controller via this.controllerFor as #KathirMagaesh suggests, I wouldn't actually advocate for this solution. It's definitely not the normal or expected Ember pattern. Furthermore, if you look at the transition api, there is no reference to transition.targetName. If this works, this is private api and thus brittle.
If you need to change a property based on the current route, you should be using the public router service which provides some useful properties for this very purpose!
For example, in your controller you could have a computed property that leverages the router service to determine what the page title should be
import Controller from '#ember/controller';
import { computed } from '#ember/object';
import { inject } from '#ember/service';
// this injects the router service into our component via Ember's DI framework
router: inject(),
export default Controller.extend({
pageTitle: computed('router.currentRouteName', function(){
let currentRoute = this.router.currentRouteName;
if(currentRoute === 'foo'){
return 'page title';
}
// do other stuff for other routes.
})
})
This leverages currentRouteName which is the period separated name like foo.bar. You can also also access the url via currentURL which would be /foo/bar
PS. Since I haven't used ES6 classes yet, I've provided the old style ember solution. You'll probably need to use the #computed decorator or #tracked where I'm using the computed function. I only know about the Octane ember style from RFCs and awesome blog posts but am not up to date with what's landed.
PPS. If you're on old ember, the current route name / URL properties are available on the application controller.
In the beforeModel hook use
this.controllerFor(currentrRouteName).set('transition', transition);
This will set transition property in controller of the current router.
For more on controllerFor()
I'd like to be able to pass some data\propagate events from a plugin on the page to my Angular 4 app.
More specifically, in my case data\events are generated inside a Silverlight plugin app that is next to the Angular app on the page.
I have the following solution in my mind:
Create a global JS function which gets called from Silverlight (since this seems to be the simplest way to get data out from Silverlight) when
there is a need to talk to Angular side.
The function, in turn, calls some Angular class method passing data
collected from Silverlight.
As an illustration to that (excluding the Silverlight part), we could have the following.
A method as an entry point on the Angular side:
export class SomeAngularClass {
public method(data: any): void {
...
}
}
And somewhere outside the Angular realm, we add a global plain JavaScript function (to be called by Silverlight):
window.somePlainJsFunction = function (data) {
// How to consume SomeAngularClass.method() from here?
}
The question is: how can we call the Angular class methods from a plain JavaScript function?
As pointed by #Dumpen, you can use #HostListener to get the custom event dispatched from javascript outside of Angular. If you also want to send parameters, then you can send them by adding them as detail object.
In Javascript:
function dispatch(email, password) {
var event = new CustomEvent('onLogin', {
detail: {
email: email,
password: password
}
})
window.dispatchEvent(event);
}
Then you can call this method on button click.
Now to listen to this dispatched event in Angular, you can create function in Angular component and add #HostListener to it like below:
#HostListener('window:onLogin', ['$event.detail'])
onLogin(detail) {
console.log('login', detail);
}
This code you can add in any component. I have added it to my root component - AppComponent
You can use a events, so you have the JavaScript send an event that you are listening for in your service.
The CustomEvent is a new feature so you might need to polyfill it: https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent/CustomEvent
JavaScript:
var event = new CustomEvent("CallAngularService");
window.dispatchEvent(event);
Angular:
#HostListener("window:CallAngularService")
onCallAngularService() {
// Run your service call here
}
I think this is what you are looking for service out side angular
You can create function/class outside Angular and provide as a value in the angular. in this way you can handle both angular and non angular stuffs together:
class SomePlainClass {
...
}
window.somePlainJsFunction = new SomePlainClass();
#NgModule({
providers: [{provide: SomePlainClass, useValue: window.somePlainJsFunction}],
...
})
class AppModule1 {}
#NgModule({
providers: [{provide: SomePlainClass, useValue: window.somePlainJsFunction}],
...
})
class AppModule2 {}
class MyComponent {
constructor(private zone:NgZone, private SomePlainClass:SharedService) {
SomePlainClass.someObservable.subscribe(data => this.zone.run(() => {
// event handler code here
}));
}
}
The way it should be done depends on particular case, especially on the precedence.
If Silverlight aplication is initialized before Angular application, and window.somePlainJsFunction is called before finishing Angular application initialization, this will result in race condition. Even if there was an acceptable way to get Angular provider instance externally, the instance wouldn't exist during somePlainJsFunction call.
If window.somePlainJsFunction callback is called after Angular bootstrap, window.somePlainJsFunction should be assigned inside Angular application, where SomeAngularClass provider instance is reachable:
window.somePlainJsFunction = function (data) {
SomeAngularClass.method();
}
If window.somePlainJsFunction callback is called before Angular bootstrap, it should provide global data for Angular application to pick up:
window.somePlainJsFunction = function (data) {
window.angularGlobalData = data;
}
Then window.angularGlobalData can be used inside Angular, either directly or as a provider.
Working Example
I'm trying to get into Nativescript + Angular2, and I read the following in the tutorial:
We’ll build this functionality as an Angular service, which is Angular’s mechanism for reusable classes that operate on data.
What they then do is to create a simple class, like this:
import { Injectable } from "#angular/core";
import { User } from "./user";
#Injectable()
export class UserService {
register(user: User) {
alert("About to register: " + user.email);
}
}
Now, I can't really see the difference between a normal class and a service - this is a very normal class definition.
So, why is it called an "Angular service"?
This creates a basic Angular service with a single method that takes an instance of the User object you created in the previous section.
Also, when using this "service" in the tutorial, it isn't clear to me when this class is instantiated - when is the construction executed? Is the object saved in memory for later use? The only call to the "userservice" in the tutorial is like this:
import { Page } from "ui/page";
import { Component, ElementRef, OnInit, ViewChild } from "#angular/core";
import { User } from "../../shared/user/user";
import { UserService } from "../../shared/user/user.service";
import { Router } from "#angular/router";
import { Color } from "color";
import { View } from "ui/core/view";
#Component({
selector: "my-app",
providers: [UserService],
templateUrl: "./pages/login/login.html",
styleUrls: ["./pages/login/login-common.css", "./pages/login/login.css"]
})
export class LoginComponent implements OnInit {
user: User;
isLoggingIn = true;
#ViewChild("container") container: ElementRef;
constructor(private router: Router, private userService: UserService, private page: Page) {
this.user = new User();
this.user.email = "bla#bla.com";
this.user.password = "1234";
}
//.... methods and stuff...
}
A class, in that context, is a regular class as in any other OO language: a "prototype" of objects which you can create instances simply using:
let myInstance = new MyClass(<arguments...>);
So, actually, an Angular service is also a class.
But consider services a special kind of class. The main difference between regular classes and service classes is their lifecycle, specially who creates their instance (who calls new).
Instances of a service are created - and managed (disposed) - by the Angular "container" (angular injector, actually).
You can also "inject" instances of service classes in constructors of other service classes (or other managed components).
A good resource in the capabilites of services is Angular's Dependency Injection Guide.
When is the construction executed?
The injector executes the construction. When? See below.
Is the object saved in memory for later use?
It could be. Depends on where you registered the service.
Before anything, know that Angular DI is a hierarchical injection system.
If you register the service with an Angular Module, the service will be created by the application's root injector. So everyone below it (aka everyone in that module) will receive the same instance of the service. In other words, Angular (will call the injector only once and) will create only one instance of the service class and pass that same instance to whoever asks for that service. And that instance will live as long as that module lives.
Now, if you register the service with a component, then the service will be registered with that component's injector. So when such component requests an instance of the service, angular will call the injector and create an instance. If any child of that component asks for an instance of such service, angular will provide the same instance. No one else, only children of the component, will receive that same instance. When that component dies, the service instance dies as well.
How does a "regular class" differ? It lacks the Injector?
The difference is not only the lack of an injector.
Angular aside, just JavaScript: you create an instance of a "regular class" by calling let instance = new MyRegularClass() somewhere in your code, right?
This instance has no "magical effects", it does nothing more than any class would (just regular JavaScript methods and properties). Example: if you need instances of other classes as arguments in the constructor, no one will "magically" create you those instances and pass them. You will have to create them manually, when calling new (e.g. new MyClass(new SomeOtherClassIDependOn(), ...)). If you want to instantiate SomeOtherClassIDependOn only once and reuse the same instance everywhere it is needed, you will have to save that instance and pass it wherever it is neeed yourself.
As services, though, angular can take some of that burden off your shoulders.
Now, before anything: since every service, deep down, is a class, someone has to call new MyServiceClass(). The difference is that someone is not you anymore. There is no new UserService() in your code. So, who is it? This someone is the Injector.
When Angular notices someone asks for a service, it calls for the injector to instantiate that service. The injector then calls let serviceInstance = new MyServiceClass(<dependencies>) and adds some "magic" to it (e.g. it can pass - inject - instances of other services to the constructor of a service), and make it available (save it) for anyone that requests that service in the scope you registered it.
Note: You can call new UserService(...) yourself, as it UserService is a class. But this instance is a regular object, not managed by angular, there is no magic (no constructor arguments will be injected, no instance is saved and reused).
I'd like to be able to pass some data\propagate events from a plugin on the page to my Angular 4 app.
More specifically, in my case data\events are generated inside a Silverlight plugin app that is next to the Angular app on the page.
I have the following solution in my mind:
Create a global JS function which gets called from Silverlight (since this seems to be the simplest way to get data out from Silverlight) when
there is a need to talk to Angular side.
The function, in turn, calls some Angular class method passing data
collected from Silverlight.
As an illustration to that (excluding the Silverlight part), we could have the following.
A method as an entry point on the Angular side:
export class SomeAngularClass {
public method(data: any): void {
...
}
}
And somewhere outside the Angular realm, we add a global plain JavaScript function (to be called by Silverlight):
window.somePlainJsFunction = function (data) {
// How to consume SomeAngularClass.method() from here?
}
The question is: how can we call the Angular class methods from a plain JavaScript function?
As pointed by #Dumpen, you can use #HostListener to get the custom event dispatched from javascript outside of Angular. If you also want to send parameters, then you can send them by adding them as detail object.
In Javascript:
function dispatch(email, password) {
var event = new CustomEvent('onLogin', {
detail: {
email: email,
password: password
}
})
window.dispatchEvent(event);
}
Then you can call this method on button click.
Now to listen to this dispatched event in Angular, you can create function in Angular component and add #HostListener to it like below:
#HostListener('window:onLogin', ['$event.detail'])
onLogin(detail) {
console.log('login', detail);
}
This code you can add in any component. I have added it to my root component - AppComponent
You can use a events, so you have the JavaScript send an event that you are listening for in your service.
The CustomEvent is a new feature so you might need to polyfill it: https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent/CustomEvent
JavaScript:
var event = new CustomEvent("CallAngularService");
window.dispatchEvent(event);
Angular:
#HostListener("window:CallAngularService")
onCallAngularService() {
// Run your service call here
}
I think this is what you are looking for service out side angular
You can create function/class outside Angular and provide as a value in the angular. in this way you can handle both angular and non angular stuffs together:
class SomePlainClass {
...
}
window.somePlainJsFunction = new SomePlainClass();
#NgModule({
providers: [{provide: SomePlainClass, useValue: window.somePlainJsFunction}],
...
})
class AppModule1 {}
#NgModule({
providers: [{provide: SomePlainClass, useValue: window.somePlainJsFunction}],
...
})
class AppModule2 {}
class MyComponent {
constructor(private zone:NgZone, private SomePlainClass:SharedService) {
SomePlainClass.someObservable.subscribe(data => this.zone.run(() => {
// event handler code here
}));
}
}
The way it should be done depends on particular case, especially on the precedence.
If Silverlight aplication is initialized before Angular application, and window.somePlainJsFunction is called before finishing Angular application initialization, this will result in race condition. Even if there was an acceptable way to get Angular provider instance externally, the instance wouldn't exist during somePlainJsFunction call.
If window.somePlainJsFunction callback is called after Angular bootstrap, window.somePlainJsFunction should be assigned inside Angular application, where SomeAngularClass provider instance is reachable:
window.somePlainJsFunction = function (data) {
SomeAngularClass.method();
}
If window.somePlainJsFunction callback is called before Angular bootstrap, it should provide global data for Angular application to pick up:
window.somePlainJsFunction = function (data) {
window.angularGlobalData = data;
}
Then window.angularGlobalData can be used inside Angular, either directly or as a provider.
Working Example