在ionic2中自定义服务：该服务需要使用到NavController

       如果在自定义的服务中的constructor中直接注入（NavControlll） private navCtrl: NavController，然后再page页面或组件中的constructor中也采用注入的方式，可能会出现这样的错误：

No provider for NavController! (YourService -> NavController)

那该如何是好呢？

简单的方法有：在page中直接传递navCtrl对象到Service或者Provider中的constructor（此时使用该服务就不能使用注入的方式）或具体方法中（作为方法的参数），然后通过该对象使用。

另一种方案呢？

既保留服务能直接在构造方法中直接注入，又能使用navController对象。

But keep in mind, this is not good practice– services should not be tied to the UI of your application.（这话需要特别注意）

参考：

Ionic 2 UI/Alert from a Angular 2 Service

Ionic 2 use NavController in Service to show Alerts

Ionic2, inject NavController to Injectable Service

Ionic 2 UI/Alert from a Angular 2 Service

When you create an Ionic 2 page/component your constructor can inject the NavController like this:

constructor(private nav:NavController) {}

However, if you have tried this from a service then you’ve probably found this error:

No provider for NavController! (YourService -> NavController)

The particular use case i was tackling is to provide a simple Alert when there is an error in my service that was using Angular 2’s http.

To fix the problem, first import IonicApp:

import {IonicApp, Alert} from 'ionic-angular';

In our constructor we inject our app:

constructor(private app: IonicApp) {}

Now to get the NavController and present some UI we can:

let alert = Alert.create({title: 'Error', message: message, buttons: ['OK']}); var nav = this.app.getActiveNav(); nav.present(alert);

Hope this helps someone out struggling with the same problem.

Ionic 2 use NavController in Service to show Alerts

June 20, 2016  Uncategorized​  ​ 3 Comments  Can Kattwinkel

Usually you inject Nav:NavController into your classes constructor to use it:

But if you are working in a service you will receive the following error:

If you want to show alerts from within a service you are required to use the NavController.  To do so simply create the private field nav with type NavController. Then use the app.getActiveNav method provided by ionic-angular. See the snippet below for a full example:

Now you can show dialogs and alerts from a service in Ionic 2. But keep in mind, this is not good practice– services should not be tied to the UI of your application.

As you can se here, @mhartington (from ionic team) says:

Just to chime in on this, you shouldn't be injecting ViewController or NavController into Service. This is not their intended purpose.

And

The service shouldn't be responsible for displaying alerts/loading/ or any component that needs to be activated by nav. A service should just be for getting and returning data.

Anything else should be done within the component.

That being said, you can obtain the nav by doing

var nav = this.app.getActiveNav();

Like you can see here.

=================================================

EDIT:

It's bad practice to change a view from a service (broken MVC). However, you could send events from services to the main controller, and the controller can use NavController (best way), or you could send NavController to your service like an attribute (not bad way...). Or you may need to create a component instead of using the service.

So, a better way to do it, would be:

observable​ in your service, to know when the dismiss

import {Injectable} from '@angular/core';
import {Platform} from 'ionic-angular';
import {Observable} from 'rxjs/Observable';

@Injectable()
export class MyCustomService {

  // Observables we're going to use
  private dismissObserver: any;
  public dismiss: any;

  constructor(private platform: Platform){
    // Your stuff
    // ...

    this.dismissObserver = null;
    this.dismiss = Observable.create(observer => {
        this.dismissObserver = observer;
    });
  }

  public yourMethod(...):void {
    // Here we send the order to go back to the home page
    this.dismissObserver.next(true);
  }
}

only, in your app.ts

initializeApp(): void {
    this.platform.ready().then(() => {
      // Okay, so the platform is ready and our plugins are available.
      // Here you can do any higher level native things you might need.
      StatusBar.styleDefault();

      // We subscribe to the dismiss observable of the service
      this.myCustomService.dismiss.subscribe((value) => {
        this.navController.setRoot(HomePage);
      });
    });
  }

ionicBootstrap

ionicBootstrap(MyApp, [MyCustomService, ...], {
  //statusbarPadding: true
});

Or, following the Angular2 Style Guide​, add it as a provider

@Component({
  templateUrl: 'build/app.html',
  directives: [...],
  providers: [MyCustomService]
})
class MyApp {
  // ...
}