guide error handler - devonfw/devon4ng GitHub Wiki

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Error Handler in angular

Angular allows us to set up a custom error handler that can be used to control the different errors and them in a correct way. Using a global error handler will avoid mistakes and provide a use friendly interface allowing us to indicate the user what problem is happening.

What is ErrorHandler

ErrorHandler is the class that Angular uses by default to control the errors. This means that, even if the application doesn’t have a ErrorHandler it is going to use the one setup by default in Angular. This can be tested by trying to find a page not existing in any app, instantly Angular will print the error in the console.

Creating your custom ErrorHandler step by step

In order to create a custom ErrorHandler three steps are going to be needed:

Creating the custom ErrorHandler class

In this first step the custom ErrorHandler class is going to be created inside the folder /app/core/errors/errors-handler.ts:

import { ErrorHandler, Injectable, Injector } from '@angular/core';
import { HttpErrorResponse } from '@angular/common/http';

@Injectable()
export class ErrorsHandler implements ErrorHandler {

    constructor(private injector: Injector) {}

    handleError(error: Error | HttpErrorResponse) {
      //  To do: Use injector to get the necessary services to redirect or
      // show a message to the user
      const classname  = error.constructor.name;
      switch ( classname )  {
        case 'HttpErrorResponse':
          console.error('HttpError:' + error.message);
          if (!navigator.onLine) {
            console.error('There's no internet connection');
            // To do: control here in internet what you wanna do if user has no internet
          } else {
            console.error('Server Error:' + error.message);
            // To do: control here if the server gave an error
          }
          break;
        default:
          console.error('Error:' + error.message);
          // To do: control here if the client/other things gave an error
      }
    }
}

This class can be used to control the different type of errors. If wanted, the classname variable could be used to add more switch cases. This would allow control of more specific situations.

Creating a ErrorInterceptor

Inside the same folder created in the last step we are going to create the ErrorInterceptor(errors-handler-interceptor.ts). This ErrorInterceptor is going to retry any failed calls to the server to make sure it is not being found before showing the error:

import { HttpInterceptor, HttpRequest, HttpHandler, HttpEvent } from '@angular/common/http';
import { Injectable } from '@angular/core';
import { Observable } from 'rxjs';
import { retry } from 'rxjs/operators';

@Injectable()
export class ErrorsHandlerInterceptor implements HttpInterceptor {

    constructor() {}
    intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
        return next.handle(req).pipe(
            retryWhen((errors: Observable<any>) => errors.pipe(
                delay(500),
                take(5),
                concatMap((error: any, retryIndex: number) => {
                    if (++retryIndex === 5) {
                        throw error;
                    }
                    return of(error);
                })
            ))
        );
    }
}

This custom made interceptor is implementing the HttpInterceptor and inside the method intercept using the method pipe,retryWhen,delay,take and concatMap from RxJs it is going to do the next things if there is errors:

  1. With delay(500) do a delay to allow some time in between requests

  2. With take(5) retry five times.

  3. With concatMap if the index that take() gives is not 5 it returns the error, else, it throws the error.

Creating a Error Module

Finally, creating a module(errors-handler.module.ts) is necessary to include the interceptor and the custom error handler. In this case, the module is going to be created in the same folder as the last two:

import { NgModule, ErrorHandler } from '@angular/core';
import { CommonModule } from '@angular/common';
import { ErrorsHandler } from './errors-handler';
import { HTTP_INTERCEPTORS } from '@angular/common/http';
import { ErrorsHandlerInterceptor } from './errors-handler-interceptor';

@NgModule({
  declarations: [], // Declare here component if you want to use routing to error component
  imports: [
    CommonModule
  ],
  providers: [
    {
      provide: ErrorHandler,
      useClass: ErrorsHandler,
    },
    {
      provide: HTTP_INTERCEPTORS,
      useClass: ErrorsHandlerInterceptor,
      multi: true,
    }
  ]
})
export class ErrorsHandlerModule { }

This module simply is providing the services that are implemented by our custom classes and then telling angular to use our custom made classes instead of the default ones. After doing this, the module has to be included in the app module app.module.ts in order to be used.

....
  imports: [
    ErrorsHandlerModule,
    ....

Handling Errors

As a final step, handling these errors is necessary. There are different ways that can be used to control the errors, here are a few:

  • Creating a custom page and using with Router to redirect to a page showing an error.

  • Creating a service in the server side or Backend to create a log with the error and calling it with HttpClient.

  • Showing a custom made SnackBar with the error message.

Using SnackBarService and NgZone

If the SnackBar is used directly, some errors can occur, this is due to SnackBar being out of the Angular zone. In order to use this service properly, NgZone is necessary. The method run() from NgZone will allow the service to be inside the Angular Zone. An example on how to use it:

import { ErrorHandler, Injectable, Injector, NgZone } from '@angular/core';
import { HttpErrorResponse } from '@angular/common/http';
import { MatSnackBar } from '@angular/material';

@Injectable()
export class ErrorsHandler implements ErrorHandler {

    constructor(private injector: Injector, private zone: NgZone) {}

    handleError(error: Error | HttpErrorResponse) {
      // Use injector to get the necessary services to redirect or
      const snackBar: MatSnackBar = this.injector.get(MatSnackBar);
      const classname  = error.constructor.name;
      let message: string;
      switch ( classname )  {
        case 'HttpErrorResponse':
          message = !(navigator.onLine) ? 'There is no internet connection' : error.message;
          break;
        default:
          message = error.message;
      }
      this.zone.run(
        () => snackBar.open(message, 'danger', { duration : 4000})
      );
    }
}

Using Injector the MatSnackBar is obtained, then the correct message is obtained inside the switch. Finally, using NgZone and run(), we open the SnackBar passing the message, and the parameters wanted.

You can find a working example of this guide in devon4ts-samples.

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