When my NgRx DefaultDataService updates, it removes the type/prototype of the objects in the store. The objects are no longer of type Todo, but are simple objects with no prototype.
The objects come from the server as json objects conforming to the Todo interface. I pipe them to be entered into the store as Todo objects like this:
#Injectable()
export class TodosDataService extends DefaultDataService<ITodo> {
baseUrl;
constructor( http: HttpClient, httpUrlGenerator: HttpUrlGenerator) {
super("Todos", http, httpUrlGenerator);
}
getAll(): Observable<Todo[]> {
//HERE - the ITodo[] from server is mapped to Todo[], then saved to store
return super.getAll().pipe(
map(todosList => todosList.map(todoData => new Todo(todoData)))
);
}
But piping them on this service's update() override method doesn't work:
update(updatedTodo: Update<Todo>): Observable<Todo> {
return super.update(updatedTodo).pipe(map(data => new Todo(data)));
}
Note: I've also tried this using my own httpClient.put request manually without DefaultDataService's super call and several other ways.
The problem problem is that when ngrx applies the deltas (changes) to the object in the store, it somehow not a todo object anymore. Here's a screenshot of the console right after running the above statement:
Todo Prototype is gone
This is causing unexpected behavior.
TL;DR: How can we make sure the update does not remove the prototype? If that's not possible, is there a simple way to intercept updates before they hit the store, in order to provide custom implementation?
This behavior is intended see the following GitHub issue for more info.
https://github.com/ngrx/platform/issues/1641
Another Observable question.
I'm working with angular translation service and Kendo.UI components.
According to the Kendo.UI documentation, I can implement a MessageService to translate its components.
To do so, I have to extend an abstract class that contains a method that return a string:
#Injectable()
export class MyMessageService extends MessageService {
private messages = {'kendo.upload.select': 'initial value'}
constructor(private readonly translate: TranslateService) {
super();
this.translate.get('my_translation_key')
.subscribe((value) => {
this.messages['kendo.upload.select'] = value;
});
}
public get(key: string): string {
return this.messages[key];
}
}
The problem is, the moment kendo injects my custom service, the observables were not resolved yet and my button never get its value changed from "'initial value'" to the translated.
From my understatement, kendo's service should accept an observable, not a string. But I'm not very familiar with promises/subjects/observables.
Is there a workaround for this?
ps.: I know TranslateService has a .instant(key) method, but the same way, the values are not loaded yet.
It sounds like you are saying it the observable is pushed before kendo subscribes. If that is the case then you should use a ReplaySubject instead of an Observable as the Replaysubject will push all the values that were given to the observable to the subscriber when it subscribes.You can tell it how many observed values to replay.
Also you could try instead to call your server after view init or even OnInint. Calling the service in the constructor is not recommended as the view is not done rendering and any html async pipes may not be set yet. Worst case you can use the Replay subject.
I'm a little bit confused as why the following snipped works as expected.
The idea of this service is to have a list of strings where if you add a string, it is removed 5 seconds later. Rxjs is used here:
#Injectable()
export class ErrorService {
private errors: Array<string> = [];
private emitErrorsChanged = new Subject<any>();
public emitErrorsChanged$ = this.emitErrorsChanged.asObservable();
constructor() {
this.emitErrorsChanged$.delay(5000).subscribe(
() => {
if (this.errors.length > 0) {
this.errors.shift();
}
}
);
}
public emitErrorChange(error: string) {
this.errors.push(`${error}`);
this.emitErrorsChanged.next(this.errors);
}
}
An error component is subscribed to this service errorService.emitErrorsChanged$.subscribe(...) and shows the strings in a list. Other components/services add strings by this.errorService.emitErrorChange(error.message).
My question is: why are the removed errors (5s) emitted to the error component? The errors are just removed from the list this.errors.shift(); but the change is not emitted by this.emitErrorsChanged.next(this.errors);
The behavior occurs because you are passing reference to your object (list in this case). The changes made by this.errors.shift(); are not emitted, but I guess you can see current state of this.errors thanks to Angular's change detection. I have prepared a demo (click) so you can see that the object reference is passed in your case - what means that the list in subscription is the exactly same array list. To prevent it you can pass a copy of your list, e.g. using spread operator like in this example:
this.emitErrorsChanged.next([...this.errors]);
I have a parent component that retrieves an array of objects using an ajax request.
This component has two children components: One of them shows the objects in a tree structure and the other one renders its content in a table format. The parent passes the array to their children through an #input property and they display the content properly. Everything as expected.
The problem occurs when you change some field within the objects: the child components are not notified of those changes. Changes are only triggered if you manually reassign the array to its variable.
I'm used to working with Knockout JS and I need to get an effect similar to that of observableArrays.
I've read something about DoCheck but I'm not sure how it works.
OnChanges Lifecycle Hook will trigger only when input property's instance changes.
If you want to check whether an element inside the input array has been added, moved or removed, you can use IterableDiffers inside the DoCheck Lifecycle Hook as follows:
constructor(private iterableDiffers: IterableDiffers) {
this.iterableDiffer = iterableDiffers.find([]).create(null);
}
ngDoCheck() {
let changes = this.iterableDiffer.diff(this.inputArray);
if (changes) {
console.log('Changes detected!');
}
}
If you need to detect changes in objects inside an array, you will need to iterate through all elements, and apply KeyValueDiffers for each element. (You can do this in parallel with previous check).
Visit this post for more information: Detect changes in objects inside array in Angular2
You can always create a new reference to the array by merging it with an empty array:
this.yourArray = [{...}, {...}, {...}];
this.yourArray[0].yourModifiedField = "whatever";
this.yourArray = [].concat(this.yourArray);
The code above will change the array reference and it will trigger the OnChanges mechanism in children components.
Read following article, don't miss mutable vs immutable objects.
Key issue is that you mutate array elements, while array reference stays the same. And Angular2 change detection checks only array reference to detect changes. After you understand concept of immutable objects you would understand why you have an issue and how to solve it.
I use redux store in one of my projects to avoid this kind of issues.
https://blog.thoughtram.io/angular/2016/02/22/angular-2-change-detection-explained.html
You can use IterableDiffers
It's used by *ngFor
constructor(private _differs: IterableDiffers) {}
ngOnChanges(changes: SimpleChanges): void {
if (!this._differ && value) {
this._differ = this._differs.find(value).create(this.ngForTrackBy);
}
}
ngDoCheck(): void {
if (this._differ) {
const changes = this._differ.diff(this.ngForOf);
if (changes) this._applyChanges(changes);
}
}
It's work for me:
#Component({
selector: 'my-component',
templateUrl: './my-component.component.html',
styleUrls: ['./my-component.component.scss']
})
export class MyComponent implements DoCheck {
#Input() changeArray: MyClassArray[]= [];
private differ: IterableDiffers;
constructor(private differs: IterableDiffers) {
this.differ = differs;
}
ngDoCheck() {
const changes = this.differ.find(this.insertedTasks);
if (changes) {
this.myMethodAfterChange();
}
}
This already appears answered. However for future problem seekers, I wanted to add something missed when I was researching and debugging a change detection problem I had. Now, my issue was a little isolated, and admittedly a stupid mistake on my end, but nonetheless relevant.
When you are updating the values in the Array or Object in reference, ensure that you are in the correct scope. I set myself into a trap by using setInterval(myService.function, 1000), where myService.function() would update the values of a public array, I used outside the service. This never actually updated the array, as the binding was off, and the correct usage should have been setInterval(myService.function.bind(this), 1000). I wasted my time trying change detection hacks, when it was a silly/simple blunder. Eliminate scope as a culprit before trying change detection solutions; it might save you some time.
Instead of triggering change detection via concat method, it might be more elegant to use ES6 destructuring operator:
this.yourArray[0].yourModifiedField = "whatever";
this.yourArray = [...this.yourArray];
You can use an impure pipe if you are directly using the array in your components template. (This example is for simple arrays that don't need deep checking)
#Pipe({
name: 'arrayChangeDetector',
pure: false
})
export class ArrayChangeDetectorPipe implements PipeTransform {
private differ: IterableDiffer<any>;
constructor(iDiff: IterableDiffers) {
this.differ = iDiff.find([]).create();
}
transform(value: any[]): any[] {
if (this.differ.diff(value)) {
return [...value];
}
return value;
}
}
<cmp [items]="arrayInput | arrayChangeDetector"></cmp>
For those time travelers among us still hitting array problems here is a reproduction of the issue along with several possible solutions.
https://stackblitz.com/edit/array-value-changes-not-detected-ang-8
Solutions include:
NgDoCheck
Using a Pipe
Using Immutable JS NPM github
i am new in Angular 2 and trying to develop a project with basic CRUD functionality.
I am facing an issue that i have a User component and trying to access it's property in ngAfterViewInit hook, but it's showing undefined.
Here is code of User component.
export class UserComponent implements OnInit, AfterViewInit
{
private users: User[];
private userRoles;
ngOnInit(){
//get users from service
this.getUserDetails();
}
ngAfterViewInit() {
console.log(this);
console.log(this.users);
}
getUserDetails() {
this._userService.getUsers().subscribe(users => this.users = users);
}
}
Below is screen shots of console. I can see that Object has property users but it shows undefined in console.
Please give suggestion if i missed anything.
Thanks in advance.
Check user property inside getUserDetails. You can use users property in your template. Check component lifecycle hooks to get better idea about component lifecycle.
getUserDetails() {
this._userService.getUsers().subscribe((users) => {
this.users = users;
console.log(this.users);
});
}
I think this is not angular 2 issue...
This is typescript issue accessing this instance globally .
We can do this by using let and declaring variable in typescripts...
When javascript object assigned is cloned to other object then changes made to one is automatically passed to other
So this issue also arrises when we want to access typescript this object inside jquery functions
let _this = this;
something.each(function() {
console.log(_this); // the lexically scoped value
console.log(this); // the library passed value
});