如何在 Angular 8 中检测 angular 中的值是由用户更改还是方法调用已更改它

Question

I have a select element in which value is getting assigned by ngOnit call.

HTML:

<select name="duration" [(ngModel)]="exercisePlan.duration" (ngModelChange)="onChange($event)"> 
    <option *ngFor="let duration of durations" [value]="duration.value">{{duration.title}}</option> 
</select> 

TS:

durations =  [{ title: "15 seconds", value: 15 }, 
              { title: "30 seconds", value: 30 }, ...]

ngOnit() {
    this.exercisePlan.duration = durations[0];
}

onChange(event) {
    console.log("How to show that change is done by user but not by some method call");
}

So the value is not assigned by user and is made available by some method (ngOnInit, in this case).

Whereas when I select a value from dropdown then how to know whether user has made the selection from select options?

Similar to that suppose if I add a Reset button in my HTML and on click of Reset I change value of my Select element, so how to distinguish in onChange() method whether the change is done by user or any other method call has made the changes to that element model?

Answer 1

Hi i would recommend to use FormControl. I would paste the correct code and then i would explain.

ngOnit() {
      this.control.setValue(
        // value to set
        durations[0], 
        // this prevent the control from notify the change (optional)
        { emitEvent: false }
      );
    }

<select name="duration" [formControl]="control">
  <option value="">Select a value</option>
  <option *ngFor="let duration of durations" [ngValue]="duration.value">{{duration.title}}</option>
</select> 

on the component:

valueDetection() {
this.formControl.valueChanges
      .subscribe(value => {
        // value is the value selected or assigned
        // here you can do logic
      });
}

In this example the FormControl change either when the user select a option or on code by a method. The FormControl is a observable so it will be activate every time the value change and you can control logic associated with that change inside the subscribe. Now i would make 2 assumption here.

First one is that you want to make code change without triggering the change. That is archived by the values in the setValue method.

Second you want to deal with logic depending if it is from code or not the change. This one is tricky. In order to know if the change is from code you must check the dirty and and touched values of the FormControl. The second choice is to make an object like this: { value: 'the value', fromCode: true} and deal with the second value on the subscribe

More info about FormControl

More info about Selects