孩子的 @Input() 值在角度 2 中不会改变

Question

I need to change the data in my Child's @Input value on click event from a parent. This is how I do it in:

Parent

update(user: SampleData): void {
        this.sampleToUpdate = user;
      }

<td style="text-align: center;">
                     <button class="btn btn-icon btn-outline-info btn-sm" (click)="update(item)">
                      <i class="nc-icon nc-badge"></i>
                     </button>
                   </td>
<app-sample *ngIf="sampleToUpdate" [sampleData]="sampleToUpdate"></app-sample>

Child

 @Input() sampleData: SampleData;
ngOnInit(): void {
    if (this.sampleData) {
      debugger;
      this.isUpdating = true;
      this.nameToUpdate = `Updating ${this.sampleData.firstName} ${this.sampleData.lastName}`
    } else {
      this.isUpdating = false;
      this.nameToUpdate = `Add New Sample`
    }
    this.createForm();
  }

  createForm(): void {
    if (!this.sampleData) {
      this.firstNameControl = new FormControl(this.sampleData.firstName, {
        validators: [Validators.required],
        updateOn: 'blur'
      });
      this.middleNameControl = new FormControl(this.sampleData.middleName);
      this.lastNameControl = new FormControl(this.sampleData.lastName, {
        validators: [Validators.required],
        updateOn: 'blur'
      });
      this. loadAmountControl = new FormControl(this.sampleData.loadAmount, {
        validators: [Validators.required, DecimalValidators.isNumberCheck],
        updateOn: 'blur'
      })
    } else {
      this.firstNameControl = new FormControl('', {
        validators: [Validators.required],
        updateOn: 'blur'
      });
      this.middleNameControl = new FormControl('');
      this.lastNameControl = new FormControl('', {
        validators: [Validators.required],
        updateOn: 'blur'
      });
      this. loadAmountControl = new FormControl(0.00, {
        validators: [Validators.required, DecimalValidators.isNumberCheck],
        updateOn: 'blur'
      })
    }

    this.form = new FormGroup({
      firstName: this.firstNameControl,
      middleName: this.middleNameControl,
      lastName: this.lastNameControl,
      loadAmount: this.loadAmountControl
    });
 }

On the first instance of the click, I get the result I wanted but on the succeeding clicks, the value does not change anymore. Can you show me how to do this right please? Thank you.

Answer 1

It is because you call createForm() on ngOnInit which is called only once when the component is inited.

You use *ngIf="sampleToUpdate" to conditionally render the child component. But if the value is changed without getting a falsy value, the child component will not be re-rendered and ngOnInit will not be called.

You can use setter at sampleData attribute to perform the logic as followings:

private _sampleData: SampleData;

@Input() 
set sampleData(value:SampleData) {
    this._sampleData = value;
    if (this._sampleData) {
      debugger;
      this.isUpdating = true;
      this.nameToUpdate = `Updating ${this._sampleData.firstName} ${this._sampleData.lastName}`
    } else {
      this.isUpdating = false;
      this.nameToUpdate = `Add New Sample`
    }
    this.createForm();
  }

---

OR

You can use ngOnChanges lifecycle hook to get the sampleData changes as followings:

ngOnChanges(changes: SimpleChanges): void {
    // changes.sampleData will have previous and current value
    if (changes.sampleData.currentValue) {
      debugger;
      this.isUpdating = true;
      this.nameToUpdate = `Updating ${this.sampleData.firstName} ${this.sampleData.lastName}`
    } else {
      this.isUpdating = false;
      this.nameToUpdate = `Add New Sample`
    }
    this.createForm();
  }

Answer 2

To detect changes in the input you can use ngOnChanges

ngOnChanges(changes: SimpleChanges) {
 if(changes.sampleData) {
 // Add your logic
 }

}