Ni nini kuma Angular lifecycle hooks kwa vipengele?

Question

Accepted Answer

**Lifecycle hooks** ni njia ambazo Angular huitwa kwa wakati maalum katika maisha ya kipengele — uundaji, mabadiliko ya ingizo, utoaji, na uharibifu. Unayatekeleza kwa kuongeza njia za interface katika darasa la kipengele chako.

## Manyumba makuu kwa utaratibu

```ts
import { Component, OnInit, OnDestroy, OnChanges, SimpleChanges } from "@angular/core";

@Component({ ... })
export class MyComponent implements OnInit, OnDestroy, OnChanges {
  ngOnChanges(changes: SimpleChanges) {
    // called when an @Input value changes (before ngOnInit the first time)
  }
  ngOnInit() {
    // called ONCE after the first ngOnChanges — do initialization, fetch data
  }
  ngOnDestroy() {
    // called just before the component is removed — CLEAN UP here
  }
}
```

## Mlolongo kamili

```text
constructor          → DI happens; inputs NOT yet available
ngOnChanges          → when @Input values are set/changed
ngOnInit             → once, after first ngOnChanges ★ (init, fetch data)
ngDoCheck            → every change detection run (custom change detection)
ngAfterContentInit   → projected content (<ng-content>) initialized
ngAfterViewInit      → the component's view + child views ready ★ (access @ViewChild)
ngAfterViewChecked   → after each view check
ngOnDestroy          → before removal ★ (unsubscribe, clear timers)
```

## Watatu unayotumia zaidi

```ts
ngOnInit() {
  this.userService.getUsers().subscribe(u => this.users = u); // init/fetch — inputs are ready
}
ngAfterViewInit() {
  this.chart.nativeElement.focus(); // the DOM/@ViewChild exists now
}
ngOnDestroy() {
  this.subscription.unsubscribe();   // prevent memory leaks!
  clearInterval(this.timer);
}
```

- **`ngOnInit`** — mahali ya kawaida kwa ajili ya kuanzisha na kunoa data (tofauti na muundo, `@Input` zinapatikana hapa).
- **`ngAfterViewInit`** — wakati mwonekano wa watoto na rejereza za `@ViewChild` zipo.
- **`ngOnDestroy`** — kusafisha: kutoka Observable, kusafisha timers, kuondoa wasikilizaji.

## Kwa nini ngOnInit badala ya muundo

Muundo unapaswa tu kufanya DI; ingizo hazijafungwa bado hapo. `ngOnInit` inaendelea baada ya Angular kusanidi kipengele na ingizo lake — mahali sahihi kwa mantiki ya kuanza.

## Kwa nini inahusu

Lifecycle hooks hukuruhusu kuendesha mantiki kwa wakati sahihi — anzisha katika `ngOnInit`, fikiri mwonekano ulioambatanishwa katika `ngAfterViewInit`, na muhimu **kusafisha katika `ngOnDestroy`** (kutoka Observable ni njia #1 kwa kuepuka mabasi ya kumbukumbu katika Angular).

Kujua utaratibu na jukumu la kila hook ni muhimu kwa ajili ya kunoa data sahihi, kufikiri DOM, na kusimamia rasilimali.