Angular 5.2.2如何获取深度2+的后代组件实例？

Hey there! Let's figure out how to get that nested ThirdLevelComponent instance in your Angular 5.2.2 app. I totally get why @ViewChildren and @ContentChildren aren't working here—let's break down the problem and fix it step by step.

Why Your Current Approach Isn't Working #

First, let's clarify why those decorators are returning empty QueryLists:

• @ViewChildren in Angular 5 only looks one level deep by default. There's no descendants option (that was added in Angular 6+), so it can't see inside the third-party SecondLevelComponent.
• @ContentChildren only targets components passed in via content projection (<ng-content>), not components that are part of a child component's internal template. Since ThirdLevelComponent lives inside SecondLevelComponent's own template, this decorator won't find it.

Solutions to Get the ThirdLevelComponent Instance #

1. Manually Traverse the Component Tree (Most Reliable for Angular 5)

You can use Angular's ApplicationRef to access all mounted components, then filter for ThirdLevelComponent and verify it's a descendant of your FirstLevelComponent.

Here's how to implement it:

import { Component, AfterViewInit, ViewChild } from '@angular/core';
import { ApplicationRef } from '@angular/core';
import { SecondLevelComponent } from 'your-third-party-library';
import { ThirdLevelComponent } from 'your-third-party-library';

@Component({
  selector: 'app-first-level',
  template: `<app-second-level></app-second-level>`
})
export class FirstLevelComponent implements AfterViewInit {
  @ViewChild(SecondLevelComponent) secondLevel: SecondLevelComponent;

  constructor(private appRef: ApplicationRef) {}

  ngAfterViewInit() {
    // Give third-party components time to finish initializing
    setTimeout(() => {
      const thirdLevelInstances = this.findDescendantComponents(ThirdLevelComponent);
      if (thirdLevelInstances.length) {
        console.log('Found ThirdLevelComponent instances:', thirdLevelInstances);
        // Do whatever you need with the instance here
      } else {
        console.log('No ThirdLevelComponent instances found');
      }
    }, 100);
  }

  private findDescendantComponents(targetType: any): any[] {
    const allComponents = this.appRef.components;
    const descendants: any[] = [];

    allComponents.forEach(comp => {
      const instance = comp.instance;
      if (instance instanceof targetType && this.isDescendant(instance, this)) {
        descendants.push(instance);
      }
    });

    return descendants;
  }

  private isDescendant(childComponent: any, parentComponent: any): boolean {
    let current = childComponent;
    // Traverse up the component tree using Angular 5's internal parent reference
    while (current) {
      if (current === parentComponent) {
        return true;
      }
      // Fall back to common internal parent properties in Angular 5
      current = current['_componentParent'] || current['parent'];
    }
    return false;
  }
}

2. Use the Third-Party Component's API (If Available)

Check the documentation for SecondLevelComponent—sometimes third-party libraries expose methods or properties to access internal components. For example:

ngAfterViewInit() {
  setTimeout(() => {
    // If SecondLevelComponent has a method like this
    const thirdLevelInstance = this.secondLevel.getThirdLevelComponent();
    if (thirdLevelInstance) {
      // Use the instance directly
    }
  }, 0);
}

This is the cleanest approach if it's an option, since it doesn't rely on Angular's internal properties.

3. DOM Query + Injector (Last Resort)

If the third-party component has a unique DOM selector, you can find its element and use the injector to grab the component instance. Note: This is fragile if the library changes its DOM structure.

import { Component, AfterViewInit, ElementRef, Injector } from '@angular/core';
import { ThirdLevelComponent } from 'your-third-party-library';

@Component({
  selector: 'app-first-level',
  template: `<app-second-level></app-second-level>`
})
export class FirstLevelComponent implements AfterViewInit {
  constructor(private elRef: ElementRef, private injector: Injector) {}

  ngAfterViewInit() {
    setTimeout(() => {
      const thirdLevelEl = this.elRef.nativeElement.querySelector('app-third-level');
      if (thirdLevelEl) {
        const componentInstance = this.injector.get(ThirdLevelComponent, null, { host: thirdLevelEl });
        if (componentInstance) {
          console.log('Found instance:', componentInstance);
        }
      }
    }, 100);
  }
}

Important Notes #

• If you ever upgrade to Angular 6+, you can simplify this with @ViewChildren(ThirdLevelComponent, { descendants: true })—this built-in option handles deep queries automatically.
• The setTimeout is a safety net to ensure third-party components have fully initialized before you query for them. You can adjust the delay if needed.
• Avoid relying on Angular's internal properties (like _componentParent) long-term, as they can change between versions.

内容的提问来源于stack exchange，提问作者Guilherme Meinlschmiedt Abdo