Angular 2 viewchild 不工作 - 无法读取未定义的属性 nativeElement

我有一个名为 multiselect.component 的组件，我访问它的 html 元素之一(<select id="category-select">)在 .ts 文件中。

多选.component.html：

<select id="category-select" multiple class="form-control">
    <option value="1">Meat</option>
    <option value="2">Dairy</option>
    <option value="3">Confectionary</option>
    <option value="4">Dessert</option>
    <option value="7">Baking</option>
    <option value="6">Grocers</option>
    <option value="5">Restaurants</option>
    <option value="8">Condiments</option>
    <option value="9">beverages</option>
</select>
<div class="form-control-icon" id="keywords-icon"></div>

多选.component.ts：

import { Component, OnInit } from '@angular/core';

@Component({
    selector: 'multiselect',
    templateUrl: 'app/shared/multiselect.component.html',
    styleUrls: [ 'app/shared/multiselect.component.css' ]
})
export class MultiselectComponent implements OnInit {
    items = 'one two three'.split(' ');
    selectedCategories;
    allSelected;
    numberOfCategories = 9;

    ngOnInit() {
        this.selectedCategories = [];
        (<any>$("#category-select")).multiselect({
            buttonWidth: '100%',
            buttonContainer: '<div style="height: 34px;" />',
            buttonClass: 'none',
            nonSelectedText: "select categories",
            nSelectedText: ' categories',
            allSelectedText: "all categories",
            selectAllNumber: false,
            includeSelectAllOption: true,
            disableIfEmpty: true,
            onSelectAll: () => {
                this.allSelected = true;
            },
            onInitialized: () => {

            },
            onChange: (option, checked) => {
                this.changed();
            }
        });
    }

    changed() {
        this.selectedCategories = [];
        var self = this;

        $('#category-select option:selected').each(function () {
            self.selectedCategories.push(
                <any>($(this).text(), $(this).val())
            )
        });
        console.log(this.selectedCategories);
        if (this.selectedCategories.length < this.numberOfCategories) {
            this.allSelected = false;
        }
    }
}

我认为使用 jquery 访问它的方式是不好的做法。我认为可以使用@viewChild来访问它。

所以我尝试像这样更改它（我首先删除第一个 jquery 元素访问器，然后稍后再执行其余操作）：

import { Component, OnInit, ViewChild } from '@angular/core';

@Component({
    selector: 'multiselect',
    templateUrl: 'app/shared/multiselect.component.html',
    styleUrls: [ 'app/shared/multiselect.component.css' ]
})
export class MultiselectComponent implements OnInit {
    items = 'one two three'.split(' ');
    selectedCategories;
    allSelected;
    numberOfCategories = 9;

    @ViewChild('category-select') select;

    ngOnInit() {
        this.selectedCategories = [];
        this.select.nativeElement.multiselect({
            buttonWidth: '100%',
            buttonContainer: '<div style="height: 34px;" />',
            buttonClass: 'none',
            nonSelectedText: "select categories",
            nSelectedText: ' categories',
            allSelectedText: "all categories",
            selectAllNumber: false,
            includeSelectAllOption: true,
            disableIfEmpty: true,
            onSelectAll: () => {
                this.allSelected = true;
            },
            onInitialized: () => {

            },
            onChange: (option, checked) => {
                this.changed();
            }
        });
    }

    changed() {
        this.selectedCategories = [];
        var self = this;

        $('#category-select option:selected').each(function () {
            self.selectedCategories.push(
                <any>($(this).text(), $(this).val())
            )
        });
        console.log(this.selectedCategories);
        if (this.selectedCategories.length < this.numberOfCategories) {
            this.allSelected = false;
        }
    }
}

我收到此控制台错误：

> EXCEPTION: Error: Uncaught (in promise): EXCEPTION: Error in app/find-page/find-form.component.html:0:56
ORIGINAL EXCEPTION: TypeError: Cannot read property 'nativeElement' of undefined
ORIGINAL STACKTRACE:
TypeError: Cannot read property 'nativeElement' of undefined
    at MultiselectComponent.System.register.context_1.execute.MultiselectComponent.ngOnInit (http://localhost:3000/js/app/shared/Multiselect.component.js:29:32)
    at DebugAppView._View_FindFormComponent0.detectChangesInternal (FindFormComponent.ngfactory.js:761:90)
    at DebugAppView.AppView.detectChanges (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12586:18)
    at DebugAppView.detectChanges (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12691:48)
    at DebugAppView.AppView.detectViewChildrenChanges (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12612:23)
    at DebugAppView._View_FindPageComponent0.detectChangesInternal (FindPageComponent.ngfactory.js:41:8)
    at DebugAppView.AppView.detectChanges (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12586:18)
    at DebugAppView.detectChanges (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12691:48)
    at DebugAppView.AppView.detectViewChildrenChanges (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12612:23)
    at DebugAppView.AppView.detectChangesInternal (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12597:18)
ERROR CONTEXT:
[object Object]

为什么我会收到这个错误？

更改为 AfterViewInit 后，我​​现在收到此错误：

platform-browser.umd.js:937 Error: Uncaught (in promise): EXCEPTION: Error in app/find-page/find-form.component.html:13:30
ORIGINAL EXCEPTION: TypeError: Cannot read property 'length' of undefined
ORIGINAL STACKTRACE:
TypeError: Cannot read property 'length' of undefined
    at DebugAppView._View_FindFormComponent0.detectChangesInternal (FindFormComponent.ngfactory.js:890:72)
    at DebugAppView.AppView.detectChanges (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12586:18)
    at DebugAppView.detectChanges (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12691:48)
    at DebugAppView.AppView.detectViewChildrenChanges (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12612:23)
    at DebugAppView._View_FindPageComponent0.detectChangesInternal (FindPageComponent.ngfactory.js:41:8)
    at DebugAppView.AppView.detectChanges (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12586:18)
    at DebugAppView.detectChanges (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12691:48)
    at DebugAppView.AppView.detectViewChildrenChanges (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12612:23)
    at DebugAppView.AppView.detectChangesInternal (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12597:18)
    at DebugAppView.AppView.detectChanges (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12586:18)
ERROR CONTEXT:
[object Object]
    at resolvePromise (http://localhost:3000/node_modules/zone.js/dist/zone.js:538:32)
    at http://localhost:3000/node_modules/zone.js/dist/zone.js:515:14
    at ZoneDelegate.invoke (http://localhost:3000/node_modules/zone.js/dist/zone.js:323:29)
    at Object.onInvoke (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:9245:45)
    at ZoneDelegate.invoke (http://localhost:3000/node_modules/zone.js/dist/zone.js:322:35)
    at Zone.run (http://localhost:3000/node_modules/zone.js/dist/zone.js:216:44)
    at http://localhost:3000/node_modules/zone.js/dist/zone.js:571:58
    at ZoneDelegate.invokeTask (http://localhost:3000/node_modules/zone.js/dist/zone.js:356:38)
    at Object.onInvokeTask (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:9236:45)
    at ZoneDelegate.invokeTask (http://localhost:3000/node_modules/zone.js/dist/zone.js:355:43)

错误似乎出在 FindFormComponent 中。

FindFormComponent 只是具有多选功能的表单（需要填写的表单，然后将查找产品）。

以下是 FindFormComponent 中使用长度的相关 html：

<div class="row has-error-text">
    <div class="col-sm-8 offset-sm-2 col-md-6 offset-md-3 col-lg-4 offset-lg-4 input-group btn-group" style="height:64px;">
        <div style="position: relative; display: inline-block; width: 100%;">
            <multiselect #multiselect></multiselect>
        </div>
    </div>
</div>
<div class="row error-text"  [style.display]="multiselect.selectedCategories.length < 1 && submitted ? 'block' : 'none'">
    <div class="col-sm-8 offset-sm-2 col-md-6 offset-md-3 col-lg-4 offset-lg-4 form-group input-group btn-group">
        <small>Please select at least 1 category.</small>
    </div>
</div>

你需要改变

ngOnInit() {
    this.selectedCategories = [];
    this.select.nativeElement.multiselect({

to

ngAfterViewInit() {
    this.selectedCategories = [];
    this.select.nativeElement.multiselect({

The nativeElement在视图初始化之前不可用。