Angular 2 viewchild无法正常工作 – 无法读取未定义的属性nativ

发布时间：2020-12-17 17:39:49 所属栏目：安全来源：网络整理

导读：我有一个名为multiselect.component的组件,我在.ts文件中访问其中一个html元素( select id =“category-select”). multiselect.component.html： select id="category-select" multiple class="form-control" option value="1"Meat/option option value="2"

我有一个名为multiselect.component的组件,我在.ts文件中访问其中一个html元素(< select id =“category-select”>).

multiselect.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>

multiselect.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;
        }
    }
}

我认为通过使用jquery,我访问它的方式是不好的做法.我认为可以使用@viewChild访问它.

所以我试图像这样改变它(我首先摆脱了第一个jquery元素访问器,然后将完成剩下的工作)：

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

@Component({
    selector: 'multiselect',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%',$(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)

该错误似乎在FindFormComponent中.

FindFormComponent只是具有multiselect的表单(填写表格然后会找到产品).

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>

解决方法

你需要改变

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

至

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

在初始化视图之前,nativeElement不可用.

（编辑：李大同）

【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容!