在实现递归ngFor循环时,空ng-template的上下文数据 [英] Empty context data of ng-template while implementing recursive ngFor loop
问题描述
我必须显示书籍类别的层次树,但在呈现的html中没有任何数据.似乎在ngTemplateOutput
上下文中有问题.
I have to show hierarchical tree of book categories but get no data in the rendered html. It seems there is a problem in ngTemplateOutput
context.
尝试使用隐式和显式方法设置上下文.明确设定时let-list="list"
类别列表的第一级已呈现,但子类别仍未呈现.
Tried setting context with implicit and explicit method. When set explicitly e.g. let-list="list"
first level of the categories list was rendered, but child categories still weren't.
角形弯角:5.2.0
Angular vesion: 5.2.0
<div class="categories">
<div *ngFor="let rootCategory of categories">
<h1>{{rootCategory.name}}</h1>
<ul>
<ng-template #recursiveList let-list>
<li *ngFor="let subCategory of list">
<div>{{subCategory.name}}</div>
<ul *ngIf="subCategory.childCategories.length > 0">
<ng-container *ngTemplateOutlet="recursiveList; context:{ $implicit: subCategory.childCategories }"></ng-container>
</ul>
</li>
</ng-template>
<ng-container *ngTemplateOutlet="recursiveList; context:{ $implicit: rootCategory.childCategories }"></ng-container>
</ul>
</div>
</div>
输出为:
<div _ngcontent-c5="" class="categories">
<!--bindings={
"ng-reflect-ng-for-of": "[object Object],[object Object"
}--><div _ngcontent-c5="">
<h1 _ngcontent-c5=""></h1>
<ul _ngcontent-c5="">
<!---->
<!--bindings={
"ng-reflect-ng-template-outlet-context": "[object Object]",
"ng-reflect-ng-template-outlet": "[object Object]"
}-->
<!--bindings={
"ng-reflect-ng-for-of": "[object Object],[object Object"
}--><li _ngcontent-c5="">
<div _ngcontent-c5=""></div>
<!---->
</li><li _ngcontent-c5="">
<div _ngcontent-c5=""></div>
<!---->
</li><li _ngcontent-c5="">
<div _ngcontent-c5=""></div>
<!---->
</li><li _ngcontent-c5="">
<div _ngcontent-c5=""></div>
<!---->
</li><li _ngcontent-c5="">
<div _ngcontent-c5=""></div>
<!---->
</li><li _ngcontent-c5="">
<div _ngcontent-c5=""></div>
<!---->
</li><li _ngcontent-c5="">
<div _ngcontent-c5=""></div>
<!---->
</li><li _ngcontent-c5="">
<div _ngcontent-c5=""></div>
<!---->
</li><li _ngcontent-c5="">
<div _ngcontent-c5=""></div>
<!---->
</li><li _ngcontent-c5="">
<div _ngcontent-c5=""></div>
<!---->
</li>
</ul>
</div><div _ngcontent-c5="">
<h1 _ngcontent-c5=""></h1>
<ul _ngcontent-c5="">
<!---->
<!---->
</ul>
</div><div _ngcontent-c5="">
<h1 _ngcontent-c5=""></h1>
<ul _ngcontent-c5="">
<!---->
<!---->
</ul>
</div><div _ngcontent-c5="">
<h1 _ngcontent-c5=""></h1>
<ul _ngcontent-c5="">
<!---->
<!---->
</ul>
</div><div _ngcontent-c5="">
<h1 _ngcontent-c5=""></h1>
<ul _ngcontent-c5="">
<!---->
<!---->
</ul>
</div>
</div>
未显示类别数据.此外,控制台在此行出现错误:
No categories data is shown. Also, an error from the console on this line:
<ul *ngIf="list.childCategories.length > 0">
错误:
ERROR TypeError: Cannot read property 'length' of undefined
at Object.eval [as updateDirectives]
简化的类别数组:
categories: Category[] = [
{
name: 'Category 1',
childCategories: [
{
name: 'Category 1.1',
childCategories: [
{
name: 'Category 1.1.1',
childCategories: [],
},
{
name: 'Category 1.1.2',
childCategories: [],
},
{
name: 'Category 1.1.3',
childCategories: [],
},
],
},
]
},
];
推荐答案
添加?
喜欢
*ngIf="list.childCategories?.length > 0"
防止错误.当list.childCategories
为null时,此表达式返回null,而不是抛出length
在null
(安全导航或Elvis运算符)上不可用
prevents the error. This expression returns null when list.childCategories
is null, instead of throwing that length
is not available on null
(safe-natigation or Elvis operator)
我认为
<ng-container *ngTemplateOutlet="recursiveList; context:{ $implicit: list.childCategories }"></ng-container>
应该是
<ng-container *ngTemplateOutlet="recursiveList; context:{ $implicit: subCategory.childCategories }"></ng-container>
subCategory
而不是list
这篇关于在实现递归ngFor循环时,空ng-template的上下文数据的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!