不了解* ngIf语句中模板变量的使用

本文介绍了不了解* ngIf语句中模板变量的使用的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

我的目标是最终进行设置，使用户可以在查看目录(spiritwoodart.com)的同时更改行数.当我遇到问题时，我什至没有走过摆弄的第一阶段.猜测其根本误会.找不到帮助时遇到麻烦(即，我的搜索很糟糕:为什么我不能在ngif语句中放入模板变量").请让我知道是否需要更多信息.感谢您提供的所有见解，甚至感谢我的新意.

所以，当我改变

此:

            <div *ngIf="(i+1) % n === 0"
                class="w-100"></div>

对此:

或者这个:

            <div #n='3' *ngIf="(i+1) % n === 0"
                class="w-100"></div>

甚至这个:

            <div *ngIf="(i+1) % {{3}} === 0"
                class="w-100"></div>

我收到如下错误:

compiler.js:485 Uncaught Error: Template parse errors:
There is no directive with "exportAs" set to "3" ("
                        [cart]="cart"></product-card>
                </div>
                <div [ERROR ->]#n='3' *ngIf="(i+1) % n === 0"
                    class="w-100"></div>
            </ng-container>
"): ng:///AppModule/ProductsComponent.html@12:21

上下文模板:

<div class="row">
    <div class="col-3">
        <product-filter [category]="category"></product-filter>
    </div>
    <div class="col">
        <div class="row"
            *ngIf="cart$ | async as cart">
            <ng-container *ngFor="let p of filteredProducts; let i = index">
                <div class="col">
                    <product-card [product]="p"
                        [cart]="cart"></product-card>
                </div>
                <div *ngIf="(i+1) % 3 === 0"
                    class="w-100"></div>
            </ng-container>
        </div>
    </div>
</div>

上下文组件:

import { Cart } from './../models/cart';
import { CartService } from './../cart.service';
import { Product } from './../models/product';
import { ProductService } from './../product.service';
import { Component, OnInit, OnDestroy } from '@angular/core';
import { ActivatedRoute } from '@angular/router';
import 'rxjs/add/operator/switchMap';
import { Subscription } from 'rxjs/Subscription';
import { Observable } from 'rxjs/Observable';
@Component({
  selector: 'app-products',
  templateUrl: './products.component.html',
  styleUrls: ['./products.component.css']
})
export class ProductsComponent implements OnInit {
  products: Product[] = [];
  filteredProducts: Product[] = [];
  category: string; // keep this field in this class in order to initialize it... then delegate it out
  cart$: Observable<Cart>;

  constructor(
    private route: ActivatedRoute,
    private productService: ProductService,
    private cartService: CartService
  ) {}

  async ngOnInit() {
    this.cart$ = await this.cartService.getCart();
    this.populateProducts();
  }

  private populateProducts() {
    this.productService
      .getAll()
      .switchMap(products => {
        this.products = products;
        return this.route.queryParamMap;
      })
      .subscribe(params => {
        this.category = params.get('category'); // initializing category field
        this.applyFilter();
      });
  }

  private applyFilter() {
    // setting the filtered products array
    this.filteredProducts = this.category
      ? this.products.filter(p => p.category === this.category)
      : this.products;
  }
}

解决方案

#是模板参考变量.它遵循DOM元素，不能那样使用.

ng-init指令存在于AngularJS中，并且可以达到这个目的，但是在Angular中并不存在，主要是因为它易于被滥用.可以重新创建它，如此答案所示.

一种解决方法是使用结构指令. 真实值的典型解决方法是ngIf结构指令:

<ng-container *ngIf="3; let n">
  <div *ngIf="(i+1) % n === 0">
    ...

如果该值是 falsy ，则它将不起作用，因为嵌套的组件将不会被编译.支持falsy值的更通用的解决方法是自定义ngVar结构指令，如此处所述.

my objective is to eventually make a setting where the user can change the number of rows while looking at the catalog (spiritwoodart.com). I didn't even get past the first stage of fiddling when I ran into a problem. Guessing its a fundamental misunderstanding. Having trouble finding help (i.e. my search sucks: "why cant i put a template variables in ngif statement"). Please let me know if more info is needed. Thanks for any and all insights, or even flames for my newbiness.

So, when I change

this:

            <div *ngIf="(i+1) % n === 0"
                class="w-100"></div>

to this:

or this:

            <div #n='3' *ngIf="(i+1) % n === 0"
                class="w-100"></div>

or even this:

            <div *ngIf="(i+1) % {{3}} === 0"
                class="w-100"></div>

I get an error that looks like this:

compiler.js:485 Uncaught Error: Template parse errors:
There is no directive with "exportAs" set to "3" ("
                        [cart]="cart"></product-card>
                </div>
                <div [ERROR ->]#n='3' *ngIf="(i+1) % n === 0"
                    class="w-100"></div>
            </ng-container>
"): ng:///AppModule/ProductsComponent.html@12:21

context-template:

<div class="row">
    <div class="col-3">
        <product-filter [category]="category"></product-filter>
    </div>
    <div class="col">
        <div class="row"
            *ngIf="cart$ | async as cart">
            <ng-container *ngFor="let p of filteredProducts; let i = index">
                <div class="col">
                    <product-card [product]="p"
                        [cart]="cart"></product-card>
                </div>
                <div *ngIf="(i+1) % 3 === 0"
                    class="w-100"></div>
            </ng-container>
        </div>
    </div>
</div>

context-component:

import { Cart } from './../models/cart';
import { CartService } from './../cart.service';
import { Product } from './../models/product';
import { ProductService } from './../product.service';
import { Component, OnInit, OnDestroy } from '@angular/core';
import { ActivatedRoute } from '@angular/router';
import 'rxjs/add/operator/switchMap';
import { Subscription } from 'rxjs/Subscription';
import { Observable } from 'rxjs/Observable';
@Component({
  selector: 'app-products',
  templateUrl: './products.component.html',
  styleUrls: ['./products.component.css']
})
export class ProductsComponent implements OnInit {
  products: Product[] = [];
  filteredProducts: Product[] = [];
  category: string; // keep this field in this class in order to initialize it... then delegate it out
  cart$: Observable<Cart>;

  constructor(
    private route: ActivatedRoute,
    private productService: ProductService,
    private cartService: CartService
  ) {}

  async ngOnInit() {
    this.cart$ = await this.cartService.getCart();
    this.populateProducts();
  }

  private populateProducts() {
    this.productService
      .getAll()
      .switchMap(products => {
        this.products = products;
        return this.route.queryParamMap;
      })
      .subscribe(params => {
        this.category = params.get('category'); // initializing category field
        this.applyFilter();
      });
  }

  private applyFilter() {
    // setting the filtered products array
    this.filteredProducts = this.category
      ? this.products.filter(p => p.category === this.category)
      : this.products;
  }
}

解决方案

# is template reference variable. It defers to DOM element and cannot be used like that.

ng-init directive exists in AngularJS and serves this purpose, but it doesn't exist in Angular, primarily because it is prone to be misused. It's possible to recreate it, as shown in this answer.

A workaround is to use a structural directive. A typical workaround for truthy value is ngIf structural directive:

<ng-container *ngIf="3; let n">
  <div *ngIf="(i+1) % n === 0">
    ...

It won't work if the value is falsy, because nested component won't be compiled. More universal workaround that supports falsy values is custom ngVar structural directive, as described here.

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