Angular learning talks about instructions and pipelines

Directive

Directives are ways provided by Angular操作DOM . Instructions are divided into属性指令and结构指令.

• Attribute directive: Modify the appearance or behavior of an existing element, wrap it with [] .

• Structural instructions: add and delete DOM nodes to modify the layout, use * as the instruction prefix. [Recommended related tutorials: "angular tutorial"]

1. Built-in instruction

1.1 *ngIf

渲染DOM nodes or移除DOM nodes based on conditions.

<div *ngIf="data.length == 0">No more data</div>

<div *ngIf="data.length > 0; then dataList else noData"></div>
<ng-template #dataList>Course List</ng-template>
<ng-template #noData>No more data</ng-template>

ng-template is used to define templates. After using ng-template to define a template, you can use ng-container and templateOutlet instructions to use it.

<ng-template #loading>
 <button (click)="login()">login</button>
 <button (click)="sigup()">sigup</button>
</ng-template>
<ng-container *ngTemplateOutlet="loading">
</ng-container>

1.2 [hidden]

显示DOM nodes or隐藏DOM nodes (display) according to conditions.

<div [hidden]="data.length == 0">Course List</div>
<div [hidden]="data.length > 0">No more data</div>

1.3 *ngFor

traverses data to generate HTML structure

interface List {
  id: number
  name: string
  age: number
}

list: List[] = [
  { id: 1, name: "Zhang San", age: 20 },
  { id: 2, name: "李思", age: 30 }
]

<li
    *ngFor="
      let item of list;
      let i = index;
      let isEven = even;
      let isOdd = odd;
      let isFirst = first;
      let isLast = last;
    "
  >
  </li>

<li *ngFor="let item of list; trackBy: identify"></li>

identify(index, item){
  return item.id; 
}

2. Custom command

requirements: Set the default background color for the element, the background color when the mouse moves in, and the background color when the mouse moves out.

<div [appHover]="{ bgColor: 'skyblue' }">Hello Angular</div>

import { AfterViewInit, Directive, ElementRef, HostListener, Input } from "@angular/core"

// Receive parameter type interface Options {
  bgColor?: string
}

@Directive({
  selector: "[appHover]"
})
export class HoverDirective implements AfterViewInit {
  //Receive parameters @Input("appHover") appHover: Options = {}
  // DOM node element to be operated on: HTMLElement
	// Get the DOM node to be operated on constructor(private elementRef: ElementRef) {
    this.element = this.elementRef.nativeElement
  }
	// Set the background color of the element after the initial completion of the component template ngAfterViewInit() {
    this.element.style.backgroundColor = this.appHover.bgColor || "skyblue"
  }
	// Add a mouse move event to the element @HostListener("mouseenter") enter() {
    this.element.style.backgroundColor = "pink"
  }
	//Add mouse out event for element @HostListener("mouseleave") leave() {
    this.element.style.backgroundColor = "skyblue"
  }
}

Pipe

The role of the pipeline is格式化组件模板数据.

1. Built-in pipeline

• date date format

• currency currency format

• uppercase Convert to

• uppercase lowercase Convert to lowercase

• json Format json data

{{ date | date: "yyyy-MM-dd" }}

2. Custom pipeline

requirements: the specified string cannot exceed Specified length

<!-- This is a... -->
{{'This is a test' | summary: 3}}

// summary.pipe.ts
import { Pipe, PipeTransform } from '@angular/core';

@Pipe({
   name: 'summary' 
});
export class SummaryPipe implements PipeTransform {
    transform (value: string, limit?: number) {
        if (!value) return null;
        let actualLimit = (limit) ? limit : 50;
        return value.substr(0, actualLimit) + '...';
    }
}

// app.module.ts
import { SummaryPipe } from './summary.pipe'
@NgModule({
    declarations: [
      SummaryPipe
    ] 
});