zoukankan      html  css  js  c++  java
  • [Angular 2] Understanding @Injectable

    In order to resolve a dependency, Angular’s DI uses type annotations. To make sure these types are preserved when transpiled to ES5, TypeScript emits metadata. In this lesson we’ll explore how the @Injectable decorator ensures metadata generation for services that have their own dependencies.

    For exmaple, the TodoService looks like this:

    export class TodoService {
        todos = [
            {id: 0, name: "eat"},
            {id: 1, name: "sleep"},
            {id: 2, name: "running"},
        ];
    
        getTodos(){
            return this.todos;
        }
    }

    Now we want to inject LoggerProvider into TodoService:

    import {LoggerProvider} from './LoggerProvider';
    
    export class TodoService {
        todos = [
            {id: 0, name: "eat"},
            {id: 1, name: "sleep"},
            {id: 2, name: "running"},
        ];
        
        constructor(private logger: LoggerProvider){
            
        }
        
    
        getTodos(){
            this.logger.debug('Items', this.todos);
            return this.todos;
        }
    }

    If we want to the code, it will show errors:

    Cannot resolve paramster in TodoService

    The problem for this is because, when TypeScript code compile to ES5 code, 'LoggerProivder' is injected into the TodoService to decreator (Angular creates it).

    We must provider decreator to the TodoService in order to let this class know how to handle DI. So to solve the problem, we just need to add '@Injectable' whcih provided by Angular:

    import {LoggerProvider} from './LoggerProvider';
    import {Injectable} from '@angular/core';
    
    @Injectable
    export class TodoService {
        todos = [
            {id: 0, name: "eat"},
            {id: 1, name: "sleep"},
            {id: 2, name: "running"},
        ];
        
        constructor(private logger: LoggerProvider){
            
        }
        
    
        getTodos(){
            this.logger.debug('Items', this.todos);
            return this.todos;
        }
    }
  • 相关阅读:
    memcached源码剖析5:并发模型
    memcached源码剖析4:并发模型
    深入剖析php执行原理(4):函数的调用
    深入剖析php执行原理(2):函数的编译
    剖析php脚本的超时机制
    strerror的坑
    深入理解php中的ini配置(2)
    深入理解php中的ini配置(1)
    一个“日期”字符串进行比较的case
    用valgrind检测php扩展内存泄露
  • 原文地址:https://www.cnblogs.com/Answer1215/p/5877838.html
Copyright © 2011-2022 走看看