Node.js async.queue()用法及代码示例

异步模块旨在与NodeJS中的异步JavaScript一起使用。 async.queue返回一个队列对象，该对象能够并发处理，即一次处理多个项目。

如何使用async.queue？

• 第1步：创建一个package.json文件。通过运行以下命令来创建package.json文件。

  npm init

• 第2步：安装异步模块。可以使用以下命令安装异步模块。

  npm i async

• 第三步：导入异步模块。可以使用以下命令导入异步模块。

  const async = require('async')

• 第四步：使用async.queue模块。 async.queue的语法。

  const queue = async.queue('function', 'concurrency value')

参数函数在添加到队列的元素上执行。并发值告诉队列在特定时间要处理的元素数。

例：请看以下示例，以更好地理解。

// Defining The queue
const queue = async.queue((task, completed) => { 
    // Here task is the current element being 
    // processed and completed is the callback function
      
    console.log("Currently Busy Processing Task " + task);
      
    // Simulating a complex process.
    setTimeout(()=>{ 
        // Number of elemets to be processed.
        const remaining = queue.length();
        completed(null, {task, remaining});
    }, 1000);
  
}, 1);
  
// The concurrency value is set to one, 
// Which means that one element is being
// Processed at a particular time

async.queue中的重要方法和属性：

1. push(element，callback)：push方法用于将元素添加到队列的尾部。以下代码演示了push方法的用法方式。

   Javascript

   
   // Takes in two parameters, the item being pushed and 
   // the callback function
   // Here the item is the element being added 
   // and other is the callback function with error 
   // and data property(destructured)
     
   queue.push(item, (error, {item, remaining}) => {
     if(error){
         console.log(`An error occurred while processing task ${task}`);
     } else {
         console.log(`Finished processing task ${task}
                . ${remaining} tasks remaining`);
     }
   });

2. length()：length方法返回队列中当前存在的元素数。以下代码演示了length方法的用法方式。

   Javascript

   
   console.log(queue.length());

3. starts属性：starts属性返回一个布尔值，该值指示队列是否已开始处理数据。以下代码演示了启动属性的工作方式。

   Javascript

   
   // Returns true if the queue has started processing the data else false
   console.log(queue.started)

4. unshift(element，callback)：unshift方法类似于push方法，但是该元素已添加到队列的开头，表明要处理的元素是重要的元素。以下代码演示了unshift方法的用法原理：-

   
   
   
   

   Javascript

   
   // Takes in two parameters, the item being pushed 
   // and the callback function
   // Here the item is the element being added 
   // and other is the callback function with error 
   // and data property(destructured)
     
   queue.unshift(item, (error, {item, remaining}) => {
     if(error){
      console.log(`An error occurred while processing task ${task}`);
     } else {
      console.log(`Finished processing task ${task}. ${remaining} tasks remaining`);
     }
   });

5. drain()方法：当队列执行完所有任务后，drain方法将运行回调函数。下面的代码演示了rain方法的用法方式。

   注意：仅当描述的函数是箭头函数时，排水方法才有效。

   Javascript

   
   // Executes when the queue is done processing all the items
   queue.drain(() => {
       console.log('Successfully processed all items');
   })

6. pause()方法：暂停方法暂停队列中元素的执行，直到调用恢复函数为止。以下代码演示了pause方法的用法方式。

   Javascript

   
   // Pauses the execution of the queue
   queue.pause()

7. resume()方法：resume方法恢复队列中元素的执行。以下代码演示了resume方法的用法方式。

   Javascript

   
   // Resumes the execution of the queue
   queue.resume()

8. kill()方法：kill方法从队列中删除所有元素，并删除掉方法的回调函数并将其强制为空闲。以下代码演示kill方法的用法方式。

   Javascript

   
   // Forces the queue into idle mode 
   // and removes the drain callback
   queue.kill()

9. idle()方法：idle()方法返回一个布尔值，指示队列是空闲还是正在处理某些内容。以下代码演示了idle方法的用法方式。

   Javascript

   
   // Returns whether the queue is idle or not
   queue.idle()

完整的代码：以下代码完整演示了async.queue的实际使用方式。

// Importing the async module
const async = require('async');
  
// Creating a tasks array
const tasks = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
  
// Defining the queue
const queue = async.queue((task, completed) => {
    console.log("Currently Busy Processing Task " + task);
      
    // Simulating a Complex task
    setTimeout(()=>{
        // The number of tasks to be processed
        const remaining = queue.length();
        completed(null, {task, remaining});
    }, 1000);
  
}, 1); // The concurrency value is 1
  
  
// The queue is idle as there are no elements
// for the queue to process
console.log(`Did the queue start ? ${queue.started}`)
  
// Adding the each task to the queue
tasks.forEach((task)=>{
  
    // Adding the 5th task to the head of the 
    // queue as it is deemed important by us
 if(task == 5){
    queue.unshift(task, (error, {task, remaining})=>{
      if(error){
       console.log(`An error occurred while processing task ${task}`);
      }else {
       console.log(`Finished processing task ${task}. ${remaining} tasks remaining`);
      }
    })      
        // Adding the task to the tail in the order of their appearance
 } else {
      queue.push(task, (error, {task, remaining})=>{
       if(error){
        console.log(`An error occurred while processing task ${task}`);
       }else {
        console.log(`Finished processing task ${task}. ${remaining} tasks remaining`);
      }
      })
    }
});
  
  
// Executes the callback when the queue is done processing all the tasks
queue.drain(() => {
    console.log('Successfully processed all items');
})
  
// The queue is not idle it is processing the tasks asynchronously
console.log(`Did the queue start ? ${queue.started}`)

输出：