Node.js async.queue()用法及代碼示例

異步模塊旨在與NodeJS中的異步JavaScript一起使用。 async.queue返回一個隊列對象，該對象能夠並發處理，即一次處理多個項目。

如何使用async.queue？

• 第1步：創建一個package.json文件。通過運行以下命令來創建package.json文件。

  npm init

• 第2步：安裝異步模塊。可以使用以下命令安裝異步模塊。

  npm i async

• 第三步：導入異步模塊。可以使用以下命令導入異步模塊。

  const async = require('async')

• 第四步：使用async.queue模塊。 async.queue的語法。

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

參數函數在添加到隊列的元素上執行。並發值告訴隊列在特定時間要處理的元素數。

例：請看以下示例，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中的重要方法和屬性：

1. push(element，callback)：push方法用於將元素添加到隊列的尾部。以下代碼演示了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方法返回隊列中當前存在的元素數。以下代碼演示了length方法的用法方式。

   Javascript

   
   console.log(queue.length());

3. starts屬性：starts屬性返回一個布爾值，該值指示隊列是否已開始處理數據。以下代碼演示了啟動屬性的工作方式。

   Javascript

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

4. unshift(element，callback)：unshift方法類似於push方法，但是該元素已添加到隊列的開頭，表明要處理的元素是重要的元素。以下代碼演示了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：當隊列執行完所有任務後，drain方法將運行回調函數。下麵的代碼演示了rain方法的用法方式。

   注意：僅當描述的函數是箭頭函數時，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：暫停方法暫停隊列中元素的執行，直到調用恢複函數為止。以下代碼演示了pause方法的用法方式。

   Javascript

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

7. resume()方法：resume方法恢複隊列中元素的執行。以下代碼演示了resume方法的用法方式。

   Javascript

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

8. kill()方法：kill方法從隊列中刪除所有元素，並刪除掉方法的回調函數並將其強製為空閑。以下代碼演示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()方法返回一個布爾值，指示隊列是空閑還是正在處理某些內容。以下代碼演示了idle方法的用法方式。

   Javascript

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

完整的代碼：以下代碼完整演示了async.queue的實際使用方式。

// 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}`)

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