node.js - 如何在异步函数中使用“this”？

Question

我有一个简单的例子：

function File(name) {
   this.name = name
   this.text = null
}

File.prototype = {
    read: function() {
         fs.readFile(this.name, function (err, data) {
     }
    },
    getContent: function() {
         return this.text
     }
}

var myfile = new File('my_file')

watch.createMonitor('my_file_dir', function (monitor) {
    monitor.files['my_file']
    monitor.on("change", function (f, stat) {
        myfile.read()
    }
})

  main program....:

   myfile.getContent() ...

我想在 this.text 变量中添加文件内容。怎么做 ？

Answer 1

• 创建局部变量并存储在那里'this'

  阅读：函数（）{ var _file = this；fs.readFile(this.name, function (err, data) { ... _file.text = data; ... }); },

• 将“this”绑定到内部函数：读取：function() {

   fs.readFile(this.name, function (err, data) {
       ...
       this.text = data;
       ...
   }.bind(this)
  

  },

Note: It's insufficiently to store data to this.text: if you read something asynchronously in yur class, you need to provide callbacks to let other objects know that you got some data in yourFile.text

Answer 2

您可以保存this对闭包外部的引用并从内部引用它：

File.prototype = {
    read: function() {
        var self = this;
        fs.readFile(this.name, function (err, data) {
            self.text = data;
        });
    },
    getContent: function() {
        return this.text
    }
}

Answer 3

你可以做几件事：

1. 创建一个作用域版本this：

   File.prototype = {
     read: function() {
       var self = this;
       fs.readFile(this.name, function (err, data) {
         self.text += data;
       });
     }
   };
   

2. 将函数绑定到 的当前值this：

   File.prototype = {
     read: function() {
       fs.readFile(this.name, function (err, data) {
         this.text += data;
       }.bind(this));
     }
   };