[英]Object-method callback loses its binding in an event handler when passed as a parameter, but not when hard-coded
I'm new to Javascript OOP and have a question about binding to callbacks in event handlers.我是 Javascript OOP 的新手,有一个关于绑定到事件处理程序中的回调的问题。
I'm trying to apply an event handler to a DOM element in my constructor function.我正在尝试将事件处理程序应用于构造函数中的 DOM 元素。 The event-handler function is a method of the object, and I'm attempting to pass a callback function (also a method of the same object) to that handler.事件处理函数是对象的方法,我试图将回调函数(也是同一对象的方法)传递给该处理程序。
When I hard code the callback inside the handler (using this.callbackMethod()) it runs as expected:当我在处理程序中硬编码回调时(使用 this.callbackMethod()),它按预期运行:
class Foo {
constructor (name){
this.name = name
this.bar();
}
callback(){
console.log(this.name + 'bar callback this:') // 'foobar callback this:
console.log(this) // Foo object with name 'foo'
}
bar(){
document.querySelector('button').addEventListener('click', function(){
console.log('bar click event this:')
console.log(this)
// this is the relevant line
this.callback()
}.bind(this));
}
}
const foo = new Foo('foo');
However, when I pass that parameter as a callback, even if I use .bind(this) on both the callback and the handler, it fails:但是,当我将该参数作为回调传递时,即使我在回调和处理程序上都使用 .bind(this) ,它也会失败:
class Foo {
constructor (name){
this.name = name
this.bar(this.callback);
}
callback(){
console.log(this.name + 'bar callback this:')// Uncaught TypeError: Cannot read property 'name' of undefined
console.log(this)
}
bar(cb){
document.querySelector('button').addEventListener('click', function(){
console.log(cb)// logs function definition
// this is the relevant line
cb().bind(this);
}.bind(this));
}
}
const foo = new Foo('foo');
Codepen examples:代码笔示例:
Hard-coded callback: https://codepen.io/RandomNeuralFiring/pen/Pgrdey硬编码回调: https : //codepen.io/RandomNeuralFiring/pen/Pgrdey
Parameter callback: https://codepen.io/RandomNeuralFiring/pen/QPXVOR参数回调: https : //codepen.io/RandomNeuralFiring/pen/QPXVOR
I want the option of re-using bar() with other callbacks, so would love to understand how I can set its context dynamically.我想要将 bar() 与其他回调一起使用的选项,所以很想了解我如何动态设置其上下文。
PS I couldn't find a suitable tag for object-binding - perhaps one should be created? PS 我找不到适合对象绑定的标签 - 也许应该创建一个?
您正在bind
cb
的返回值- 尝试先绑定函数然后调用它:
cb.bind(this)();
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