Android中的回调是什么? [英] What is callback in Android?

问题描述

我想了解回调的概念.我在互联网上搜索了有关回调的信息，并且有很多使用接口的示例，一个类正在使用该接口调用另一个类的方法.但是还是没搞懂回调的主要概念，使用回调的目的是什么?

I want to understand the concept of callback. I have searched on internet about the callbacks and there are many examples using interface, and one class is calling a method of another class using that interface. But still I can't get the main concept of callbacks, what is the purpose of using callbacks?

推荐答案

这里有一个很好的 教程，很好地描述了回调和用例.

Here is a nice tutorial, which describes callbacks and the use-case well.

回调的概念是通知一个类同步/异步另一个类中的某些工作是否完成.有人称之为好莱坞原则:不要叫我们我们叫你".

The concept of callbacks is to inform a class synchronous / asynchronous if some work in another class is done. Some call it the Hollywood principle: "Don't call us we call you".

举个例子:

class A implements ICallback {
     MyObject o;
     B b = new B(this, someParameter);

     @Override
     public void callback(MyObject o){
           this.o = o;
     }
}

class B {
     ICallback ic;
     B(ICallback ic, someParameter){
         this.ic = ic;
     }

    new Thread(new Runnable(){
         public void run(){
             // some calculation
             ic.callback(myObject)
         }
    }).start(); 
}

interface ICallback{
    public void callback(MyObject o);
}

A 类调用 B 类以在线程中完成一些工作.如果 Thread 完成工作，它将通过回调通知 Class A 并提供结果.所以不需要轮询什么的.您将在结果可用后立即获得结果.

Class A calls Class B to get some work done in a Thread. If the Thread finished the work, it will inform Class A over the callback and provide the results. So there is no need for polling or something. You will get the results as soon as they are available.

在 Android 中使用回调函数 f.e.活动和片段之间.因为 Fragment 应该是模块化的，所以你可以在 Fragment 中定义一个回调来调用 Activity 中的方法.

In Android Callbacks are used f.e. between Activities and Fragments. Because Fragments should be modular you can define a callback in the Fragment to call methods in the Activity.

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