概述

我们先看一下维基百科对回调函数的定义:

在计算机程序设计中，回调函数，或简称回调(Callback 即call then back 被主函数调用运算后会返回主函数)，是指通过函数参数传递到其它代码的，某一块可执行代码的引用。这一设计允许了底层代码调用在高层定义的子程序。

举个栗子

我们看一下android中最经典的View.setOnClickListener()源码:

1.定义接口

/**

* Interface definition for a callback to be invoked when a view is clicked.

*/

public interface OnClickListener {

/**

* Called when a view has been clicked.

*

* @param v The view that was clicked.

*/

void onClick(View v);

}

2.注册回调函数

/**

* Register a callback to be invoked when this view is clicked. If this view is not

* clickable, it becomes clickable.

*

* @param l The callback that will run

*

* @see #setClickable(boolean)

*/

public void setOnClickListener(@Nullable OnClickListener l) {

if (!isClickable()) {

setClickable(true);

}

getListenerInfo().mOnClickListener = l;

}

3.执行回调函数

/**

* Call this view's OnClickListener, if it is defined. Performs all normal

* actions associated with clicking: reporting accessibility event, playing

* a sound, etc.

*

* @return True there was an assigned OnClickListener that was called, false

* otherwise is returned.

*/

public boolean performClick() {

final boolean result;

final ListenerInfo li = mListenerInfo;

if (li != null && li.mOnClickListener != null) {

playSoundEffect(SoundEffectConstants.CLICK);

li.mOnClickListener.onClick(this);

result = true;

} else {

result = false;

}

sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);

return result;

}

4.调用View的注册回调函数方法

view.setOnClickListener(new View.OnClickListener() {

@Override

public void onClick(View v) {

//do something

}

});

ok了,这样就完成了一次完整的回调函数.

总结一下

首先,在MainActivity中通过View中的setOnclickListener方法的参数将OnClickListener 接口的实现者传给了View,最终在被点击时在performClick方法中,执行了实现者的onClick方法.

是不是就如维基百科上的回调定义:通过函数参数传递到其它代码的,某一块可执行代码的引用.O(∩_∩)O.