Android到处都在使用的回调分析

回调函数：

回调函数就是一个通过函数指针调用的函数。如果你把函数的指针(地址)作为参数传递给另一个函数，当这个指针被用为调用它所指向的函数时，我们就说这是回调函数。回调函数不是由该函数的实现方直接调用，而是在特定的事件或条件发生时由另外的一方调用的，用于对该事件或条件进行响应。

Java 中没有指针的概念，通过接口和内部类的方式实现回调的功能:

1. 定义接口 Callback ,包含回调方法 callback()

2. 在一个类Caller 中声明一个Callback接口对象 mCallback

3. 在程序中赋予Caller对象的接口成员(mCallback) 一个内部类对象如

new Callback（）{

callback（）{

//函数的具体实现

}

这样,在需要的时候,可用Caller对象的mCallback接口成员调用callback()方法,完成回调.

Android事件侦听器是视图View类的接口，包含一个单独的回调方法。这些方法将在视图中注册的侦听器被用户界面操作触发时由Android框架调用。回调方法被包含在Android事件侦听器接口中：

例如,Android 的view 对象都含有一个命名为 OnClickListener 接口成员变量，用户的点击操作都会交给 OnClickListener的 OnClick() 方法进行处理。

开发者若需要对点击事件做处理，可以定义一个 OnClickListener 接口对象，赋给需要被点击的 view的接口成员变量OnClickListener，一般是用 view 的setOnClickListener() 函数来完成这一操作。

当有用户点击事件时，系统就会回调被点击view的OnClickListener接口成员的OnClick()方法。

实例(对于Android界面上Button点击事件监听的模拟):

1．定义接口

public interface OnClickListener {

public void OnClick(Button b);

}

2．定义Button

public class Button {

OnClickListener listener;

public void click() {

listener.OnClick(this);

}

public void setOnClickListener(OnClickListener listener) {

this.listener = listener;

}

3．将接口对象OnClickListener 赋给 Button的接口成员

public class Activity {

public Activity() {

}

public static void main(String[] args) {

Button button = new Button();

button.setOnClickListener(new OnClickListener(){

@Override

public void OnClick(Button b) {

System.out.println("clicked");

}

});

button.click(); //user click,System call button.click();

}

要定义这些方法并处理你的事件，在你的活动中实现这个嵌套接口或定义它为一个匿名类：

采用匿名类方式实现：

//CreateananonymousimplementationofOnClickListener
privateOnClickListenermCorkyListener=newOnClickListener(){
publicvoidonClick(Viewv){
//dosomethingwhenthebuttonisclicked
}
};
protectedvoidonCreate(BundlesavedValues){
...
//Captureourbuttonfromlayout
Buttonbutton=(Button)findViewById(R.id.corky);
//RegistertheonClicklistenerwiththeimplementationabove
button.setOnClickListener(mCorkyListener);
...
}

采用嵌套接口实现：

publicclassExampleActivityextendsActivityimplementsOnClickListener{
protectedvoidonCreate(BundlesavedValues){
...
Buttonbutton=(Button)findViewById(R.id.corky);
button.setOnClickListener(this);
}
//ImplementtheOnClickListenercallback
publicvoidonClick(Viewv){
//dosomethingwhenthebuttonisclicked
}
...
}

下面以MMS模块中的实际例子为例进行说明：

例子1：

//ConversationListAdapter.javaprivate OnContentChangedListener mOnContentChangedListener; public interface OnContentChangedListener {        void onContentChanged(ConversationListAdapter adapter);    }    public void setOnContentChangedListener(OnContentChangedListener l) {        mOnContentChangedListener = l;    }    @Override    protected void onContentChanged() {        if (mCursor != null && !mCursor.isClosed()) {            if (mOnContentChangedListener != null) {                mOnContentChangedListener.onContentChanged(this);            }        }    }//ConversationList.java  private final ConversationListAdapter.OnContentChangedListener mContentChangedListener =        new ConversationListAdapter.OnContentChangedListener() {        public void onContentChanged(ConversationListAdapter adapter) {            startAsyncQuery();        }    };    private void initListAdapter() {        mListAdapter = new ConversationListAdapter(this, null);        mListAdapter.setOnContentChangedListener(mContentChangedListener);        setListAdapter(mListAdapter);        getListView().setRecyclerListener(mListAdapter);    }

例子2：

//WorkingMessage.java private final MessageStatusListener mStatusListener; public interface MessageStatusListener {        /**         * Called when the protocol for sending the message changes from SMS         * to MMS, and vice versa.         *         * @param mms If true, it changed to MMS.  If false, to SMS.         */        void onProtocolChanged(boolean mms);        /**         * Called when an attachment on the message has changed.         */        void onAttachmentChanged();        /**         * Called just before the process of sending a message.         */        void onPreMessageSent();        /**         * Called once the process of sending a message, triggered by         * {@link send} has completed. This doesn't mean the send succeeded,         * just that it has been dispatched to the network.         */        void onMessageSent();        /**         * Called if there are too many unsent messages in the queue and we're not allowing         * any more Mms's to be sent.         */        void onMaxPendingMessagesReached();        /**         * Called if there's an attachment error while resizing the images just before sending.         */        void onAttachmentError(int error);    }    private WorkingMessage(ComposeMessageActivity activity) {        mActivity = activity;        mContentResolver = mActivity.getContentResolver();        mStatusListener = activity; //设置        mAttachmentType = TEXT;        mText = "";    }  public static WorkingMessage createEmpty(ComposeMessageActivity activity) {        // Make a new empty working message.        WorkingMessage msg = new WorkingMessage(activity);        return msg;    }  private void sendSmsWorker(String msgText, String semiSepRecipients, long threadId) {        String[] dests = TextUtils.split(semiSepRecipients, ";");        if (LogTag.VERBOSE || Log.isLoggable(LogTag.TRANSACTION, Log.VERBOSE)) {            LogTag.debug("sendSmsWorker sending message: recipients=" + semiSepRecipients +                    ", threadId=" + threadId);        }        MessageSender sender = new SmsMessageSender(mActivity, dests, msgText, threadId);        try {            sender.sendMessage(threadId);            // Make sure this thread isn't over the limits in message count            Recycler.getSmsRecycler().deleteOldMessagesByThreadId(mActivity, threadId);        } catch (Exception e) {            Log.e(TAG, "Failed to send SMS message, threadId=" + threadId, e);        }        mStatusListener.onMessageSent();  //调用    }//ComposeMessageActivity.javapublic class ComposeMessageActivity extends Activity        implements View.OnClickListener, TextView.OnEditorActionListener,        MessageStatusListener/*实现该接口*/, Contact.UpdateListener {     ....       public void onProtocolChanged(final boolean mms) {//实现onProtocolChanged接口        // Have to make sure we're on the UI thread. This function can be called off of the UI        // thread when we're adding multi-attachments        runOnUiThread(new Runnable() {            public void run() {                toastConvertInfo(mms);                setSendButtonText(mms);            }        });    }    ....     public void initialize(Bundle savedInstanceState, long originalThreadId) {        Intent intent = getIntent();        // Create a new empty working message.        mWorkingMessage = WorkingMessage.createEmpty(this); //}

参考：

回调机制在 Android 监听用户界面操作中的体现 http://www.cnblogs.com/greatstar/archive/2011/03/02/1968999.html

百度百科: 回调函数 http://baike.baidu.com/view/414773.html?fromTaglist

java中回调函数的实例说明 http://www.blogjava.net/songfei/articles/126093.html

Android事件侦听浅谈 http://developer.51cto.com/art/201001/180846.htm

Android到处都在使用的回调分析

回调机制在 Android 监听用户界面操作中的体现 http://www.cnblogs.com/greatstar/archive/2011/03/02/1968999.html

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