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?Android到处都在使用的回调分析
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回调函数：
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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。
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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.
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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):
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1．定义接口
public interface OnClickListener {
??? public void OnClick(Button b);
}
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2． 定义Button
public class Button {
? OnClickListener listener;
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? public void click() {
??? listener.OnClick(this);
? }
? public void setOnClickListener(OnClickListener listener) {
??? this.listener = listener;
? }
}
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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();
? }
}
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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：
- //?Create?an?anonymous?implementation?of?OnClickListener ?
- private?OnClickListener?mCorkyListener?=?new?OnClickListener()?{ ?
- public?void?onClick(View?v)?{ ?
- //?do?something?when?the?button?is?clicked ?
- } ?
- }; ?
- protected?void?onCreate(Bundle?savedValues)?{ ?
- ... ?
- //?Capture?our?button?from?layout ?
- Button?button?=?(Button)findViewById(R.id.corky); ?
- //?Register?the?onClick?listener?with?the?implementation?above ?
- button.setOnClickListener(mCorkyListener); ?
- ... ?
- }?
采用嵌套接口实现：
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- public?class?ExampleActivity?extends?Activity?implements?OnClickListener?{ ?
- protected?void?onCreate(Bundle?savedValues)?{ ?
- ... ?
- Button?button?=?(Button)findViewById(R.id.corky); ?
- button.setOnClickListener(this); ?
- } ?
- //?Implement?the?OnClickListener?callback ?
- public?void?onClick(View?v)?{ ?
- //?do?something?when?the?button?is?clicked ?
- } ?
- ... ?
- }?
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下面以MMS模块中的实际例子为例进行说明：
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?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：
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//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 * [email protected] 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：
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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
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