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Android到处都在使用的回调分析

博客分类：

Android

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：

// 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：

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
}
...
}

下面以MMS模块中的实际例子为例进行说明：

例子1：

//ConversationListAdapter.java

private 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.java

public 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：