Android开源代码解读のOnScrollListener实现ListActivity滚屏首字母提示

OnScrollListener接口是定义在AbsListView中的，而AbsListView的直接子类有GridView和ListView，非直接子类有ExpandableListView。OnScrollListener的完整路径是frameworks\base\core\java\android\widget\AbsListView.java，代码如下：

/**
* Interface definition for a callback to be invoked when the list or grid
* has been scrolled.
*/
public interface OnScrollListener {

/**
* The view is not scrolling. Note navigating the list using the trackball counts as
* being in the idle state since these transitions are not animated.
*/
public static int SCROLL_STATE_IDLE = 0;

/**
* The user is scrolling using touch, and their finger is still on the screen
*/
public static int SCROLL_STATE_TOUCH_SCROLL = 1;

/**
* The user had previously been scrolling using touch and had performed a fling. The
* animation is now coasting to a stop
*/
public static int SCROLL_STATE_FLING = 2;

/**
* Callback method to be invoked while the list view or grid view is being scrolled. If the
* view is being scrolled, this method will be called before the next frame of the scroll is
* rendered. In particular, it will be called before any calls to
* {@link Adapter#getView(int, View, ViewGroup)}.
*
* @param view The view whose scroll state is being reported
*
* @param scrollState The current scroll state. One of {@link #SCROLL_STATE_IDLE},
* {@link #SCROLL_STATE_TOUCH_SCROLL} or {@link #SCROLL_STATE_IDLE}.
*/
public void onScrollStateChanged(AbsListView view, int scrollState);

/**
* Callback method to be invoked when the list or grid has been scrolled. This will be
* called after the scroll has completed
* @param view The view whose scroll state is being reported
* @param firstVisibleItem the index of the first visible cell (ignore if
*        visibleItemCount == 0)
* @param visibleItemCount the number of visible cells
* @param totalItemCount the number of items in the list adaptor
*/
public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount,
int totalItemCount);
}

该接口定义了三种滚动状态和两个回调函数，下面的代码使用onScroll来实现滚屏时提示内容首字母功能，当然，重点在例子的实现分析上面，并不局限于onScroll的使用。直接先看MainActivity的实现吧：(代码来自APIDemos，有修改)

package hust.iprai.asce1885;

import android.app.ListActivity;
import android.content.Context;
import android.graphics.PixelFormat;
import android.os.Bundle;
import android.os.Handler;
import android.view.LayoutInflater;
import android.view.View;
import android.view.WindowManager;
import android.view.ViewGroup.LayoutParams;
import android.widget.AbsListView;
import android.widget.AbsListView.OnScrollListener;
import android.widget.ArrayAdapter;
import android.widget.TextView;

/**
* 直接继承ListActivity，免去了ListView的继承，同时实现OnScrollListener接口
*
*/
public class MainActivity extends ListActivity implements OnScrollListener {

private final class RemoveWindow implements Runnable {

public void run() {
removeWindow();
}

}

private RemoveWindow mRemoveWindow = new RemoveWindow();
Handler mHandler = new Handler();
private WindowManager mWindowManager;
private TextView mDialogText;
private boolean mShowing; //是否显示mDialogText
private boolean mReady; //mDialogText是否已准备好
private char mPrevLetter = Character.MIN_VALUE;

@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);

//获取Window窗口管理服务
mWindowManager = (WindowManager) getSystemService(Context.WINDOW_SERVICE);

//使用ArrayAdapter将字符串数组中的数据映射到TextView中
setListAdapter(new ArrayAdapter<String>(this,
android.R.layout.simple_list_item_1, mStrings));

//设置滚动监听器ScrossListener
getListView().setOnScrollListener(this);

//获取LayoutInflater服务
LayoutInflater inflate = (LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);

//从xml文件生成TextView对象
mDialogText = (TextView) inflate.inflate(R.layout.list_position, null);
mDialogText.setVisibility(View.INVISIBLE);

//将Runnable添加到消息队列中
mHandler.post(new Runnable() {

public void run() {
mReady = true;
//设置mDialogText的WindowManager.LayoutParams参数
WindowManager.LayoutParams lp = new WindowManager.LayoutParams(
LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT,
WindowManager.LayoutParams.TYPE_APPLICATION,
WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE
| WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE,
PixelFormat.TRANSLUCENT);
mWindowManager.addView(mDialogText, lp);
}

});

}

@Override
protected void onDestroy() {
super.onDestroy();
mWindowManager.removeView(mDialogText);
mReady = false;
}

@Override
protected void onPause() {
super.onPause();
removeWindow();
mReady = false;
}

@Override
protected void onResume() {
super.onResume();
mReady = true;
}

//OnScrollListener的回调函数
public void onScroll(AbsListView view, int firstVisibleItem,
int visibleItemCount, int totalItemCount) {
if (mReady) {
//取得当前ListView中第一项内容的首字母
char firstLetter = mStrings[firstVisibleItem].charAt(0);

//当mDialogText还不可见且当前首字母于前次滚动后首字母不一致时
//将mDialogText设置为可见
if (!mShowing && firstLetter != mPrevLetter) {
mShowing = true;
mDialogText.setVisibility(View.VISIBLE);
}

//将首字母显示出来
mDialogText.setText(((Character)firstLetter).toString());
//将消息队列中还在等待post的mRemoveWindow清除
mHandler.removeCallbacks(mRemoveWindow);
//将Runnable mRemoveWindow添加到消息队列中，并延迟3s后运行
//实现的功能就是在mDialogText显示3s后，将它设置为不可见
mHandler.postDelayed(mRemoveWindow, 3000);
mPrevLetter = firstLetter;
}
}

//OnScrollListener的回调函数
public void onScrollStateChanged(AbsListView view, int scrollState) {

}

//将mDialogText设置为不可见
private void removeWindow() {
if (mShowing) {
mShowing = false;
mDialogText.setVisibility(View.INVISIBLE);
}
}

//用于显示的模拟数据
private String[] mStrings = Cheeses.sCheeseStrings;
}

mDialogText的布局文件list_position.xml如下：

<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
android:textSize="50sp"
android:textColor="#99FFFFFF"
android:typeface="monospace"
android:background="#BB000000"
android:minWidth="70dip"
android:maxWidth="70dip"
android:padding="10dip"
android:gravity="center"
/>

程序运行结果如下图所示：