本文介绍如何在Android应用中实现下拉刷新功能。通过自定义ListView控件并结合触摸及滚动监听，实现实时更新数据的效果。具体步骤包括创建自定义ListView、添加头部刷新视图及实现刷新逻辑。
相信大家都已经见过人人、QQ、新浪微博等客户端的下拉刷新功能，感觉用起来是不是特别爽，现在我就从网上copy了一段代码过来和大家一起学习学习。先把代码贴出来，等你们在自己机子上运行了先。
新建项目：com.markupartist.android.example.pulltorefresh，Activity名为：PullToRefreshActivity
再建立一个包：com.markupartist.android.widget，里面建一个文件：PullToRefreshListView
XML文件，文件名：pull_to_refresh.xml：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
>
<!--
此处是自定义ListView控件
-->
<com.markupartist.android.widget.PullToRefreshListView
android:id="@+id/usalist"
android:layout_height="fill_parent"
android:layout_width="fill_parent"
>
</com.markupartist.android.widget.PullToRefreshListView>
</LinearLayout>
[代码]文件名：pull_to_refresh_header.xml：
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:paddingTop="10dip"
android:paddingBottom="15dip"
android:gravity="center"
android:id="@+id/pull_to_refresh_header"
>
<ProgressBar
android:id="@+id/pull_to_refresh_progress"
android:indeterminate="true"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginLeft="30dip"
android:layout_marginRight="20dip"
android:layout_marginTop="10dip"
android:visibility="gone"
android:layout_centerVertical="true"
style="@drawable/icon"
/>
<ImageView
android:id="@+id/pull_to_refresh_image"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginLeft="30dip"
android:layout_marginRight="20dip"
android:visibility="gone"
android:layout_gravity="center"
android:gravity="center"
android:src="@drawable/goicon"
/>
<TextView
android:id="@+id/pull_to_refresh_text"
android:text="@string/pull_to_refresh_tap_label"
android:textAppearance="?android:attr/textAppearanceMedium"
android:textStyle="bold"
android:paddingTop="5dip"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:gravity="center"
/>
<TextView
android:id="@+id/pull_to_refresh_updated_at"
android:layout_below="@+id/pull_to_refresh_text"
android:visibility="gone"
android:textAppearance="?android:attr/textAppearanceSmall"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:gravity="center"
/>
</RelativeLayout>
还需要一张图片
，这个就是下拉箭头。
[代码]PullToRefreshActivity.java：
package com.markupartist.android.example.pulltorefresh;
import java.util.Arrays;
import java.util.LinkedList;
import android.app.Activity;
import android.app.ListActivity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.widget.ArrayAdapter;
import com.markupartist.android.widget.PullToRefreshListView;
import com.markupartist.android.widget.PullToRefreshListView.OnRefreshListener;
public class PullToRefreshActivity extends Activity {
private LinkedList<String> mListItems;
//此处开始初始化mlistview，包括一些事件
private PullToRefreshListView mlistview;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.pull_to_refresh);
// Set a listener to be invoked when the list should be refreshed.
mlistview = ((PullToRefreshListView)PullToRefreshActivity.this.findViewById(R.id.usalist) );
mlistview.setOnRefreshListener(new OnRefreshListener() {
@Override
public void onRefresh() {
// TODO Auto-generated method stub
new GetDataTask().execute();
}
});/*() {
@Override
public void onRefresh() {
// Do work to refresh the list here.
new GetDataTask().execute();
}
});*/
mListItems = new LinkedList<String>();
mListItems.addAll(Arrays.asList(mStrings));
ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
android.R.layout.simple_list_item_1, mListItems);
mlistview.setAdapter(adapter);
}
//执行异步的操作
private class GetDataTask extends AsyncTask<Void, Void, String[]> {
@Override
protected String[] doInBackground(Void... params) {
// Simulates a background job.
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
;
}
return mStrings;
}
@Override
protected void onPostExecute(String[] result) {
mListItems.addFirst("Added after refresh...");
// Call onRefreshComplete when the list has been refreshed.
mlistview.onRefreshComplete();
super.onPostExecute(result);
}
}
private String[] mStrings = {
"Abbaye de Belloc", "Abbaye du Mont des Cats", "Abertam",
"Abondance", "Ackawi", "Acorn", "Adelost", "Affidelice au Chablis",
"Afuega'l Pitu", "Airag", "Airedale", "Aisy Cendre",
"Allgauer Emmentaler"};
}
[代码]PullToRefreshListView.java：
package com.markupartist.android.widget;
import com.markupartist.android.example.pulltorefresh.R;
import android.content.Context;
import android.util.AttributeSet;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;
import android.view.animation.LinearInterpolator;
import android.view.animation.RotateAnimation;
import android.widget.AbsListView;
import android.widget.ImageView;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.ProgressBar;
import android.widget.RelativeLayout;
import android.widget.TextView;
import android.widget.AbsListView.OnScrollListener;
public class PullToRefreshListView extends ListView implements OnScrollListener {
/**初始状态*/
private static final int TAP_TO_REFRESH = 1;
/**拉动刷新*/
private static final int PULL_TO_REFRESH = 2;
/**释放刷新**/
private static final int RELEASE_TO_REFRESH = 3;
/**正在刷新*/
private static final int REFRESHING = 4;
private static final String TAG = "PullToRefreshListView";
//刷新接口
private OnRefreshListener mOnRefreshListener;
//箭头图片
private static int REFRESHICON = R.drawable.goicon;
/**
* listview 滚动监听器
*/
private OnScrollListener mOnScrollListener;
//视图索引器
private LayoutInflater mInflater;
/**
* 头部视图 内容 -- start
*/
private RelativeLayout mRefreshView;
private TextView mRefreshViewText;
private ImageView mRefreshViewImage;
private ProgressBar mRefreshViewProgress;
private TextView mRefreshViewLastUpdated;
/**
* 头部视图 内容 -- end
*/
/**当前listivew 的滚动状态*/
private int mCurrentScrollState;
/**当前listview 的刷新状态*/
private int mRefreshState;
//动画效果
/**变为向下的箭头*/
private RotateAnimation mFlipAnimation;
/**变为逆向的箭头*/
private RotateAnimation mReverseFlipAnimation;
//头视图的高度
private int mRefreshViewHeight;
//头视图 原始的 top padding 属性值
private int mRefreshOriginalTopPadding;
//
private int mLastMotionY;
//是否反弹
private boolean mBounceHack;
public PullToRefreshListView(Context context) {
super(context);
init(context);
}
public PullToRefreshListView(Context context, AttributeSet attrs) {
super(context, attrs);
//默认此处构造函数
init(context);
}
public PullToRefreshListView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
init(context);
}
private void init(Context context) {
// Load all of the animations we need in code rather than through XML
//初始化动画
//
mFlipAnimation = new RotateAnimation(0, -180,RotateAnimation.RELATIVE_TO_SELF, 0.5f,RotateAnimation.RELATIVE_TO_SELF, 0.5f);
mFlipAnimation.setInterpolator(new LinearInterpolator());//设置速度模式为LinearInterpolator
mFlipAnimation.setDuration(250);//设置持续时间
mFlipAnimation.setFillAfter(true);//设置为填充状态
mReverseFlipAnimation = new RotateAnimation(-180, 0,RotateAnimation.RELATIVE_TO_SELF, 0.5f,RotateAnimation.RELATIVE_TO_SELF, 0.5f);
mReverseFlipAnimation.setInterpolator(new LinearInterpolator());
mReverseFlipAnimation.setDuration(250);
mReverseFlipAnimation.setFillAfter(true);
mInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);//设置minflater为获取XML文件格式
mRefreshView = (RelativeLayout) mInflater.inflate(R.layout.pull_to_refresh_header, this, false);//(R.layout.pull_to_refresh_header, null);
mRefreshViewText =(TextView) mRefreshView.findViewById(R.id.pull_to_refresh_text);
mRefreshViewImage =(ImageView) mRefreshView.findViewById(R.id.pull_to_refresh_image);
mRefreshViewProgress = (ProgressBar) mRefreshView.findViewById(R.id.pull_to_refresh_progress);
mRefreshViewLastUpdated = (TextView) mRefreshView.findViewById(R.id.pull_to_refresh_updated_at);
mRefreshViewImage.setMinimumHeight(50);
//mRefreshView.setOnClickListener(new OnClickRefreshListener());//此处毫无用处，不会调用此代码
mRefreshOriginalTopPadding = mRefreshView.getPaddingTop();
System.out.println("设置为初始状态");
mRefreshState = TAP_TO_REFRESH;//初始状态
addHeaderView(mRefreshView);//将mRefreshView加入ListView头部
super.setOnScrollListener(this);
measureView(mRefreshView);
mRefreshViewHeight = mRefreshView.getMeasuredHeight(); //获取头文件的测量高度
}
@Override
protected void onAttachedToWindow() {
setSelection(1);
}
@Override
public void setAdapter(ListAdapter adapter) {
super.setAdapter(adapter);
setSelection(1);
}
/**
* Set the listener that will receive notifications every time the list
* scrolls.
*
* @param l The scroll listener.
*/
@Override
public void setOnScrollListener(AbsListView.OnScrollListener l) {
System.out.println("时刻刷新");
mOnScrollListener = l;
}
/**
* Register a callback to be invoked when this list should be refreshed.
*
* @param onRefreshListener The callback to run.
*/
public void setOnRefreshListener(OnRefreshListener onRefreshListener) {
mOnRefreshListener = onRefreshListener;
}
/**
* Set a text to represent when the list was last updated.
* @param lastUpdated Last updated at.
*/
public void setLastUpdated(CharSequence lastUpdated) {
if (lastUpdated != null) {
mRefreshViewLastUpdated.setVisibility(View.VISIBLE);
mRefreshViewLastUpdated.setText(lastUpdated);
} else {
mRefreshViewLastUpdated.setVisibility(View.GONE);
}
}
@Override
public boolean onTouchEvent(MotionEvent event) {
//当前手指的Y值
final int y = (int) event.getY();
//Log.i(TAG, "触屏的Y值"+y);
mBounceHack = false; //不反弹
switch (event.getAction()) {
case MotionEvent.ACTION_UP:
//将垂直滚动条设置为可用状态
if (!isVerticalScrollBarEnabled()) {
setVerticalScrollBarEnabled(true);
}
//如果头部刷新条出现，并且不是正在刷新状态
if (getFirstVisiblePosition() == 0 && mRefreshState != REFRESHING) {
if ((mRefreshView.getBottom() >= mRefreshViewHeight
|| mRefreshView.getTop() >= 0)
&& mRefreshState == RELEASE_TO_REFRESH) { //如果头部视图处于拉离顶部的情况
// Initiate the refresh
mRefreshState = REFRESHING; //将标量设置为，正在刷新
prepareForRefresh(); //准备刷新
onRefresh(); //刷新
} else if (mRefreshView.getBottom() < mRefreshViewHeight
|| mRefreshView.getTop() <= 0) {
// Abort refresh and scroll down below the refresh view
// 停止刷新，并且滚动到头部刷新视图的下一个视图
resetHeader();
setSelection(1); //定位在第二个列表项
}
}
break;
case MotionEvent.ACTION_DOWN:
mLastMotionY = y; //跟踪手指的Y值
break;
case MotionEvent.ACTION_MOVE:
//更行头视图的toppadding 属性
applyHeaderPadding(event);
break;
}
return super.onTouchEvent(event);
}
/****
* 不断的头部的top padding 属性
* @param ev
*/
private void applyHeaderPadding(MotionEvent ev) {
//获取累积的动作数
int pointerCount = ev.getHistorySize();
// Log.i(TAG, "获取累积的动作数"+pointerCount);
for (int p = 0; p < pointerCount; p++) {
if (mRefreshState == RELEASE_TO_REFRESH) { //如果是释放将要刷新状态
if (isVerticalFadingEdgeEnabled()) {
setVerticalScrollBarEnabled(false);
}
//历史累积的高度
int historicalY = (int) ev.getHistoricalY(p);
//Log.i(TAG, "单个动作getHistoricalY值："+historicalY);
// Calculate the padding to apply, we divide by 1.7 to
// simulate a more resistant effect during pull.
int topPadding = (int) (((historicalY - mLastMotionY)
- mRefreshViewHeight) / 1.7);
mRefreshView.setPadding(
mRefreshView.getPaddingLeft(),
topPadding,
mRefreshView.getPaddingRight(),
mRefreshView.getPaddingBottom());
}
}
}
/**
* Sets the header padding back to original size.
* 使头部视图的 toppadding 恢复到初始值
*/
private void resetHeaderPadding() {
mRefreshView.setPadding(
mRefreshView.getPaddingLeft(),
mRefreshOriginalTopPadding,
mRefreshView.getPaddingRight(),
mRefreshView.getPaddingBottom());
}
/**
* Resets the header to the original state.
* 初始化头部视图 状态
*/
private void resetHeader() {
if (mRefreshState != TAP_TO_REFRESH) {
mRefreshState = TAP_TO_REFRESH; //初始刷新状态
//使头部视图的 toppadding 恢复到初始值
resetHeaderPadding();
// Set refresh view text to the pull label
//将文字初始化
mRefreshViewText.setText(R.string.pull_to_refresh_tap_label);
// Replace refresh drawable with arrow drawable
//设置初始图片
mRefreshViewImage.setImageResource(REFRESHICON);
// Clear the full rotation animation
// 清除动画
mRefreshViewImage.clearAnimation();
// Hide progress bar and arrow.
//隐藏头视图
mRefreshViewImage.setVisibility(View.GONE);
//隐藏进度条
mRefreshViewProgress.setVisibility(View.GONE);
}
}
//测量视图的高度
private void measureView(View child) {
//获取头部视图属性
ViewGroup.LayoutParams p = child.getLayoutParams();
if (p == null) {
p = new ViewGroup.LayoutParams(
ViewGroup.LayoutParams.FILL_PARENT,
ViewGroup.LayoutParams.WRAP_CONTENT);
}
int childWidthSpec = ViewGroup.getChildMeasureSpec(0,
0 + 0, p.width);
int lpHeight = p.height;
int childHeightSpec;
//不懂MeasureSpec------------------------------------------------------------------------------------------
if (lpHeight > 0) { //如果视图的高度大于0
childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
} else {
childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);
}
child.measure(childWidthSpec, childHeightSpec);
//不懂MeasureSpec------------------------------------------------------------------------------------------
}
/**
* 滑动监听事件
*/
@Override
public void onScroll(AbsListView view, int firstVisibleItem,int visibleItemCount, int totalItemCount) {
// When the refresh view is completely visible, change the text to say
// "Release to refresh..." and flip the arrow drawable.
//System.out.println("是这个this？");
if (mCurrentScrollState == SCROLL_STATE_TOUCH_SCROLL //如果是接触滚动状态,并且不是正在刷新的状态
&& mRefreshState != REFRESHING) {
if (firstVisibleItem == 0) { //如果显示出来了第一个列表项
//显示刷新图片
mRefreshViewImage.setVisibility(View.VISIBLE);
if ((mRefreshView.getBottom() >= mRefreshViewHeight + 20
|| mRefreshView.getTop() >= 0)
&& mRefreshState != RELEASE_TO_REFRESH) { //如果下拉了listiview,则显示上拉刷新动画
mRefreshViewText.setText(R.string.pull_to_refresh_release_label);
mRefreshViewImage.clearAnimation();
mRefreshViewImage.startAnimation(mFlipAnimation);
mRefreshState = RELEASE_TO_REFRESH;
Log.i(TAG, "现在处于下拉状态");
} else if (mRefreshView.getBottom() < mRefreshViewHeight + 20
&& mRefreshState != PULL_TO_REFRESH) { //如果没有到达，下拉刷新距离，则回归原来的状态
mRefreshViewText.setText(R.string.pull_to_refresh_pull_label);
if (mRefreshState != TAP_TO_REFRESH) {
mRefreshViewImage.clearAnimation();
mRefreshViewImage.startAnimation(mReverseFlipAnimation);
mRefreshViewText.setText("非刷新状态，开始回弹");
Log.i(TAG, "现在处于回弹状态");
}
mRefreshState = PULL_TO_REFRESH;
}
} else {
System.out.println("隐藏刷新图片");
mRefreshViewImage.setVisibility(View.GONE); //隐藏刷新图片
resetHeader(); //初始化，头部
}
} else if (mCurrentScrollState == SCROLL_STATE_FLING //如果是自己滚动状态+ 第一个视图已经显示 + 不是刷新状态
&& firstVisibleItem == 0
&& mRefreshState != REFRESHING) {
setSelection(1);
mBounceHack = true; //状态为回弹
Log.i(TAG, "现在处于自由滚动到顶部的状态");
} else if (mBounceHack && mCurrentScrollState == SCROLL_STATE_FLING) {
setSelection(1);
Log.i(TAG, "现在处于自由滚动到顶部的状态");
}
if (mOnScrollListener != null) {
mOnScrollListener.onScroll(view, firstVisibleItem,
visibleItemCount, totalItemCount);
}
}
//滚动状态改变
@Override
public void onScrollStateChanged(AbsListView view, int scrollState) {
mCurrentScrollState = scrollState;
if (mCurrentScrollState == SCROLL_STATE_IDLE) { //如果滚动停顿
mBounceHack = false;
}
if (mOnScrollListener != null) {
mOnScrollListener.onScrollStateChanged(view, scrollState);
}
}
//准备刷新
public void prepareForRefresh() {
resetHeaderPadding(); //初始化，头部文件
mRefreshViewImage.setVisibility(View.GONE);
// We need this hack, otherwise it will keep the previous drawable.
mRefreshViewImage.setImageDrawable(null);
mRefreshViewProgress.setVisibility(View.VISIBLE);
// Set refresh view text to the refreshing label
mRefreshViewText.setText(R.string.pull_to_refresh_refreshing_label);
mRefreshState = REFRESHING;
}
//刷新
public void onRefresh() {
Log.d(TAG, "执行刷新");
if (mOnRefreshListener != null) {
mOnRefreshListener.onRefresh();
}
}
/**
* 刷新完成 的回调函数
* Resets the list to a normal state after a refresh.
* @param lastUpdated Last updated at.
*/
public void onRefreshComplete(CharSequence lastUpdated) {
setLastUpdated(lastUpdated);
onRefreshComplete();
}
/**
* 刷新完成回调函数
* Resets the list to a normal state after a refresh.
*/
public void onRefreshComplete() {
Log.d(TAG, "onRefreshComplete");
resetHeader();
// If refresh view is visible when loading completes, scroll down to
// the next item.
if (mRefreshView.getBottom() > 0) {
invalidateViews(); //重绘视图
setSelection(1);
}
}
/**
* Invoked when the refresh view is clicked on. This is mainly used when
* there's only a few items in the list and it's not possible to drag the
* list.
*/
private class OnClickRefreshListener implements OnClickListener {
@Override
public void onClick(View v) {
if (mRefreshState != REFRESHING) {
//准备刷新
System.out.println("准备刷新");
prepareForRefresh();
System.out.println("开始刷新");
//刷新
onRefresh();
}
}
}
/**
* 刷新方法接口
*/
public interface OnRefreshListener {
public void onRefresh();
}
}好了，所有代码都在这里（突然感觉好长啊，为什么不能用附件呢，表示疑惑下），下面就来分析下原理吧。此技术主要使用了ListView中setOnScrollListener功能，它实现一个方法OnScroll，来监听ListView上下滚动功能（只要有滚动动作都会重新执行一遍此方法，总之，执行比较频繁，可以满足实时监听），并对此作出一些限制，详细情况请参看代码。此外，最为重要的就是在ListView头部增加一个自定义项，它拥有自定义的XML文件。当ListView向下滚动将头部文件都拉出时会修改当前状态，在回滚时取消头部隐藏，同时执行刷新动作，刷新完毕后再重置为初始状态。
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