該效果是一名國外工程師（johannilsson）的代碼，拿來研究瞭下，自己整合瞭一下，現在拿出來，跟大傢一起分享。
再次感謝這位國外工程師（johannilsson），謝謝！
新浪微博，和QQ空間裡面，都有那個下拉刷新的效果，另很多人眼前一亮，細細分析，原理原來如此。
在原作者的基礎上，寫瞭一些註釋，和幫助大傢更好的閱讀理解，（可能其中有些地方註釋不準，歡迎指正，謝謝）
下面，就亮出關鍵代碼：
**** 自定義listivew （關鍵代碼）
public class PullToRefreshListView extends ListView implements OnScrollListener {
private static final int TAP_TO_REFRESH = 1; // 初始狀態
private static final int PULL_TO_REFRESH = 2; //拉動刷新
private static final int RELEASE_TO_REFRESH = 3; //釋放刷新
private static final int REFRESHING = 4; //正在刷新
private static final String TAG = "PullToRefreshListView";
//刷新接口
private OnRefreshListener mOnRefreshListener;
//箭頭圖片
private static int REFRESHICON = R.drawable.goicon;
/**
* listview 滾動監聽器
*/
private OnScrollListener mOnScrollListener;
//視圖索引器
private LayoutInflater mInflater;
/**
* 頭部視圖 內容 — start
*/
private RelativeLayout mRefreshView;
private TextView mRefreshViewText;
private ImageView mRefreshViewImage;
private ProgressBar mRefreshViewProgress;
private TextView mRefreshViewLastUpdated;
/**
* 頭部視圖 內容 — end
*/
//當前listivew 的滾動狀態
private int mCurrentScrollState;
//當前listview 的刷新狀態
private int mRefreshState;
//動畫效果
//變為向下的箭頭
private RotateAnimation mFlipAnimation;
//變為逆向的箭頭
private RotateAnimation mReverseFlipAnimation;
//頭視圖的高度
private int mRefreshViewHeight;
//頭視圖 原始的top padding 屬性值
private int mRefreshOriginalTopPadding;
//
private int mLastMotionY;
//是否反彈
private boolean mBounceHack;
public PullToRefreshListView(Context context) {
super(context);
init(context);
}
public PullToRefreshListView(Context context, AttributeSet attrs) {
super(context, attrs);
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
//初始化動畫
//
mFlipAnimation = new RotateAnimation(0, -180,
RotateAnimation.RELATIVE_TO_SELF, 0.5f,
RotateAnimation.RELATIVE_TO_SELF, 0.5f);
mFlipAnimation.setInterpolator(new LinearInterpolator());
mFlipAnimation.setDuration(250);
mFlipAnimation.setFillAfter(true);
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);
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());
mRefreshOriginalTopPadding = mRefreshView.getPaddingTop();
mRefreshState = TAP_TO_REFRESH;
addHeaderView(mRefreshView);
super.setOnScrollListener(this);
measureView(mRefreshView);
mRefreshViewHeight = mRefreshView.getMeasuredHeight(); //獲取頭文件的測量高度
}
@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) {
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) {
//當前手指的Y值
final int y = (int) event.getY();
//Log.i(TAG, "觸屏的Y值"+y);
mBounceHack = false; //不反彈
switch (event.getAction()) {
case MotionEvent.ACTION_UP:
//將垂直滾動條設置為可用狀態
if (!isVerticalScrollBarEnabled()) {
setVerticalScrollBarEnabled(true);
}
//如果頭部刷新條出現，並且不是正在刷新狀態
if (getFirstVisiblePosition() == 0 && mRefreshState != REFRESHING) {
if ((mRefreshView.getBottom() >= mRefreshViewHeight
|| mRefreshView.getTop() >= 0)
&& mRefreshState == RELEASE_TO_REFRESH) { //如果頭部視圖處於拉離頂部的情況
// Initiate the refresh
mRefreshState = REFRESHING; //將標量設置為，正在刷新
prepareForRefresh(); //準備刷新
onRefresh(); //刷新
} else if (mRefreshView.getBottom() < mRefreshViewHeight
|| mRefreshView.getTop() <= 0) {
// Abort refresh and scroll down below the refresh view
// 停止刷新，並且滾動到頭部刷新視圖的下一個視圖
resetHeader();
setSelection(1); //定位在第二個列表項
}
}
break;
case MotionEvent.ACTION_DOWN:
mLastMotionY = y; //跟蹤手指的Y值
break;
case MotionEvent.ACTION_MOVE:
//更行頭視圖的toppadding 屬性
applyHeaderPadding(event);
break;
}
return super.onTouchEvent(event);
}
/****
* 不斷的頭部的top padding 屬性
* @param ev
*/
private void applyHeaderPadding(MotionEvent ev) {
//獲取累積的動作數
int pointerCount = ev.getHistorySize();
// Log.i(TAG, "獲取累積的動作數"+pointerCount);
for (int p = 0; p < pointerCount; p++) {
if (mRefreshState == RELEASE_TO_REFRESH) { //如果是釋放將要刷新狀態
if (isVerticalFadingEdgeEnabled()) {
setVerticalScrollBarEnabled(false);
}
//歷史累積的高度
int historicalY = (int) ev.getHistoricalY(p);
//Log.i(TAG, "單個動作getHistoricalY值："+historicalY);
// Calculate the padding to apply, we pide 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.
* 使頭部視圖的toppadding 恢復到初始值
*/
private void resetHeaderPadding() {
mRefreshView.setPadding(
mRefreshView.getPaddingLeft(),
mRefreshOriginalTopPadding,
mRefreshView.getPaddingRight(),
mRefreshView.getPaddingBottom());
}
/**
* Resets the header to the original state.
* 初始化頭部視圖 狀態
*/
private void resetHeader() {
if (mRefreshState != TAP_TO_REFRESH) {
mRefreshState = TAP_TO_REFRESH; //初始刷新狀態
//使頭部視圖的toppadding 恢復到初始值
resetHeaderPadding();
// Set refresh view text to the pull label
//將文字初始化
mRefreshViewText.setText(R.string.pull_to_refresh_tap_label);
// Replace refresh drawable with arrow drawable
//設置初始圖片
mRefreshViewImage.setImageResource(REFRESHICON);
// Clear the full rotation animation
// 清除動畫
mRefreshViewImage.clearAnimation();
// Hide progress bar and arrow.
//隱藏頭視圖
mRefreshViewImage.setVisibility(View.GONE);
//隱藏進度條
mRefreshViewProgress.setVisibility(View.GONE);
}
}
//測量視圖的高度
private void measureView(View child) {
//獲取頭部視圖屬性
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) { //如果視圖的高度大於0
childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
} else {
childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);
}
child.measure(childWidthSpec, childHeightSpec);
//不懂MeasureSpec——————————————————————————————
}
/****
* 滑動事件
*/
@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.
if (mCurrentScrollState == SCROLL_STATE_TOUCH_SCROLL //如果是接觸滾動狀態,並且不是正在刷新的狀態
&& mRefreshState != REFRESHING) {
if (firstVisibleItem == 0) { //如果顯示出來瞭第一個列表項
//顯示刷新圖片
mRefreshViewImage.setVisibility(View.VISIBLE);
if ((mRefreshView.getBottom() >= mRefreshViewHeight + 20
|| mRefreshView.getTop() >= 0)
&& mRefreshState != RELEASE_TO_REFRESH) { //如果下拉瞭listiview,則顯示上拉刷新動畫
mRefreshViewText.setText(R.string.pull_to_refresh_release_label);
mRefreshViewImage.clearAnimation();
mRefreshViewImage.startAnimation(mFlipAnimation);
mRefreshState = RELEASE_TO_REFRESH;
Log.i(TAG, "現在處於下拉狀態");
} else if (mRefreshView.getBottom() < mRefreshViewHeight + 20
&& mRefreshState != PULL_TO_REFRESH) { //如果沒有到達，下拉刷新距離，則回歸原來的狀態
mRefreshViewText.setText(R.string.pull_to_refresh_pull_label);
if (mRefreshState != TAP_TO_REFRESH) {
mRefreshViewImage.clearAnimation();
mRefreshViewImage.startAnimation(mReverseFlipAnimation);
Log.i(TAG, "現在處於回彈狀態");
}
mRefreshState = PULL_TO_REFRESH;
}
} else {
mRefreshViewImage.setVisibility(View.GONE); //隱藏刷新圖片
resetHeader(); //初始化，頭部
}
} else if (mCurrentScrollState == SCROLL_STATE_FLING //如果是自己滾動狀態+ 第一個視圖已經顯示+ 不是刷新狀態
&& firstVisibleItem == 0
&& mRefreshState != REFRESHING) {
setSelection(1);
mBounceHack = true; //狀態為回彈
Log.i(TAG, "現在處於自由滾動到頂部的狀態");
} else if (mBounceHack && mCurrentScrollState == SCROLL_STATE_FLING) {
setSelection(1);
Log.i(TAG, "現在處於自由滾動到頂部的狀態");
}
if (mOnScrollListener != null) {
mOnScrollListener.onScroll(view, firstVisibleItem,
visibleItemCount, totalItemCount);
}
}
//滾動狀態改變
@Override
public void onScrollStateChanged(AbsListView view, int scrollState) {
mCurrentScrollState = scrollState;
if (mCurrentScrollState == SCROLL_STATE_IDLE) { //如果滾動停頓
mBounceHack = false;
}
if (mOnScrollListener != null) {
mOnScrollListener.onScrollStateChanged(view, scrollState);
}
}
//準備刷新
public void prepareForRefresh() {
resetHeaderPadding(); //初始化，頭部文件
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, "執行刷新");
if (mOnRefreshListener != null) {
mOnRefreshListener.onRefresh();
}
}
/**
* 刷新完成 的回調函數
* Resets the list to a normal state after a refresh.
* @param lastUpdated Last updated at.
*/
public void onRefreshComplete(CharSequence lastUpdated) {
setLastUpdated(lastUpdated);
onRefreshComplete();
}
/**
* 刷新完成回調函數
* 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(); //重繪視圖
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) {
//準備刷新
prepareForRefresh();
//刷新
onRefresh();
}
}
}
/**
* 刷新方法接口
*/
public interface OnRefreshListener {
public void onRefresh();
}
* 如果你還是沒有弄明白的話，那就點擊下面的鏈接，來下載整個demo項目:
http://download.csdn.net/detail/zjl5211314/3775209
原作者：johannilsson
選自：https://github.com/johannilsson/android-pulltorefresh
