android 下拉 支持多选

关于listview的操作五花八门，有下拉刷新，分级显示，分页列表，逐页加载等，以后会陆续和大家分享这些技术，今天讲下下拉加载这个功能的实现。

最初的下拉加载应该是ios上的效果，现在很多应用如新浪微博等都加入了这个操作。即下拉listview刷新列表，这无疑是一个非常友好的操作。今天就和大家分享下这个操作的实现。

先看下运行效果：

代码参考国外朋友Johan Nilsson的实现，http://johannilsson.com/2011/03/13/android-pull-to-refresh-update.html。

主要原理为监听触摸和滑动操作，在listview头部加载一个视图。那要做的其实很简单：1.写好加载到listview头部的view 2.重写listview，实现onTouchEvent方法和onScroll方法，监听滑动状态。计算headview全部显示出来即可实行加载动作，加载完成即刷新列表。重新隐藏headview。

首先写下headview的xml代码：

<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="?android:attr/progressBarStyleSmall" /> <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/ic_pulltorefresh_arrow" /> <TextView android:id="@+id/pull_to_refresh_text" 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>

代码比较简单，即headview包括一个进度条一个箭头和两段文字（一个显示加载状态，另一个显示最后刷新时间，本例就不设置了）。

而后重写listview，代码如下：

package com.notice.pullrefresh; import android.content.Context; import android.util.AttributeSet; 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.AbsListView.OnScrollListener; import android.widget.ImageView; import android.widget.ListAdapter; import android.widget.ListView; import android.widget.ProgressBar; import android.widget.RelativeLayout; import android.widget.TextView; public class PullToRefreshListView extends ListView implements OnScrollListener { // 状态 private static final int TAP_TO_REFRESH = 1; private static final int PULL_TO_REFRESH = 2; private static final int RELEASE_TO_REFRESH = 3; private static final int REFRESHING = 4; private OnRefreshListener mOnRefreshListener; // 监听对listview的滑动动作 private OnScrollListener mOnScrollListener; private LayoutInflater mInflater; //顶部刷新时出现的控件 private RelativeLayout mRefreshView; private TextView mRefreshViewText; private ImageView mRefreshViewImage; private ProgressBar mRefreshViewProgress; private TextView mRefreshViewLastUpdated; // 当前滑动状态 private int mCurrentScrollState; // 当前刷新状态 private int mRefreshState; // 箭头动画效果 private RotateAnimation mFlipAnimation; private RotateAnimation mReverseFlipAnimation; private int mRefreshViewHeight; private int mRefreshOriginalTopPadding; private int mLastMotionY; 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); } /** * 初始化控件和箭头动画（这里直接在代码中初始化动画而不是通过xml） */ private void init(Context context) { 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); 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); mRefreshOriginalTopPadding = mRefreshView.getPaddingTop(); mRefreshState = TAP_TO_REFRESH; //为listview头部增加一个view addHeaderView(mRefreshView); super.setOnScrollListener(this); measureView(mRefreshView); mRefreshViewHeight = mRefreshView.getMeasuredHeight(); } @Override protected void onAttachedToWindow() { setSelection(1); } @Override public void setAdapter(ListAdapter adapter) { super.setAdapter(adapter); setSelection(1); } /** * 设置滑动监听器 * */ @Override public void setOnScrollListener(AbsListView.OnScrollListener l) { mOnScrollListener = l; } /** * 注册一个list需要刷新时的回调接口 * */ public void setOnRefreshListener(OnRefreshListener onRefreshListener) { mOnRefreshListener = onRefreshListener; } /** * 设置标签显示何时最后被刷新 * * @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); } } // 实现该方法处理触摸 @Override public boolean onTouchEvent(MotionEvent event) { final int y = (int) event.getY(); mBounceHack = false; switch (event.getAction()) { case MotionEvent.ACTION_UP: if (!isVerticalScrollBarEnabled()) { setVerticalScrollBarEnabled(true); } if (getFirstVisiblePosition() == 0 && mRefreshState != REFRESHING) { // 拖动距离达到刷新需要 if ((mRefreshView.getBottom() >= mRefreshViewHeight || mRefreshView.getTop() >= 0) && mRefreshState == RELEASE_TO_REFRESH) { // 把状态设置为正在刷新 mRefreshState = REFRESHING; // 准备刷新 prepareForRefresh(); // 刷新 onRefresh(); } else if (mRefreshView.getBottom() < mRefreshViewHeight || mRefreshView.getTop() <= 0) { // 中止刷新 resetHeader(); setSelection(1); } } break; case MotionEvent.ACTION_DOWN: // 获得按下y轴位置 mLastMotionY = y; break; case MotionEvent.ACTION_MOVE: // 计算边距 applyHeaderPadding(event); break; } return super.onTouchEvent(event); } // 获得header的边距 private void applyHeaderPadding(MotionEvent ev) { int pointerCount = ev.getHistorySize(); for (int p = 0; p < pointerCount; p++) { if (mRefreshState == RELEASE_TO_REFRESH) { if (isVerticalFadingEdgeEnabled()) { setVerticalScrollBarEnabled(false); } int historicalY = (int) ev.getHistoricalY(p); // 计算申请的边距，除以1.7使得拉动效果更好 int topPadding = (int) (((historicalY - mLastMotionY) - mRefreshViewHeight) / 1.7); mRefreshView.setPadding( mRefreshView.getPaddingLeft(), topPadding, mRefreshView.getPaddingRight(), mRefreshView.getPaddingBottom()); } } } /** * 将head的边距重置为初始的数值 */ private void resetHeaderPadding() { mRefreshView.setPadding( mRefreshView.getPaddingLeft(), mRefreshOriginalTopPadding, mRefreshView.getPaddingRight(), mRefreshView.getPaddingBottom()); } /** * 重置header为之前的状态 */ private void resetHeader() { if (mRefreshState != TAP_TO_REFRESH) { mRefreshState = TAP_TO_REFRESH; resetHeaderPadding(); // 将刷新图标换成箭头 mRefreshViewImage.setImageResource(R.drawable.ic_pulltorefresh_arrow); // 清除动画 mRefreshViewImage.clearAnimation(); // 隐藏图标和进度条 mRefreshViewImage.setVisibility(View.GONE); mRefreshViewProgress.setVisibility(View.GONE); } } // 估算headview的width和height private void measureView(View child) { 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; if (lpHeight > 0) { childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY); } else { childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED); } child.measure(childWidthSpec, childHeightSpec); } @Override public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) { // 在refreshview完全可见时，设置文字为松开刷新，同时翻转箭头 if (mCurrentScrollState == SCROLL_STATE_TOUCH_SCROLL && mRefreshState != REFRESHING) { if (firstVisibleItem == 0) { mRefreshViewImage.setVisibility(View.VISIBLE); if ((mRefreshView.getBottom() >= mRefreshViewHeight + 20 || mRefreshView.getTop() >= 0) && mRefreshState != RELEASE_TO_REFRESH) { mRefreshViewText.setText("松开加载..."); mRefreshViewImage.clearAnimation(); mRefreshViewImage.startAnimation(mFlipAnimation); mRefreshState = RELEASE_TO_REFRESH; } else if (mRefreshView.getBottom() < mRefreshViewHeight + 20 && mRefreshState != PULL_TO_REFRESH) { mRefreshViewText.setText("下拉刷新..."); if (mRefreshState != TAP_TO_REFRESH) { mRefreshViewImage.clearAnimation(); mRefreshViewImage.startAnimation(mReverseFlipAnimation); } mRefreshState = PULL_TO_REFRESH; } } else { mRefreshViewImage.setVisibility(View.GONE); resetHeader(); } } else if (mCurrentScrollState == SCROLL_STATE_FLING && firstVisibleItem == 0 && mRefreshState != REFRESHING) { setSelection(1); mBounceHack = true; } else if (mBounceHack && mCurrentScrollState == SCROLL_STATE_FLING) { setSelection(1); } if (mOnScrollListener != null) { mOnScrollListener.onScroll(view, firstVisibleItem, visibleItemCount, totalItemCount); } } @Override public void onScrollStateChanged(AbsListView view, int scrollState) { mCurrentScrollState = scrollState; if (mCurrentScrollState == SCROLL_STATE_IDLE) { mBounceHack = false; } if (mOnScrollListener != null) { mOnScrollListener.onScrollStateChanged(view, scrollState); } } public void prepareForRefresh() { resetHeaderPadding();// 恢复header的边距 mRefreshViewImage.setVisibility(View.GONE); // 注意加上，否则仍然显示之前的图片 mRefreshViewImage.setImageDrawable(null); mRefreshViewProgress.setVisibility(View.VISIBLE); // 设置文字 mRefreshViewText.setText("加载中..."); mRefreshState = REFRESHING; } public void onRefresh() { if (mOnRefreshListener != null) { mOnRefreshListener.onRefresh(); } } /** * 重置listview为普通的listview，该方法设置最后更新时间 * * @param lastUpdated * Last updated at. */ public void onRefreshComplete(CharSequence lastUpdated) { setLastUpdated(lastUpdated); onRefreshComplete(); } /** * 重置listview为普通的listview，不设置最后更新时间 */ public void onRefreshComplete() { resetHeader(); // 如果refreshview在加载结束后可见，下滑到下一个条目 if (mRefreshView.getBottom() > 0) { invalidateViews(); setSelection(1); } } /** * 刷新监听器接口 */ public interface OnRefreshListener { /** * list需要被刷新时调用 */ public void onRefresh(); } }

相信我注释已经写的比较详细了，主要注意onTouchEvent和onScroll方法，在这里面计算头部边距，从而通过用户的手势实现“下拉刷新”到“松开加载”以及“加载”三个状态的切换。其中还有一系列和header有关的方法，用来设置header的显示以及取得header的边距。于此同时，代码留出了接口以供调用。

那么现在写一个测试Activity来试验下效果：

package com.notice.pullrefresh; import java.util.Arrays; import java.util.LinkedList; import android.app.ListActivity; import android.os.AsyncTask; import android.os.Bundle; import android.widget.ArrayAdapter; import com.notice.pullrefresh.PullToRefreshListView.OnRefreshListener; public class PullrefreshActivity extends ListActivity { private LinkedList<String> mListItems; ArrayAdapter<String> adapter; /** Called when the activity is first created. */ @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.pull_to_refresh); // list需要刷新时调用 ((PullToRefreshListView) getListView()) .setOnRefreshListener(new OnRefreshListener() { @Override public void onRefresh() { // 在这执行后台工作 new GetDataTask().execute(); } }); mListItems = new LinkedList<String>(); mListItems.addAll(Arrays.asList(mStrings)); adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, mListItems); setListAdapter(adapter); } private class GetDataTask extends AsyncTask<Void, Void, String[]> { @Override protected String[] doInBackground(Void... params) { // 在这里可以做一些后台工作 try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } return mStrings; } @Override protected void onPostExecute(String[] result) { // 下拉后增加的内容 mListItems.addFirst("Added after refresh..."); // 刷新完成调用该方法复位 ((PullToRefreshListView) getListView()).onRefreshComplete(); super.onPostExecute(result); } } private String[] mStrings = { "normal data1", "normal data2", "nomal data3", "normal data4", "norma data5", "normal data6" }; }

代码通过asyncTask实现一个异步操作，并通过设置onRefreshListener监听器调用onRefresh方法实现下拉时刷新，并在刷新完成后调用onRefreshComplete做复位处理。

今天就和大家分享这些，有问题欢迎留言交流。