(转)android UI进阶之实现listview的下拉加载

 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。