自定义控件pickView

package com.example.healthembed.util;

import java.util.ArrayList;
import java.util.List;
import java.util.Timer;
import java.util.TimerTask;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.Paint.Align;
import android.graphics.Paint.FontMetricsInt;
import android.graphics.Paint.Style;
import android.os.Handler;
import android.os.Message;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.view.View;

/**
 * 滚动选择器
 *
 * @author chenjing
 *
 */
public class PickerView extends View
{

    public static final String TAG = "PickerView";
    /**
     * text之间间距和minTextSize之比
     */
    public static final float MARGIN_ALPHA = 2.8f;
    /**
     * 自动回滚到中间的速度
     */
    public static final float SPEED = 2;

    private List<String> mDataList;
    /**
     * 选中的位置，这个位置是mDataList的中心位置，一直不变
     */
    private int mCurrentSelected;
    private Paint mPaint;

    private float mMaxTextSize = 80;
    private float mMinTextSize = 40;

    private float mMaxTextAlpha = 255;
    private float mMinTextAlpha = 120;

    private int mColorText = 0x333333;

    private int mViewHeight;
    private int mViewWidth;

    private float mLastDownY;
    /**
     * 滑动的距离
     */
    private float mMoveLen = 0;
    private boolean isInit = false;
    private onSelectListener mSelectListener;
    private Timer timer;
    private MyTimerTask mTask;

    Handler updateHandler = new Handler()
    {

        @Override
        public void handleMessage(Message msg)
        {
            if (Math.abs(mMoveLen) < SPEED)
            {
                mMoveLen = 0;
                if (mTask != null)
                {
                    mTask.cancel();
                    mTask = null;
                    performSelect();
                }
            } else
            {
                // 这里mMoveLen / Math.abs(mMoveLen)是为了保有mMoveLen的正负号，以实现上滚或下滚
                mMoveLen = mMoveLen - mMoveLen / Math.abs(mMoveLen) * SPEED;
            }

            invalidate();//重绘
        }

    };

    public PickerView(Context context)
    {
        super(context);
        init();
    }

    public PickerView(Context context, AttributeSet attrs)
    {
        super(context, attrs);
        init();
    }

    public void setOnSelectListener(onSelectListener listener)
    {
        mSelectListener = listener;
    }

    private void performSelect()
    {
        if (mSelectListener != null)
            mSelectListener.onSelect(mDataList.get(mCurrentSelected));
        //选中的位置，这个位置是mDataList的中心位置，一直不变
    }

    public void setData(List<String> datas)
    {
        mDataList = datas;
        mCurrentSelected = datas.size() / 2;
        invalidate();
    }

    public void setSelected(int selected)
    {
        mCurrentSelected = selected;
    }

    private void moveHeadToTail()
    {
        String head = mDataList.get(0);
        mDataList.remove(0);
        mDataList.add(head);
    }

    private void moveTailToHead()
    {
        String tail = mDataList.get(mDataList.size() - 1);
        mDataList.remove(mDataList.size() - 1);
        mDataList.add(0, tail);
        //Inserts the specified element at the specified position in this list (optional operation)
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)
    {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        mViewHeight = getMeasuredHeight();
        Log.e(TAG,String.valueOf(mViewHeight));
        mViewWidth = getMeasuredWidth();
        Log.e(TAG,String.valueOf(mViewWidth));
        // 按照View的高度计算字体大小
        mMaxTextSize = mViewWidth / 9.0f;
        mMinTextSize = mMaxTextSize / 2f;
        isInit = true;
        invalidate();//调用invalidate()方法，请求重新draw()
    }

    private void init()
    {
        timer = new Timer();
        mDataList = new ArrayList<String>();
        mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);//抗锯齿
        mPaint.setStyle(Style.FILL);
        mPaint.setTextAlign(Align.CENTER);//设置绘制文字的对齐方向
        mPaint.setColor(mColorText);
    }
     public void initDataHigh() {

        for(int j = 0; j<35; j++) {
            moveTailToHead();
        }

    }

    public void initDataLow() {

        for(int j = 0; j<50; j++) {
            moveTailToHead();
        }

    }

    @Override
    protected void onDraw(Canvas canvas)
    {
        super.onDraw(canvas);
        // 根据index绘制view
        if (isInit)
            drawData(canvas);
    }

    private void drawData(Canvas canvas)
    {
        // 先绘制选中的text再往上往下绘制其余的text
        float scale = parabola(mViewHeight / 4.0f, mMoveLen);
        float size = (mMaxTextSize - mMinTextSize) * scale + mMinTextSize;
        mPaint.setTextSize(size);
        mPaint.setAlpha((int) ((mMaxTextAlpha - mMinTextAlpha) * scale + mMinTextAlpha));
        // text居中绘制，注意baseline的计算才能达到居中，y值是text中心坐标
    /*    float x = (float) (mViewWidth / 2.0);
        float y = (float) (mViewHeight / 2.0 + mMoveLen);
        FontMetricsInt fmi = mPaint.getFontMetricsInt();
        float baseline = (float) (y - (fmi.bottom / 2.0 + fmi.top / 2.0));
*/

        float x = (float) (mViewWidth / 2.0+ mMoveLen);
        float y = (float) (mViewHeight / 2.0 );
        FontMetricsInt fmi = mPaint.getFontMetricsInt();
        float baseline = (float) (x - (fmi.bottom+fmi.descent)/2);
        canvas.drawText(mDataList.get(mCurrentSelected), baseline, y, mPaint);
        // 绘制上方data
        for (int i = 1; (mCurrentSelected - i) >= 0; i++)
        {
            drawOtherText(canvas, i, -1);
            Log.e(TAG,String.valueOf(mCurrentSelected));
        }
        // 绘制下方data
        for (int i = 1; (mCurrentSelected + i) < mDataList.size(); i++)
        {
            drawOtherText(canvas, i, 1);
        }

    }

    /**
     * @param canvas
     * @param position
     *            距离mCurrentSelected的差值
     * @param type
     *            1表示向下绘制，-1表示向上绘制
     */
    private void drawOtherText(Canvas canvas, int position, int type)
    {
        float d = (float) (MARGIN_ALPHA * mMinTextSize * position + type
                * mMoveLen);
        float scale = parabola(mViewHeight / 4.0f, d);
        float size = (mMaxTextSize - mMinTextSize) * scale + mMinTextSize;
        mPaint.setTextSize(size);
        mPaint.setAlpha((int) ((mMaxTextAlpha - mMinTextAlpha) * scale + mMinTextAlpha));
        /*float y = (float) (mViewHeight / 2.0 + type * d);*/
        float x = (float) (mViewWidth/ 2.0 + type * d);
        FontMetricsInt fmi = mPaint.getFontMetricsInt();
        float baseline = (float) (x - (fmi.bottom+fmi.descent));
        canvas.drawText(mDataList.get(mCurrentSelected + type * position),
                baseline,(float) (mViewHeight / 2.0),  mPaint);
    }

    /**
     * 抛物线
     *
     * @param zero
     *            零点坐标
     * @param x
     *            偏移量
     * @return scale
     */
    private float parabola(float zero, float x)
    {
        float f = (float) (1 - Math.pow(x / zero, 2));//x的y次方
        return f < 0 ? 0 : f;
    }

    @Override
    public boolean onTouchEvent(MotionEvent event)
    {
        switch (event.getActionMasked())
        {
        case MotionEvent.ACTION_DOWN://表示用户开始触摸
            doDown(event);
            break;
        case MotionEvent.ACTION_MOVE:
            doMove(event);
            break;
        case MotionEvent.ACTION_UP://表示用户抬起了手指
            doUp(event);
            break;
        }
        return true;
    }

    private void doDown(MotionEvent event)
    {
        if (mTask != null)
        {
            mTask.cancel();
            mTask = null;
        }
        mLastDownY = event.getX();
    }

    private void doMove(MotionEvent event)
    {

        mMoveLen += (event.getX() - mLastDownY);

        //mMoveLen为正为向下滑
        if (mMoveLen > MARGIN_ALPHA * mMinTextSize / 2)
        {
            // 往下滑超过离开距离
            moveTailToHead();
            mMoveLen = mMoveLen - MARGIN_ALPHA * mMinTextSize;
        } else if (mMoveLen < -MARGIN_ALPHA * mMinTextSize / 2)
        {
            // 往上滑超过离开距离
            moveHeadToTail();
            mMoveLen = mMoveLen + MARGIN_ALPHA * mMinTextSize;
        }

        mLastDownY = event.getX();
        invalidate();
    }

    /*
        task - task to be scheduled.
        firstTime - First time at which task is to be executed.
        period - time in milliseconds between successive task executions.
    */
    private void doUp(MotionEvent event)
    {
        // 抬起手后mCurrentSelected的位置由当前位置move到中间选中位置
        if (Math.abs(mMoveLen) < 0.0001)
        {
            mMoveLen = 0;
            return;
        }
        if (mTask != null)
        {
            mTask.cancel();
            mTask = null;
        }
        mTask = new MyTimerTask(updateHandler);
        timer.schedule(mTask, 0, 1000);
        //第三个参数,第一次调用之后，从第二次开始每隔多长的时间调用一次 run() 方法
    }

    class MyTimerTask extends TimerTask
    {
        Handler handler;

        public MyTimerTask(Handler handler)
        {
            this.handler = handler;
        }

        @Override
        public void run()
        {
            handler.sendMessage(handler.obtainMessage());
        }

    }

    public interface onSelectListener
    {
        void onSelect(String text);
    }
}

难点1：

字体随距离的渐变。可以看到，text随离中心位置的距离变化而变化，这里变化的是透明度alpha和字体大小TexSize，这两个值我都设置了Max和Min值，通过其与中心点的距离计算scale。

难点2：

循环滚动。为了解决循环滚动的问题我把存放text的List从中间往上下摊开，通过不断地moveHeadToTail和moveTailToHead使选中的text始终是list的中间position的值。

效果图：