Android自定义控件实战——滚动选择器PickerView续

最近做项目，遇到一个需要实现滚动选择的需求。以前做过一个自定义实现滚动选择的view(https://blog.csdn.net/u014763302/article/details/53104814)，但发现不大符合需求。本着快速实现的目标，就找到了这为大神的博客（https://blog.csdn.net/zhongkejingwang/article/details/38513301），看了下实现的差不多了，就拿过来用，顺便加了一些小功能，并对存在的bug做了一些优化，特记录。

目前需求：

• 上下可循环滚动选择
• 点击按钮自动上下滚动一格

效果如下：

思路讲解：

由于代码本身是基于上面博文的基础上来的，逻辑大部分还是没变，需要细节的请移步开头的链接，博文中思路细节大神讲的很明白。
说下我添加的功能及优化的地方：

• 点击按钮自动滚动一格：原来的滚动实现是基于触摸事件+记录滑动距离，因此自动滚动也基于一样的逻辑：主动调用触摸方法模拟触摸事件。
• 原博文存在瞬滑时回滚很多造成不友好现象，修复是在ACTION_UP时通过计算矫正滑动位置和data item的对应关系。

下面上代码。

package com.example.wheelview;

import android.animation.Animator;
import android.animation.AnimatorListenerAdapter;
import android.animation.ValueAnimator;
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.view.MotionEvent;
import android.view.View;
import android.view.animation.LinearInterpolator;

import java.util.ArrayList;
import java.util.List;

/**
 * 滚动选择器,https://blog.csdn.net/zhongkejingwang/article/details/38513301
 *
 * @author huanghp
 */
public class PickerView extends View {
    public static final String TAG = "PickerView";
    //minTextSize和text之间间距之比
    public static final float MARGIN_ALPHA = 1.56f;
    //自动回滚到中间的速度
    public static final byte SPEED = 2;
    public static final byte PERIOD = 10;
    public static final byte CODE = 0X10;
    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 = 48;
    private float mMinTextSize = 32;

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

    private int mColorText = 0xffffff;

    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 volatile boolean isInAnimate;
    private Handler updateHandler = new Handler() {

        @Override
        public void handleMessage(Message msg) {
            if (Math.abs(mMoveLen) < SPEED) {
                mMoveLen = 0;
                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;
                sendEmptyMessageDelayed(CODE, PERIOD);
            }
            invalidate();
        }
    };

    public PickerView(Context context) {
        this(context, null);
    }

    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));
    }

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

    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);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        mViewHeight = getMeasuredHeight();
        mViewWidth = getMeasuredWidth();
        // 按照View的高度计算字体大小
//        mMaxTextSize = mViewHeight / 4.0f;
//        mMinTextSize = mMaxTextSize / 2f;
        isInit = true;
        invalidate();
    }

    private void init() {
        mDataList = new ArrayList<>();
        mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
        mPaint.setStyle(Style.FILL);
        mPaint.setTextAlign(Align.CENTER);
        mPaint.setColor(mColorText);
//        mPaint.setTypeface(Typeface.create("eurostile", Typeface.NORMAL));
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        if (isInit) drawData(canvas);
    }

    private void drawData(Canvas canvas) {
        if (mDataList.isEmpty()) return;
        // 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));//这里是经过简化后的公式
        canvas.drawText(mDataList.get(mCurrentSelected), x, baseline, mPaint);
        // 绘制上方data
        for (int i = 1; (mCurrentSelected - i) >= 0; i++) {
            drawOtherText(canvas, i, -1);
        }
        // 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);
        FontMetricsInt fmi = mPaint.getFontMetricsInt();
        float baseline = (float) (y - (fmi.bottom / 2.0 + fmi.top / 2.0));
        canvas.drawText(mDataList.get(mCurrentSelected + type * position), (float) (mViewWidth / 2.0), baseline, 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));
        return f < 0 ? 0 : f;
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        switch (event.getActionMasked()) {
            case MotionEvent.ACTION_DOWN:
                doDown(event);
                break;
            case MotionEvent.ACTION_MOVE:
                doMove(event);
                break;
            case MotionEvent.ACTION_UP:
                doUp(event);
                break;
        }
        return true;
    }

    private void doDown(MotionEvent event) {
        updateHandler.removeMessages(CODE);
        mLastDownY = event.getY();
    }

    private void doMove(MotionEvent event) {
        mMoveLen += (event.getY() - mLastDownY);
        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.getY();
        invalidate();
    }

    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;
        }
        //这段是为了修正瞬滑导致回滚较多现象，屏蔽亦可
        if (Math.abs(mMoveLen) > MARGIN_ALPHA * mMinTextSize / 2) {
            int m = (int) (Math.abs(mMoveLen) / (MARGIN_ALPHA * mMinTextSize / 2));
            for (int i = 0; i < m; i++) {
                if (mMoveLen > 0) moveTailToHead();
                else moveHeadToTail();
            }
            if (mMoveLen > 0) mMoveLen -= MARGIN_ALPHA * mMinTextSize * m / 2;
            else mMoveLen += MARGIN_ALPHA * mMinTextSize * m / 2;
        }
        updateHandler.removeMessages(CODE);
        updateHandler.sendEmptyMessage(CODE);
    }

    /**
     * @param up 是否向上滑动
     */
    public void autoScroll(boolean up) {
        if (isInAnimate) return;
        isInAnimate = true;
        final float[] values = up ? new float[]{MARGIN_ALPHA * mMinTextSize + 1, 0} : new float[]{0, MARGIN_ALPHA * mMinTextSize + 1};
        ValueAnimator va = ValueAnimator.ofFloat(values);
        va.setDuration(100);
        va.setInterpolator(new LinearInterpolator());
        va.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
            @Override
            public void onAnimationUpdate(ValueAnimator animation) {
                float v = (float) animation.getAnimatedValue();
                doMove(MotionEvent.obtain(System.currentTimeMillis(), 0, 0, 0f, v, 0));
            }
        });
        va.addListener(new AnimatorListenerAdapter() {
            @Override
            public void onAnimationEnd(Animator animation) {
                doUp(null);
                isInAnimate = false;
            }

            @Override
            public void onAnimationStart(Animator animation) {
                doDown(MotionEvent.obtain(System.currentTimeMillis(), 0, 0, 0f, values[0], 0));
            }
        });
        va.start();
    }

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

谢谢！

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