Android自定义控件实战——滚动选择器PickerView

  手机里设置闹钟需要选择时间，那个选择时间的控件就是滚动选择器，前几天用手机刷了MIUI，发现自带的那个时间选择器效果挺好看的，于是就自己仿写了一个，权当练手。先来看效果：

                                                                 

效果还行吧？实现思路就是自定义一个PickerView，单独滚动的是一个PickerView，显然上图中有分和秒的选择所以在布局里用了两个PickerView。由于这里不涉及到text的点击事件，所以只需要继承View就行了，直接把text用canvas画上去。PickerView的实现的主要难点：

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。我用的是变化曲线是抛物线scale=1-ax^2(x<=Height/4),scale = 0(x>Height/4)，a=(4/Height)^2。x就是距离View中心的偏移量。用图片表示如下：

难点2：

     text的居中。绘制text的时候不仅要使其在x方向上居中，还要在y方向上居中，在x方向上比较简单，设置Paint的Align为Align.CENTER就行了，但是y方向上很蛋疼，需要计算text的baseline。

难点3：

    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。

  

     以上就是几个难点，了解了之后可以来看PickerView的代码了：

 package com.jingchen.timerpicker;

 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.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();
         }

     };

     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));
     }

     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);
     }

     @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()
     {
         timer = new Timer();
         mDataList = new ArrayList<String>();
         mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
         mPaint.setStyle(Style.FILL);
         mPaint.setTextAlign(Align.CENTER);
         mPaint.setColor(mColorText);
     }

     @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));

         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)
     {
         if (mTask != null)
         {
             mTask.cancel();
             mTask = null;
         }
         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;
         }
         if (mTask != null)
         {
             mTask.cancel();
             mTask = null;
         }
         mTask = new MyTimerTask(updateHandler);
         timer.schedule(mTask, 0, 10);
     }

     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);
     }
 }

代码里的注释都写的很清楚了。接下来，我们就用写好的PickerView实现文章开头的图片效果吧~

首先看MainActivity的布局：

 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:background="#000000" >

     <RelativeLayout
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_centerInParent="true"
         android:background="#ffffff" >

         <com.jingchen.timerpicker.PickerView
             android:id="@+id/minute_pv"
             android:layout_width="80dp"
             android:layout_height="160dp" />

         <TextView
             android:id="@+id/minute_tv"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:layout_centerVertical="true"
             android:layout_toRightOf="@id/minute_pv"
             android:text="分"
             android:textColor="#ffaa33"
             android:textSize="26sp"
             android:textStyle="bold" />

         <com.jingchen.timerpicker.PickerView
             android:id="@+id/second_pv"
             android:layout_width="80dp"
             android:layout_height="160dp"
             android:layout_toRightOf="@id/minute_tv" />

         <TextView
             android:id="@+id/second_tv"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:layout_centerVertical="true"
             android:layout_toRightOf="@id/second_pv"
             android:text="秒"
             android:textColor="#ffaa33"
             android:textSize="26sp"
             android:textStyle="bold" />
     </RelativeLayout>

 </RelativeLayout>

两个PickerView两个TextView，很简单。

下面是MainActivity的代码：

 package com.jingchen.timerpicker;

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

 import com.jingchen.timerpicker.PickerView.onSelectListener;

 import android.app.Activity;
 import android.os.Bundle;
 import android.view.Menu;
 import android.widget.TextView;
 import android.widget.Toast;

 public class MainActivity extends Activity
 {

     PickerView minute_pv;
     PickerView second_pv;

     @Override
     protected void onCreate(Bundle savedInstanceState)
     {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
         minute_pv = (PickerView) findViewById(R.id.minute_pv);
         second_pv = (PickerView) findViewById(R.id.second_pv);
         List<String> data = new ArrayList<String>();
         List<String> seconds = new ArrayList<String>();
         for (int i = 0; i < 10; i++)
         {
             data.add("0" + i);
         }
         for (int i = 0; i < 60; i++)
         {
             seconds.add(i < 10 ? "0" + i : "" + i);
         }
         minute_pv.setData(data);
         minute_pv.setOnSelectListener(new onSelectListener()
         {

             @Override
             public void onSelect(String text)
             {
                 Toast.makeText(MainActivity.this, "选择了 " + text + " 分",
                         Toast.LENGTH_SHORT).show();
             }
         });
         second_pv.setData(seconds);
         second_pv.setOnSelectListener(new onSelectListener()
         {

             @Override
             public void onSelect(String text)
             {
                 Toast.makeText(MainActivity.this, "选择了 " + text + " 秒",
                         Toast.LENGTH_SHORT).show();
             }
         });
     }

     @Override
     public boolean onCreateOptionsMenu(Menu menu)
     {
         getMenuInflater().inflate(R.menu.main, menu);
         return true;
     }

 }