前言：像这类的自定义控件有非常多的开源项目，但还是没有找到我项目想要的，所以简单实现了一个，下面简单讲讲实现原理。

效果图：

实现思路：

首先画固定背景尺子，而实现这个则要计算刻度线的宽度、刻度线间的距离，以及要确定刻度线的总是，根据这些可以求出第一条刻度线的x坐标，使得整个尺子居中；下图为尺子尺寸的计算方法：

贴上关键代码：

/** * 画固定的尺子 * @param canvas */ private void drawLine(Canvas canvas) { canvas.save(); int height = mHeight; int drawCount = 0;//已经画了刻度线的个数 float xPosition; for(int i=0; drawCount<=mMaxLineCount; i++){ xPosition = (mLineDivider*mDensity + mLineWidth)*drawCount + mLeftWidth; if(i%5 == 0 && i%10 != 0){//刻度为5的倍数，但同时不是10的倍数 canvas.drawLine(xPosition,height*0.85f-mPaddingBottom,xPosition,height*0.15f+mPaddingTop,mLinePaint); }else if(i%10 == 0){//刻度为10的倍数 canvas.drawLine(xPosition,height-mPaddingBottom,xPosition,mPaddingTop,mLinePaint); }else {//普通的刻度 canvas.drawLine(xPosition,height*0.75f-mPaddingBottom,xPosition,height*0.25f+mPaddingTop,mLinePaint); } drawCount++; } canvas.restore(); }

然后画出可以拖动的刻度线（首图粉红色线），要实现该功能其实不难，第一种情况：通过在onTouch里面获取event.getX()坐标，而在这其中用到PointF类来保存x坐标，然后根据x坐标在onDraw()里面绘制即可；第二种情况：自动搜台，这其实很简单，开启子线程每Thread.sleep(200)就累加一定x值即可实现；

最后通过回调把计算好的值传递到Activity中，任务完成！

要是不太清楚回调原理的可看我另外一篇博客：Android回调与观察者模式的实现原理

下面贴上View的源码：

package com.xhunmon.radiorule;import android.content.Context;import android.content.res.TypedArray;import android.graphics.Canvas;import android.graphics.Paint;import android.graphics.PointF;import android.util.AttributeSet;import android.util.SparseArray;import android.view.MotionEvent;import android.view.View;/** * user: uidq0530 ,date: 2017-04-16. * description：收音机FM搜台尺子 * * @author xhunmon */public class RadioRulerView extends View { private static final String tag = "RadioRulerView"; private int mHeight; //view的高度 private int mWidth; //view的宽度 private Paint mLinePaint; //固定的尺子画笔 private int mLineWidth;//尺子刻度线的宽 private int mLineColor;//固定尺子刻度线的颜色 private int mMoveLineColor;//移动尺子刻度线的颜色 private float mDensity; private int mLineDivider; //两条刻度线间的距离 private float mLeftWidth; //尺子离view左边的距离 private int mMaxLineCount = 220; //总共要画多少条刻度 private Paint mMoveLinePaint; //移动尺子的画笔 private int mValue; //尺子被选中的值 private float mMaxX; //onTouch中能触摸的最大x值 private float mMinX; //onTouch中能触摸的最小x值 private OnValueChangeListener mListener; private SparseArray<PointF> activePointers; private PointF xPoint; private int mPaddingBottom; private int mPaddingTop; private boolean mIsAuto = false; public RadioRulerView(Context context) { this(context,null); } public RadioRulerView(Context context, AttributeSet attrs) { this(context, attrs,0); } public RadioRulerView(Context context, AttributeSet attrs, int defStyleAttr) { this(context, attrs, defStyleAttr,0); } public RadioRulerView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) { super(context, attrs, defStyleAttr, defStyleRes); mDensity = context.getResources().getDisplayMetrics().density; TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.RadioRulerView, defStyleAttr, defStyleRes); mLineWidth = (int) a.getDimension(R.styleable.RadioRulerView_line_width,5*mDensity); mLineDivider = (int) a.getDimension(R.styleable.RadioRulerView_line_divider,15*mDensity); mLineColor = a.getColor(R.styleable.RadioRulerView_line_color,0xff888888); mMoveLineColor = a.getColor(R.styleable.RadioRulerView_move_line_color,0xffff0000); a.recycle(); init(); } private void init() { activePointers = new SparseArray<>(); mLinePaint = new Paint(); mLinePaint.setAntiAlias(true); mLinePaint.setColor(mLineColor); mLinePaint.setStrokeWidth(mLineWidth); mLinePaint.setStyle(Paint.Style.STROKE); mMoveLinePaint = new Paint(); mMoveLinePaint.setAntiAlias(true); mMoveLinePaint.setColor(mMoveLineColor); mMoveLinePaint.setStrokeWidth(mLineWidth); mMoveLinePaint.setStyle(Paint.Style.STROKE); } //此方法在view的尺寸确定后调用 @Override protected void onSizeChanged(int w, int h, int oldw, int oldh) { super.onSizeChanged(w, h, oldw, oldh); mHeight = getHeight(); mWidth = getWidth(); mPaddingBottom = getPaddingBottom(); mPaddingTop = getPaddingTop(); mLeftWidth = (mWidth - mMaxLineCount*(mLineWidth +mLineDivider))/2; mMaxX = mMaxLineCount*(mLineWidth +mLineDivider) + mLeftWidth; mMinX = mLeftWidth; } @Override protected void onDraw(Canvas canvas) { super.onDraw(canvas); drawLine(canvas); drawMoveLine(canvas); } /** * 画固定的尺子 * @param canvas */ private void drawLine(Canvas canvas) { canvas.save(); int height = mHeight; int drawCount = 0;//已经画了刻度线的个数 float xPosition; for(int i=0; drawCount<=mMaxLineCount; i++){ xPosition = (mLineDivider*mDensity + mLineWidth)*drawCount + mLeftWidth; if(i%5 == 0 && i%10 != 0){//刻度为5的倍数，但同时不是10的倍数 canvas.drawLine(xPosition,height*0.85f-mPaddingBottom,xPosition,height*0.15f+mPaddingTop,mLinePaint); }else if(i%10 == 0){//刻度为10的倍数 canvas.drawLine(xPosition,height-mPaddingBottom,xPosition,mPaddingTop,mLinePaint); }else {//普通的刻度 canvas.drawLine(xPosition,height*0.75f-mPaddingBottom,xPosition,height*0.25f+mPaddingTop,mLinePaint); } drawCount++; } canvas.restore(); } /** * 搜索FM频道的刻度线 * @param canvas */ private void drawMoveLine(Canvas canvas) { canvas.save(); xPoint = activePointers.valueAt(0); if (xPoint != null) { canvas.drawLine(xPoint.x,mHeight-mPaddingBottom, xPoint.x,mPaddingTop,mMoveLinePaint); setValue(eventXValue(xPoint.x)); }else { canvas.drawLine(mMinX,mHeight-mPaddingBottom, mMinX,mPaddingTop,mMoveLinePaint); } canvas.restore(); } @Override public boolean onTouchEvent(MotionEvent event) { int pointerIndex = event.getActionIndex(); int pointerId = event.getPointerId(pointerIndex); switch (event.getActionMasked()) { case MotionEvent.ACTION_DOWN: case MotionEvent.ACTION_POINTER_DOWN: { float downX = event.getX(pointerIndex); if(downX > mMaxX || downX < mMinX) break; PointF position = new PointF(downX, event.getY(pointerIndex)); activePointers.put(pointerId, position); break; } case MotionEvent.ACTION_MOVE: { int pointerCount = event.getPointerCount(); for (int i = 0; i < pointerCount; i++) { PointF point = activePointers.get(event.getPointerId(i)); if (point == null) continue; float moveX = event.getX(i); if(moveX > mMaxX || moveX < mMinX) break; point.x = event.getX(i); point.y = event.getY(i); } break; } case MotionEvent.ACTION_UP: case MotionEvent.ACTION_POINTER_UP: case MotionEvent.ACTION_CANCEL: { int pointerCount = event.getPointerCount(); PointF point = activePointers.get(event.getPointerId(pointerCount-1)); if (point == null) break; float upX = event.getX(pointerCount-1); if(upX > mMaxX || upX < mMinX) break; point.x = eventXValue(event.getX(pointerCount-1)); point.y = event.getY(pointerCount-1); break; } } invalidate(); return true; } /** *作用：使得放手后MoveLine和Line重合；精确mValue * @param x onTouch中的event.getX() * @return */ public int eventXValue(float x){ mLineDivider = (int) (mLineDivider*mDensity); return (int) ((x-mLeftWidth)%(mLineWidth +mLineDivider)>((mLineWidth +mLineDivider)/2) ? (((mLineWidth +mLineDivider)*((int)((x-mLeftWidth)/(mLineWidth +mLineDivider))+1))+mLeftWidth) : (((mLineWidth +mLineDivider)*((int)((x-mLeftWidth)/(mLineWidth +mLineDivider))))+mLeftWidth)); } /** * 设置最大刻度线个数 * @param count */ public void setMaxLineCount(int count) { mMaxLineCount = count; } /** * 设置是否启用自动搜索功能 * @param isAuto */ public void setAutoSearchFM(boolean isAuto){ this.mIsAuto = isAuto; } /** * 开始自动搜台 */ public void startAutoSeachFM(){ if(mIsAuto) new Thread(new SeachThread()).start(); } /** * 搜台要在开启子线程 */ private class SeachThread implements Runnable{ @Override public void run() { while(mIsAuto){ xPoint = activePointers.valueAt(0); if(xPoint != null){ xPoint.x += (mLineWidth + mLineDivider); if(xPoint.x > mMaxX) xPoint.x = mLeftWidth; }else { PointF position = new PointF(mLeftWidth, mHeight); activePointers.put(0, position); } try { Thread.sleep(200); } catch (InterruptedException e) { e.printStackTrace(); } postInvalidate(); } } } /*****************************值传递的回调*************************************/ public interface OnValueChangeListener { void onValueChange(float value); } public void setOnValueChangeListener(OnValueChangeListener listener){ mListener = listener; } private void setValue(float value) { if(mListener != null){ mValue = (int) ((value - mLeftWidth)/(mLineDivider*mDensity + mLineWidth)); //FM的范围从88.0 ~ 108.0 mListener.onValueChange(mValue/10f + 88); } } /******************************************************************/}

贴上Activity代码：

package com.xhunmon.radiorule;import android.app.Activity;import android.os.Bundle;import android.view.View;import android.widget.Button;import android.widget.CheckBox;import android.widget.CompoundButton;import android.widget.TextView;public class MainActivity extends Activity implements RadioRulerView.OnValueChangeListener, CompoundButton.OnCheckedChangeListener, View.OnClickListener { private TextView mShow; private RadioRulerView mRule; private CheckBox mCbAuto; private Button mBtStart; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); mShow = (TextView) findViewById(R.id.tv); mRule = (RadioRulerView) findViewById(R.id.rule); mCbAuto = (CheckBox) findViewById(R.id.cb_auto); mBtStart = (Button) findViewById(R.id.bt_start); mRule.setMaxLineCount(200);//FM从88.0 ~ 108.0总共有200频道 mRule.setOnValueChangeListener(this); mCbAuto.setOnCheckedChangeListener(this); mBtStart.setOnClickListener(this); } @Override public void onValueChange(float value) { mShow.setText("FM："+value); } @Override public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) { if(isChecked){ mRule.setAutoSearchFM(true); }else { mRule.setAutoSearchFM(false); } } @Override public void onClick(View v) { if(v.getId() == R.id.bt_start){ mRule.startAutoSeachFM(); } }}

整个项目都放在github上面了，欢迎做客与讨论：
https://github.com/xhunmon/RadioRule

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。