Android实现简单时钟View的方法

通过Canvas的平移与旋转简化绘图逻辑是一个非常有用的技巧，下面的时钟view就是利用这个方法完成的，省去了使用三角函数计算坐标的麻烦。

package com.example.swt369.simpleclock;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.support.annotation.Nullable;
import android.util.AttributeSet;
import android.view.View;
import android.view.ViewTreeObserver;
import android.widget.Toast;

import java.util.Calendar;

/**
 * Created by swt369 on 2017/8/20.
 */

public class ClockView extends View {
 //时钟各个部件的长度
 private float mScaleLengthLong;
 private float mScaleLengthShort;
 private float mTickLengthHour;
 private float mTickLengthMinute;
 private float mTickLengthSecond;
 //view的宽和高
 private float mWidth;
 private float mHeight;
 //时钟半径
 private float mRadius;

 //绘制各部件时用的Paint
 private Paint mPaintScaleLong;
 private Paint mPaintScaleShort;
 private Paint mPaintOutline;
 private Paint mPaintNum;
 private Paint mPaintTickHour;
 private Paint mPaintTickMinute;
 private Paint mPaintTickSecond;

 private ViewTreeObserver.OnPreDrawListener onPreDrawListener;
 public ClockView(final Context context, @Nullable AttributeSet attrs) {
  super(context, attrs);

  onPreDrawListener = new ViewTreeObserver.OnPreDrawListener() {
   //获取view宽高并计算各个部件的长度
   @Override
   public boolean onPreDraw() {
    mWidth = getMeasuredWidth();
    mHeight = getMeasuredHeight();
    mRadius = Math.min(mWidth,mHeight) / 2 * 0.95f;
    mScaleLengthLong = mRadius * 0.1f;
    mScaleLengthShort = mRadius * 0.05f;
    mTickLengthHour = mRadius * 0.3f;
    mTickLengthMinute = mRadius * 0.45f;
    mTickLengthSecond = mRadius * 0.6f;
    return true;
   }
  };
  getViewTreeObserver().addOnPreDrawListener(onPreDrawListener);

  //点击显示具体时间
  setOnClickListener(new OnClickListener() {
   @Override
   public void onClick(View v) {
    Calendar calendar = Calendar.getInstance();
    String time = String.format("当前时间：%02d:%02d:%02d",
      calendar.get(Calendar.HOUR_OF_DAY),calendar.get(Calendar.MINUTE),calendar.get(Calendar.SECOND));
    Toast.makeText(context,time, Toast.LENGTH_SHORT).show();
   }
  });

  //初始化所有Paint对象
  initializePaints();
 }

 @Override
 protected void onDraw(Canvas canvas) {
  if(mWidth == 0 || mHeight == 0){
   return;
  }
  if(onPreDrawListener != null){
   getViewTreeObserver().removeOnPreDrawListener(onPreDrawListener);
   onPreDrawListener = null;
  }
  //绘制时钟
  drawClock(canvas);
  //一秒后重绘
  postInvalidateDelayed(1000);
 }

 private void drawClock(Canvas canvas) {
  //保存原始状态
  canvas.save();

  //将坐标系原点移到中心，并逆时针旋转90度。完成后x轴朝上。
  canvas.translate(mWidth / 2,mHeight / 2);
  canvas.rotate(-90);

  //画外围轮廓
  canvas.drawCircle(0,0,mRadius, mPaintOutline);

  //画刻度
  for(int i = 0 ; i < 12 ; i++){
   String num = String.valueOf(i == 0 ? 12 : i);
   if(i % 3 == 0){
    canvas.drawLine(mRadius,0,mRadius - mScaleLengthLong,0, mPaintScaleLong);
   }else {
    canvas.drawLine(mRadius,0,mRadius - mScaleLengthShort,0, mPaintScaleShort);
   }
   canvas.drawText(num,mRadius - mScaleLengthLong - mPaintNum.measureText(num) * 2,0, mPaintNum);

   //顺时针旋转30度
   canvas.rotate(30);
  }

  Calendar calendar = Calendar.getInstance();

  //画时针
  int hour = calendar.get(Calendar.HOUR);
  canvas.save();
  canvas.rotate(hour * 30);
  canvas.drawLine(0,0,mTickLengthHour,0,mPaintTickHour);
  canvas.restore();

  //画分针
  int minute = calendar.get(Calendar.MINUTE);
  canvas.save();
  canvas.rotate(minute * 6);
  canvas.drawLine(0,0,mTickLengthMinute,0,mPaintTickMinute);
  canvas.restore();

  //画秒针
  int second = calendar.get(Calendar.SECOND);
  canvas.save();
  canvas.rotate(second * 6);
  canvas.drawLine(0,0,mTickLengthSecond,0,mPaintTickSecond);
  canvas.restore();

  //恢复原始状态
  canvas.restore();
 }

 private void initializePaints(){
  mPaintScaleLong = new Paint();
  mPaintScaleLong.setAntiAlias(true);
  mPaintScaleLong.setStrokeWidth(5);

  mPaintScaleShort = new Paint();
  mPaintScaleShort.setAntiAlias(true);
  mPaintScaleShort.setStrokeWidth(3);

  mPaintOutline = new Paint();
  mPaintOutline.setStyle(Paint.Style.STROKE);
  mPaintOutline.setAntiAlias(true);
  mPaintOutline.setStrokeWidth(5);

  mPaintNum = new Paint();
  mPaintNum.setTextSize(30);

  mPaintTickHour = new Paint();
  mPaintTickHour.setAntiAlias(true);
  mPaintTickHour.setStrokeWidth(6);

  mPaintTickMinute = new Paint();
  mPaintTickMinute.setAntiAlias(true);
  mPaintTickMinute.setStrokeWidth(4);

  mPaintTickSecond = new Paint();
  mPaintTickSecond.setAntiAlias(true);
  mPaintTickSecond.setStrokeWidth(2);
 }
}

代码注释已经比较详细了，下面把比较重要的部分再说明一下：

//保存原始状态
canvas.save();

//省略中间部分...

//恢复原始状态
canvas.restore();

在绘图开始时调用canvas.save()，可以保存下未经任何平移、旋转操作的原始画布状态。在所有绘图工作完成后调用canvas.restore()，可以恢复到上一次保存的状态（类似进出栈的感觉）。

//将坐标系原点移到中心，并逆时针旋转90度。完成后x轴朝上。
canvas.translate(mWidth / 2,mHeight / 2);
canvas.rotate(-90);

画布的平移与旋转可能比较抽象，可以想象成坐标系的平移与旋转。调用canvas.translate(dx,dy)相当于将坐标原点向x,y方向移动了dx,dy的距离，调用canvas.rotate(degree)相当于坐标系顺时针旋转了degree°。

//画刻度
for(int i = 0 ; i < 12 ; i++){
 String num = String.valueOf(i == 0 ? 12 : i);
 if(i % 3 == 0){
  canvas.drawLine(mRadius,0,mRadius - mScaleLengthLong,0, mPaintScaleLong);
 }else {
  canvas.drawLine(mRadius,0,mRadius - mScaleLengthShort,0, mPaintScaleShort);
 }
 canvas.drawText(num,mRadius - mScaleLengthLong - mPaintNum.measureText(num) * 2,0, mPaintNum);

 //顺时针旋转30度
 canvas.rotate(30);
}

根据i值决定是画大刻度（90°的倍数）还是小刻度。每次画完一条之后将坐标系顺时针旋转30°，这样保证每次画的刻度线的坐标不变。

//一秒后重绘
postInvalidateDelayed(1000);

作用是在1000ms（1s）后再次执行View的draw流程，产生指针运动的效果。

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呐喊教程。

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