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。

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