Canvas画圆弧

public void drawArc(RectF oval, float startAngle, float sweepAngle, boolean useCenter, Paint paint)

1.oval :指定圆弧的外轮廓矩形区域。
2.startAngle: 圆弧起始角度，单位为度。
3.sweepAngle: 圆弧扫过的角度，顺时针方向，单位为度。
4.useCenter: 如果为True时，在绘制圆弧时将圆心包括在内，通常用来绘制扇形。
5.paint: 绘制圆弧的画板属性，如颜色，是否填充等。

private RectF rectF = new RectF();
private float strokeWidth = 2;

@Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(widthMeasureSpec, heightMeasureSpec);
        //获取总宽度,是包含padding值
        int width = MeasureSpec.getSize(widthMeasureSpec);
        rectF.set(strokeWidth / 2f, strokeWidth / 2f, width - strokeWidth / 2f, MeasureSpec.getSize(heightMeasureSpec) - strokeWidth / 2f);
        float radius = width / 2f; // 半径
        float angle = (360 - arcAngle) / 2f;  // 角度
    }

@Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
//        float startAngle = 270 - arcAngle / 2f;
        float startAngle = 0;
        float finishedSweepAngle = progress / (float) getMax() * arcAngle;
        float finishedStartAngle = startAngle;
        if(progress == 0) finishedStartAngle = 0.01f;
        paint.setColor(unfinishedStrokeColor);
//        canvas.drawArc(rectF, startAngle, arcAngle, false, paint);  // 画出最原始的圆弧
        canvas.drawArc(rectF, startAngle, 360, false, paint);  // 画出最原始的圆弧
        paint.setColor(finishedStrokeColor);

        /**
         * 参数1  指定圆弧的外轮廓矩形区域。
         * 参数2  圆弧起始角度，单位为度
         * 参数3  圆弧扫过的角度，顺时针方向，单位为度。
         * 参数4  如果为True时，在绘制圆弧时将圆心包括在内，通常用来绘制扇形。
         */
        canvas.drawArc(rectF, finishedStartAngle, finishedSweepAngle, false, paint);  // 画出进度走过的圆弧
        }

上面是从0开始绘制的，进度默认设置了20，效果见下图；

从上图可以看到，画圆弧的时候0度正好对应3点整点。

画一个开口的圆弧（一个圆的80%）

public class ArcOProgress extends View {

    private Paint paint;
    private float endArc = 360 * 0.8f;
    private RectF rect = new RectF();
    private int progress = 20;  // 默认的进度
    private float strokeWidth;  // 画笔宽度

    public ArcOProgress(Context context) {
        super(context);
        initPaint();
    }

    public ArcOProgress(Context context, AttributeSet attrs) {
        super(context, attrs);
        initPaint();
    }

    public ArcOProgress(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        initPaint();

    }

    private void initPaint() {
        paint = new Paint();
        paint.setAntiAlias(true);
        paint.setStyle(Paint.Style.STROKE);
        paint.setStrokeWidth(Utils.dp2px(getResources(),4));
        paint.setDither(true);
        paint.setStrokeCap(Paint.Cap.ROUND);
        strokeWidth = Utils.dp2px(getResources(),4);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        // 获取总宽度
        int sizeWidth = MeasureSpec.getSize(widthMeasureSpec);
        // 获取总高度
        int sizeHeight = MeasureSpec.getSize(heightMeasureSpec);
        rect.set(strokeWidth/2, strokeWidth/2, sizeWidth - strokeWidth/2, sizeHeight - strokeWidth/2);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        float startArc = 270 - endArc / 2f;
        float currentProgress = progress / (float) 100 * endArc;
        paint.setColor(Color.parseColor("#ffbbcc"));
        canvas.drawArc(rect,startArc,endArc,false,paint);
        paint.setColor(Color.GREEN);
        canvas.drawArc(rect,startArc,currentProgress,false,paint);
    }

    @Override
    public void invalidate() {
        initPaint();
        super.invalidate();
    }
}

效果图：

加入绘制文字：

public class ArcOProgress extends View {

    private Paint paint;  // 圆弧的画笔
    private float endArc = 360 * 0.8f;   // 圆弧的总弧度
    private RectF rect = new RectF();
    private int progress = 20;  // 默认的进度
    private float strokeWidth;  // 画笔宽度

    private Paint textPaint; // 绘制文字
    private int sizeWidth;  // 控件的宽度
    private int sizeHeight; // 控件高度

    private Paint textBottomPaint; // 圆弧底部的文字绘制画笔

    public ArcOProgress(Context context) {
        super(context);
        initPaint();
    }

    public ArcOProgress(Context context, AttributeSet attrs) {
        super(context, attrs);
        initPaint();
    }

    public ArcOProgress(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        initPaint();

    }

    private void initPaint() {
        paint = new Paint();
        paint.setAntiAlias(true);
        paint.setStyle(Paint.Style.STROKE);
        paint.setStrokeWidth(Utils.dp2px(getResources(),4));
        paint.setDither(true);
        paint.setStrokeCap(Paint.Cap.ROUND);
        strokeWidth = Utils.dp2px(getResources(),4);

        textPaint = new Paint();
        textPaint.setTextSize(50);
        textPaint.setAntiAlias(true);
        textPaint.setColor(Color.RED);
        textPaint.setStrokeWidth(strokeWidth);
        textPaint.setStyle(Paint.Style.FILL);
        textPaint.setTextAlign(Paint.Align.CENTER);

        textBottomPaint = new Paint();
        textBottomPaint.setStyle(Paint.Style.FILL);
        textBottomPaint.setTextSize(70);
        textBottomPaint.setAntiAlias(true);
//        textBottomPaint.setDither(true);
        textBottomPaint.setColor(Color.BLUE);
        textBottomPaint.setStrokeWidth(strokeWidth);
        textBottomPaint.setTextAlign(Paint.Align.CENTER);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        // 获取总宽度
        sizeWidth = MeasureSpec.getSize(widthMeasureSpec);
        // 获取总高度
        sizeHeight = MeasureSpec.getSize(heightMeasureSpec);
        rect.set(strokeWidth/2, strokeWidth/2, sizeWidth - strokeWidth/2, sizeHeight - strokeWidth/2);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        float startArc = 270 - endArc / 2f;
        float currentProgress = progress / (float) 100 * endArc;
        paint.setColor(Color.parseColor("#ffbbcc"));
        canvas.drawArc(rect,startArc,endArc,false,paint);
        paint.setColor(Color.GREEN);
        canvas.drawArc(rect,startArc,currentProgress,false,paint);

        float baseLine = (getHeight() - (textPaint.descent() + textPaint.ascent())) / 2.0f;

        canvas.drawText("爱奇艺%",sizeWidth/2,baseLine,textPaint);

        float baseLineBottom = getHeight() + ((textBottomPaint.descent() + textBottomPaint.ascent()) / 2.0f);
//        float baseLineBottom = getHeight();
        canvas.drawText("爱奇艺",sizeWidth/2,baseLineBottom,textBottomPaint);
    }

    @Override
    public void invalidate() {
        initPaint();
        super.invalidate();
    }
}

/**
 * DP、SP 转换为 PX 的工具类
 */
public class Util {

    public static int px2dip(Context context, float pxValue) {
        final float scale = context.getResources().getDisplayMetrics().density;
        return (int) (pxValue / scale + 0.5f);
    }

    public static int dip2px(Context context, float dipValue) {
        final float scale = context.getResources().getDisplayMetrics().density;
        return (int) (dipValue * scale + 0.5f);
    }

    public static int px2sp(Context context, float pxValue) {
        final float fontScale = context.getResources().getDisplayMetrics().scaledDensity;
        return (int) (pxValue / fontScale + 0.5f);
    }

    public static int sp2px(Context context, float spValue) {
        final float fontScale = context.getResources().getDisplayMetrics().scaledDensity;
        return (int) (spValue * fontScale + 0.5f);
    }
}