SemiCircleProgressBar 实现并支持渐变

实现一个支持渐变的半圆形进度条可以使用自定义 View 的方式，主要思路如下：

1. 继承 View，重写 onDraw 方法，在方法中绘制半圆形和进度条。

2. 在 onDraw 方法中使用 Paint 绘制半圆形和进度条。绘制半圆形可以使用 drawArc 方法，绘制渐变进度条可以使用 Shader。

3. 创建一个 SweepGradient 对象，设置渐变的颜色和位置。

4. 将 SweepGradient 对象设置给 Paint 对象的 Shader 属性，使得进度条使用渐变颜色绘制。

下面是一个简单的实现示例：

public class SemiCircleProgressBar extends View {

    private static final int DEFAULT_START_ANGLE = 180;
    private static final int DEFAULT_SWEEP_ANGLE = 180;

    private Paint mBackgroundPaint;
    private Paint mProgressPaint;

    private int mStartAngle = DEFAULT_START_ANGLE;
    private int mSweepAngle = DEFAULT_SWEEP_ANGLE;
    private int mProgressColor = Color.RED;
    private int mBackgroundColor = Color.GRAY;

    public SemiCircleProgressBar(Context context) {
        super(context);
        init();
    }

    public SemiCircleProgressBar(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    public SemiCircleProgressBar(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init();
    }

    private void init() {
        mBackgroundPaint = new Paint();
        mBackgroundPaint.setAntiAlias(true);
        mBackgroundPaint.setColor(mBackgroundColor);
        mBackgroundPaint.setStyle(Paint.Style.STROKE);

        mProgressPaint = new Paint();
        mProgressPaint.setAntiAlias(true);
        mProgressPaint.setStyle(Paint.Style.STROKE);
        mProgressPaint.setStrokeCap(Paint.Cap.ROUND);
    }

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

        int width = getWidth();
        int height = getHeight();

        int strokeWidth = Math.min(width, height) / 10;
        mBackgroundPaint.setStrokeWidth(strokeWidth);
        mProgressPaint.setStrokeWidth(strokeWidth);

        RectF rectF = new RectF(strokeWidth / 2, strokeWidth / 2,
                width - strokeWidth / 2, height - strokeWidth / 2);

        // 绘制半圆形背景
        canvas.drawArc(rectF, mStartAngle, mSweepAngle, false, mBackgroundPaint);

        // 绘制渐变进度条
        int[] colors = {Color.GREEN, Color.YELLOW, Color.RED};
        float[] positions = {0f, 0.5f, 1f};
        SweepGradient sweepGradient = new SweepGradient(width / 2, height / 2,
                colors, positions);
        mProgressPaint.setShader(sweepGradient);
        canvas.drawArc(rectF, mStartAngle, mSweepAngle, false, mProgressPaint);
    }

    public void setProgressColor(int progressColor) {
        mProgressColor = progressColor;
        mProgressPaint.setColor(mProgressColor);
        invalidate();
    }

    public void setBackgroundColor(int backgroundColor) {
        mBackgroundColor = backgroundColor;
        mBackgroundPaint.setColor(mBackgroundColor);
        invalidate();
    }

    public void setStartAngle(int startAngle) {
        mStartAngle = startAngle;
        invalidate();
    }

    public void setSweepAngle(int sweepAngle) {
        mSweepAngle = sweepAngle;
        invalidate();
    }
}

在代码中，我们使用了 SweepGradient 实现渐变，设置了颜色和位置，然后将 SweepGradient 对象设置给进度条的 Paint 对象的 Shader 属性。最终我们可以得到一个支持渐变的半圆形进度条。