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1.简介

SweepGradient可以实现扫描渐变渲染，类似雷达扫描图，渐变圆弧，渐变进度条等，构造函数有两个

/**
 * A Shader that draws a sweep gradient around a center point.
 *
 * @param cx       The x-coordinate of the center
 * @param cy       The y-coordinate of the center
 * @param colors   The colors to be distributed between around the center.
 *                 There must be at least 2 colors in the array.
 * @param positions May be NULL. The relative position of
 *                 each corresponding color in the colors array, beginning
 *                 with 0 and ending with 1.0. If the values are not
 *                 monotonic, the drawing may produce unexpected results.
 *                 If positions is NULL, then the colors are automatically
 *                 spaced evenly.
 */
public SweepGradient(float cx, float cy,
        @NonNull @ColorInt int colors[], @Nullable float positions[])；
/**
 * A Shader that draws a sweep gradient around a center point.
 *
 * @param cx       The x-coordinate of the center
 * @param cy       The y-coordinate of the center
 * @param color0   The color to use at the start of the sweep
 * @param color1   The color to use at the end of the sweep
 */
public SweepGradient(float cx, float cy, @ColorInt int color0, @ColorInt int color1) ；

参数说明：
cx,cy 渐变中心坐标。
color0,color1：渐变开始结束颜色。
colors，positions：类似LinearGradient,用于多颜色渐变,positions为null时，根据颜色线性渐变。

2.两种颜色渐变

构造函数：
SweepGradient(float cx, float cy, @ColorInt int color0, @ColorInt int color1)

SweepGradient sweepGradient1 = new SweepGradient(400, 400, Color.RED, Color.GREEN);
mPaint.setShader(sweepGradient1);
canvas.drawCircle(400, 400, 200, mPaint);

3.多颜色扫描渐变

构造函数：
SweepGradient(float cx, float cy,@NonNull @ColorInt int colors[], @Nullable float positions[])

int[] colors = {Color.BLUE, Color.RED, Color.YELLOW};
SweepGradient sweepGradient2 = new SweepGradient(400, 800, colors, null);
mPaint.setShader(sweepGradient2);
canvas.drawCircle(400, 800, 200, mPaint);

设置position：

position数组设置主要作用是特定位置对应颜色数组，位置取值【0-1】，0表示开始位置，1表示结束位置，数组和颜色数组一一对应。

int[] colors = {Color.BLUE, Color.RED, Color.YELLOW, Color.GREEN};
float[] positions = {0f, 0.2f, 0.6f, 1.0f};
SweepGradient sweepGradient2 = new SweepGradient(400, 400, colors, positions);
mPaint.setShader(sweepGradient2);
canvas.drawCircle(400, 400, 200, mPaint);

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