SweepGradient掃描漸變

SweepGradient可以實現掃描漸變渲染，類似雷達掃描圖，漸變圓弧，漸變進度條等，建構函式有兩個：

/**
 * 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) ；

引數說明：
cx,cy 漸變中心座標。
color0,color1：漸變開始結束顏色。
colors，positions：類似LinearGradient,用於多顏色漸變,positions為null時，根據顏色線性漸變。

兩種顏色漸變

建構函式：
SweepGradient(float cx, float cy, @ColorInt int color0, @ColorInt int color1)

sweepGradient =
 
 new SweepGradient(450,450,Color.RED, Color.GREEN);
mPaint.setShader(sweepGradient);
canvas.drawCircle(450,450,450,mPaint);

多顏色掃描漸變

建構函式：
SweepGradient(float cx, float cy,@NonNull @ColorInt int colors[], @Nullable float positions[])

int [] colors = {Color.BLACK,Color.RED, Color.BLUE,Color.GREEN};
sweepGradient = new SweepGradient(450,450,colors,null);
mPaint.setShader(sweepGradient);
canvas.drawCircle(450,450,450,mPaint);

設定position：

position陣列設定主要作用是特定位置對應顏色陣列，位置取值【0-1】，0表示開始位置，1表示結束位置，陣列和顏色陣列一一對應。

int [] colors = {Color.BLACK,Color.RED, Color.BLUE,Color.GREEN};
float[] postions = {0f,0.1f,0.7f,1.0f};
sweepGradient = new SweepGradient(450,450,colors,postions);
mPaint.setShader(sweepGradient);
canvas.drawCircle(450,450,450,mPaint);

可以明顯看到BLUE藍色變多。

旋轉掃描例項：

此處只是示例程式碼，如果要真實現可以利用屬性動畫或者handler更新，

@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    startAngle = startAngle +1;

    if (startAngle == 360){
        startAngle = 0;
    }
    
    int [] colors = {Color.parseColor("#2300FF00"),Color.parseColor("#7F00FF00")};
    float[] position = new float[2];
    position[0] = (startAngle * 1f)/360f;
    position[1] = ((startAngle + 60)* 1.0f)/360f;
    sweepGradient = new SweepGradient(450,450,colors,position);
    mPaint.setShader(sweepGradient);
    canvas.drawCircle(450,450,450,mPaint2);
    canvas.drawCircle(450,450,350,mPaint2);
    canvas.drawCircle(450,450,200,mPaint2);
    canvas.drawCircle(450,450,100,mPaint2);
    float[] lines = {0, 450, 450, 450, 450,450,900, 450, 450, 0, 450, 450,450,450, 450, 900};
    canvas.drawLines(lines,mPaint2);

    canvas.drawArc(0,0,900,900,startAngle,endAngle,true,mPaint);

    invalidate();
}

繪製漸變圓弧

此處只是示例程式碼，如果要真實現可以利用屬性動畫或者handler更新，

endAngle = endAngle +1;

if (endAngle >= 360){
    endAngle = 0;
}

int [] colors = {Color.parseColor("#FFFFFF00"),Color.parseColor("#FFFF0000")};
float[] position = new float[2];
position[0] = 0f;
position[1] = ((endAngle)* 1.0f)/360f;
sweepGradient = new SweepGradient(470,470,colors,position);
mPaint.setShader(sweepGradient);
canvas.drawCircle(470,470,450,mPaint2);

canvas.drawArc(20,20,920,920,startAngle,endAngle,false,mPaint);

invalidate();

可以看到，由於設定了positions陣列，繪製多少圓弧，顏色是從圓弧開始繪製的地方漸變到圓弧結束弧度。
positions陣列的設定也有講究，需要開始繪製的顏色對應positions陣列值為0f，結束位置為((endAngle)* 1.0f)/360f;

如果不設定positions陣列引數，結果如下：

 endAngle = endAngle +1;

        if (endAngle >= 360){
            endAngle = 0;
        }

        int [] colors = {Color.parseColor("#FFFFFF00"),Color.parseColor("#FFFF0000")};
        float[] position = new float[2];
        position[0] = 0f;
        position[1] = ((endAngle)* 1.0f)/360f;
        sweepGradient = new SweepGradient(470,470,colors,null);
        mPaint.setShader(sweepGradient);
        canvas.drawCircle(470,470,450,mPaint2);

        canvas.drawArc(20,20,920,920,startAngle,endAngle,false,mPaint);

        invalidate();

可以看到圓弧的漸變顏色一直是從開始到整個圓結束，所以和不設定positions陣列有較大區別。