Android：使用渐变填充颜色在canvas api中绘制弧

Question

I want to draw an arc using canvas using a gradient fill. How can achieve this?

Answer 1

Hey I stole this from here: Draw an arc with a SweepGradient in Android

but it works fine, I used a LinearGradient instead.

Shader gradient = new SweepGradient (0,getMeasuredHeight()/2, Color.RED, Color.WHITE);
lightRed.setShader(gradient);
canvas.drawArc(rectf, -90, 360, false, lightRed);

Answer 2

In my cause i I had to draw someone like this:

Maybe you too.

So, let's think! How does Sweep Gradient works? If you draw rect via this:

     private val colors = intArrayOf(ContextCompat.getColor(context, R.color.progress_from_color),
 ContextCompat.getColor(context, R.color.progress_to_color))

    private var positions = floatArrayOf(0.0f, 1.0f)
    private var sweepGradient : SweepGradient? = null

    sweepGradient = SweepGradient(w / 2F,h / 2F,colors,  positions)

will be:

So the idea is to rotate it!

 sweepGradient.apply {
            val rotate = 270f
            val gradientMatrix = Matrix()
            gradientMatrix.preRotate(rotate, mWidth / 2F, mHeight / 2F)
            setLocalMatrix(gradientMatrix)
        }

Finally we can draw our arc:

 mPaint.shader = gradient
 canvas.drawArc(rectF, startAngle, finishedSweepAngle, false, mPaint)

See my full custom view source code on github repository .

Answer 3

You can also use an array of colors and variable positions. For example, define 10 colors, one each 10% progress:

<color name="color_0">#3C3C3F41</color>
<color name="color_10">#1AFF2323</color>
<color name="color_20">#33FF2323</color>
<color name="color_30">#4DFF2323</color>
<color name="color_40">#66FF2323</color>
<color name="color_50">#80FF2323</color>
<color name="color_60">#99FF2323</color>
<color name="color_70">#B3FF2323</color>
<color name="color_80">#CCFF2323</color>
<color name="color_90">#E6FF2323</color>
<color name="color_100">#FFFF2323</color>

Put all of these colors inside a colors intArray like this:

var colors = intArrayOf(
            ContextCompat.getColor(context, R.color.color_0),
            ContextCompat.getColor(context, R.color.color_10),
            ContextCompat.getColor(context, R.color.color_20),
            ContextCompat.getColor(context, R.color.color_30),
            ContextCompat.getColor(context, R.color.color_40),
            ContextCompat.getColor(context, R.color.color_50),
            ContextCompat.getColor(context, R.color.color_60),
            ContextCompat.getColor(context, R.color.color_70),
            ContextCompat.getColor(context, R.color.color_80),
            ContextCompat.getColor(context, R.color.color_90),
            ContextCompat.getColor(context, R.color.color_100)
        )

Then, define positions. Positions sweep from 0.0 to 1.0 (positions 0.1 corresponds to color_10, position 0.2 to color_20 etc.)

var positions = floatArrayOf(0.0f, 0.1f, 0.2f, 0.3f, 0.4f, 0.5f, 0.6f, 0.7f, 0.8f, 0.9f, 1.0f)

Once we've defined positions, we can set the SweepGradient to our paint

Shader gradient = new SweepGradient (0,getMeasuredHeight()/2, colors, positions);
lightRed.setShader(gradient);

Finally we can draw our arc with our shader paint:

 canvas.drawArc(rectf, -90, 360, false, lightRed);

Final effect in my custom view: