Android Gradient border?

Question

Task:

Want to accomplish gradient BORDER...

Start color starts from blue circle edge and end color ends on red ring edge.

I tried with shape oval, and ring and radial gradient. I need gradient that starts from edge of the circle (not from the centre) and expand to the end color. I am interested is it even possible to do that with radial gradient. What am I missing?

So far I made it like this:

<!-- Added shadow -->
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">

    <item
        android:width="85dp"
        android:height="85dp">

        <shape
            android:shape="ring"
            android:thickness="10dp"
            android:useLevel="false">

            <gradient
                android:endColor="@android:color/transparent"
                android:gradientRadius="37.5dp"
                android:startColor="@color/colorBlack"
                android:type="radial"></gradient>
        </shape>
    </item>

    <item
        android:width="64dp"
        android:height="64dp"
        android:left="11dp"
        android:top="11dp">

        <shape android:shape="oval">
            <solid android:color="@color/colorPrimaryBlue" />
        </shape>
    </item>
</layer-list>

OUTPUT:

Original problem is not solved. The main problem is that radial gradient starts from the centre of the circle and not from the edge.

Any more ideas?

Answer 1

I understand that you want what can be described as a corona around the disk with the disk a solid color and the corona starting with the color black at the disk's edge and transitioning to transparency radially.

I think you will need to implement a drawable for the corona with a RadialGradient using the constructors that allows the specification of colors and color stops from the center of the disk.

Mark Allison has a blog post entitled RadialGradient – Gradients that discusses the use of RadialGradients and their implementation.

You can look at a similar implementation that is transparent in the center and starts with black at the edge of the disk and transitions to a separate color. You would have to compute the starting position of the black as a fraction of the overall size of the image.

---

---

The following is a custom drawable that produces the desired effect:

class CoronaDrawable : Drawable() {
    private val mPaint = Paint()
    private var mRadius = 0f
    private val mColors =
        intArrayOf(Color.TRANSPARENT, Color.TRANSPARENT, Color.BLACK, Color.BLACK and 0xFFFFFF)
    private val mStops = floatArrayOf(0f, 0.85f, 0.85f, 1.0f)

    override fun onBoundsChange(bounds: Rect) {
        super.onBoundsChange(bounds)
        mRadius = min(bounds.width(), bounds.height()) / 2f
        mPaint.shader = shaderFactory(mRadius, mRadius, mColors, mStops)
    }

    override fun draw(c: Canvas) {
        c.drawCircle(mRadius, mRadius, mRadius, mPaint)
    }

    override fun setAlpha(alpha: Int) {
    }

    override fun setColorFilter(filter: ColorFilter?) {
    }

    override fun getOpacity(): Int {
        return PixelFormat.OPAQUE
    }

    private fun shaderFactory(centerX: Float, centerY: Float, colors: IntArray, stops: FloatArray) =
        RadialGradient(
            centerX, centerY, min(centerX, centerY), colors, stops, Shader.TileMode.CLAMP
        )
}

which looks like this in the Android Studio layout designer as the background to a simple View:

The key take-away here is that the blank area in the center is set to transition from transparent to transparent or, otherwise, no change. This gives the empty center. At 85% of the radius the color abruptly changes to black and transitions to a transparent black.

For API 24+, you can place this drawable into a drawable resource file as follows and use it like a regular drawable.

corona.xml

<drawable class="com.example.radialgradientwithstops.CoronaDrawable"/>

You will have to determine how best to incorporate this drawable into your layer list or other means of presentation in your app.