Hi I made this using patterns from other people. In this snippet, I am making an rx implementation of opengles renderer.

OnSubscribeRenderer.kt

class OnSubscribeRenderer(private val surfaceView: GLSurfaceView,
                          private val previewSize: Size) : Observable<SurfaceEvent>() {

	// TODO make a github gist and ask to community for insights

	override fun subscribeActual(observer: Observer<in SurfaceEvent>) {
		val listener = Listener(observer, previewSize)
		observer.onSubscribe(listener)

		surfaceView.setEGLContextClientVersion(2)
		surfaceView.setRenderer(listener)
		surfaceView.renderMode = GLSurfaceView.RENDERMODE_CONTINUOUSLY
	}

	private class Listener(private val observer: Observer<in SurfaceEvent>,
	                       private val previewSize: Size) : GLSurfaceView.Renderer, Disposable {

		private val mMVPMatrix = FloatArray(16)

		private val mProjMatrix = FloatArray(16)

		private val mVMatrix = FloatArray(16)

		private var disposed = false

		override fun isDisposed() = disposed

		override fun dispose() {
			disposed = true
			observer.onNext(Destroyed())
			observer.onComplete()
		}

		override fun onDrawFrame(gl: GL10?) {
			GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT)

			observer.onNext(Draw(mMVPMatrix))
		}

		override fun onSurfaceChanged(gl: GL10?, glwidth: Int, glheight: Int) {
			GLES20.glViewport(0, 0, glwidth, glheight)

			val psheight = previewSize.width
			val pswidth = previewSize.height
			val glRatio = glheight.toFloat() / glwidth.toFloat()
			val psRatio = psheight.toFloat() / pswidth.toFloat()
			// CENTER CROPPING THE VIEW
			if(glRatio > psRatio) {
				// this means gl surface is taller than the preview size
				val scaleWidth = glheight.toFloat() / psRatio
				val difference = scaleWidth - glwidth.toFloat()
				val percentage = (difference / scaleWidth)
				// adjust both left and right edge of the frustum
				Matrix.frustumM(mProjMatrix, 0, -1f + percentage, 1f - percentage, -1f, 1f, 3f, 7f)
			} else {
				// this means preview size is taller than the gl surface
				val scaledHeight = glwidth.toFloat() / psRatio
				val difference = scaledHeight - glheight.toFloat()
				val percentage = (difference / scaledHeight)
				// adjust both top and bottom edge of the frustum
				Matrix.frustumM(mProjMatrix, 0, -1f, 1f, -1f + percentage, 1f - percentage, 3f, 7f)
			}
			Matrix.setLookAtM(mVMatrix, 0, 0f, 0f, -3f, 0f, 0f, 0f, 0f, 1f, 0f)
			Matrix.multiplyMM(mMVPMatrix, 0, mProjMatrix, 0, mVMatrix, 0)

			observer.onNext(Changed(previewSize))
		}

		override fun onSurfaceCreated(gl: GL10?, config: EGLConfig?) {
			GLES20.glClearColor(1.0f, 1.0f, 1.0f, 1.0f)
			observer.onNext(Created(config))
		}
	}

	companion object {
		@JvmStatic
		private val TAG: String = OnSubscribeRenderer::class.java.simpleName
	}
}