private void createShader() {
    ...

    Bitmap bitmap = Bitmap.createBitmap(getWidth(), getHeight(), Bitmap.Config.ARGB_8888);
    Canvas canvas = new Canvas(bitmap);

    // Draw default waves into the bitmap
    // y=Asin(ωx+φ)+h
    float waveX1 = 0;
    final float wave2Shift = mDefaultWaveLength / 4;
    final float endX = getWidth();
    final float endY = getHeight();

    ...

    while (waveX1 < endX) {
        double wx = waveX1 * mDefaultAngularFrequency;
        int startY = (int) (mDefaultWaterLevel + mDefaultAmplitude * Math.sin(wx));

        // draw bottom wave with the alpha 40
        canvas.drawLine(waveX1, startY, waveX1, endY, wavePaint1);
        // draw top wave with the alpha 60
        float waveX2 = (waveX1 + wave2Shift) % endX;
        canvas.drawLine(waveX2, startY, waveX2, endY, wavePaint2);

        waveX1++;
    }

    // use the bitamp to create the shader
    mWaveShader = new BitmapShader(bitmap, Shader.TileMode.REPEAT, Shader.TileMode.CLAMP);
    mViewPaint.setShader(mWaveShader);
}

// sacle shader according to mWaveLengthRatio and mAmplitudeRatio
// this decides the size(mWaveLengthRatio for width, mAmplitudeRatio for height) of waves
mShaderMatrix.setScale(
        mWaveLengthRatio / DEFAULT_WAVE_LENGTH_RATIO,
        mAmplitudeRatio / DEFAULT_AMPLITUDE_RATIO,
        0,
        mDefaultWaterLevel);
// translate shader according to mWaveShiftRatio and mWaterLevelRatio this decides the start position(mWaveShiftRatio for x, mWaterLevelRatio for
// this decides the start position(mWaveShiftRatio for x, mWaterLevelRatio for y) of waves
mShaderMatrix.postTranslate(
        mWaveShiftRatio * getWidth(),
        (DEFAULT_WATER_LEVEL_RATIO - mWaterLevelRatio) * getHeight());

// assign matrix to invalidate the shader
mWaveShader.setLocalMatrix(mShaderMatrix);

float radius = getWidth() / 2f
        - (mBorderPaint == null ? 0f : mBorderPaint.getStrokeWidth());
canvas.drawCircle(getWidth() / 2f, getHeight() / 2f, radius, mViewPaint);

// horizontal animation.
// wave waves infinitely.
ObjectAnimator waveShiftAnim = ObjectAnimator.ofFloat(
        mWaveView, "waveShiftRatio", 0f, 1f);
waveShiftAnim.setRepeatCount(ValueAnimator.INFINITE);
waveShiftAnim.setDuration(1000);
waveShiftAnim.setInterpolator(new LinearInterpolator());
animators.add(waveShiftAnim);

// vertical animation.
// water level increases from 0 to center of WaveView
ObjectAnimator waterLevelAnim = ObjectAnimator.ofFloat(
        mWaveView, "waterLevelRatio", 0f, 0.5f);
waterLevelAnim.setDuration(10000);
waterLevelAnim.setInterpolator(new DecelerateInterpolator());
animators.add(waterLevelAnim);

// amplitude animation.
// wave grows big then grows small, repeatedly
ObjectAnimator amplitudeAnim = ObjectAnimator.ofFloat(
        mWaveView, "amplitudeRatio", 0f, 0.05f);
amplitudeAnim.setRepeatCount(ValueAnimator.INFINITE);
amplitudeAnim.setRepeatMode(ValueAnimator.REVERSE);
amplitudeAnim.setDuration(5000);
amplitudeAnim.setInterpolator(new LinearInterpolator());
animators.add(amplitudeAnim);