rainbowview.java

MainActivity.java

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        getWindow().getDecorView().getViewTreeObserver().addOnPreDrawListener(
                new ViewTreeObserver.OnPreDrawListener() {
            @Override
            public boolean onPreDraw() {
                final View view = findViewById(R.id.rainbow_view);
                ((ViewGroup) view.getParent()).getOverlay().add(new Drawable() {
                    {
                        setBounds(view.getLeft(), view.getTop(), view.getRight(), view.getBottom());
                    }
                    @Override
                    public void draw(Canvas canvas) {
                        Rect bounds = getBounds();
                        float x = bounds.width() / 2.0f;
                        float y = bounds.height() / 2.0f;
                        Paint p = new Paint();
                        p.setShader(new RadialGradient(x, y, Math.max(x, y), new int[] {
                                0x9fff0000,
                                0x9fffff00,
                                0x9f00ff00,
                                0x9f00ffff,
                                0x9f0000ff,
                                0x9fff00ff
                        }, new float[] {
                                0.0f, 0.20f, 0.40f, 0.60f, 0.80f, 1.00f
                        }, Shader.TileMode.REPEAT));
                        // p.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SCREEN));
                        canvas.drawRect(bounds, p);
                    }

                    @Override
                    public void setAlpha(int i) {
                    }

                    @Override
                    public void setColorFilter(ColorFilter colorFilter) {
                    }

                    @Override
                    public int getOpacity() {
                        return PixelFormat.TRANSLUCENT;
                    }
                });
                getWindow().getDecorView().getViewTreeObserver().removeOnPreDrawListener(this);
                return true;
            }
        });
    }
}