一般我們給button或者imageview設定點選效果時，會使用一個xml的選擇器selector。但是整個應用下來會建立很多個xml，甚至很多圖片。
一般我們要得點選效果只是讓透明度有些變化而已，那麼針對透明度變化自定義一個元件。

public class AutoBgButton extends Button {

    public AutoBgButton(Context context) {
        super(context);
    }

    public AutoBgButton(Context context, AttributeSet attrs) {
        super
 
(context, attrs);
    }

    public AutoBgButton(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public void setBackground(Drawable d) {
        // Replace the original background drawable (e.g. image) with a
        // LayerDrawable that
 

        // contains the original drawable.
        SAutoBgButtonBackgroundDrawable layer = new SAutoBgButtonBackgroundDrawable(d);
        super.setBackground(layer);
    }

    protected class SAutoBgButtonBackgroundDrawable extends LayerDrawable {

        // The color filter to apply when the button is pressed
 

        protected ColorFilter _pressedFilter = new LightingColorFilter(Color.LTGRAY, 1);
        // Alpha value when the button is disabled
        protected int _disabledAlpha = 200;

        public SAutoBgButtonBackgroundDrawable(Drawable d) {
            super(new Drawable[]{d});
        }

        @Override
        protected boolean onStateChange(int[] states) {
            boolean enabled = false;
            boolean pressed = false;

            for (int state : states) {
                if (state == android.R.attr.state_enabled)
                    enabled = true;
                else if (state == android.R.attr.state_pressed)
                    pressed = true;
            }

            mutate();
            if (enabled && pressed) {
                setColorFilter(_pressedFilter);
            } else if (!enabled) {
                setColorFilter(null);
                setAlpha(_disabledAlpha);
            } else {
                setColorFilter(null);
            }

            invalidateSelf();

            return super.onStateChange(states);
        }

        @Override
        public boolean isStateful() {
            return true;
        }
    }
}