設置當前窗口的透明度

pop 提示窗背景，有時需要設置窗口的透明度

    private void setLightMode(int lightMode) {
        WindowManager.LayoutParams layoutParams = getWindow().getAttributes();
        switch (lightMode) {
            case LIGHT_OFF:
                layoutParams.alpha = 0.7f;
                break;
            case LIGHT_ON:
                layoutParams.alpha = 1.0f;
                break;
        }
        getWindow().setAttributes(layoutParams);
    }