看了许久，没研究清楚下面这个代码中哪里订阅合约？
+
我看所有的示例策略中都没有 合约 这个参数，示例策略中也没有订阅过程，也就是策略和合约是完全分离的。。。疑惑
+

def run_child():
    """
    Running in the child process.
    """
    SETTINGS["log.file"] = True

    event_engine = EventEngine()
    main_engine = MainEngine(event_engine)
    main_engine.add_gateway(CtpGateway)
    cta_engine = main_engine.add_app(CtaStrategyApp)
    main_engine.write_log("主引擎创建成功")

    log_engine = main_engine.get_engine("log")
    event_engine.register(EVENT_CTA_LOG, log_engine.process_log_event)
    main_engine.write_log("注册日志事件监听")

    main_engine.connect(ctp_setting, "CTP")
    main_engine.write_log("连接CTP接口")

    sleep(10)

    cta_engine.init_engine()
    main_engine.write_log("CTA策略初始化完成")

    cta_engine.init_all_strategies()
    sleep(60)   # Leave enough time to complete strategy initialization
    main_engine.write_log("CTA策略全部初始化")

    cta_engine.start_all_strategies()
    main_engine.write_log("CTA策略全部启动")

    while True:
        sleep(10)

        trading = check_trading_period()
        if not trading:
            print("关闭子进程")
            main_engine.close()
            sys.exit(0)

策略初始化的时候会去订阅合约，可以自己去vnpy.app.cta_strategy.engine下去看