returning). There are many styles of asynchronous interfaces: • Callback argument • Return a placeholder (Future, Promise, Deferred) • Deliver to a queue • Callback registry (e.g. POSIX signals) Regardless do not actually make things asynchronous). Asynchronous operations in Tornado generally return placeholder objects (Futures), with the exception of some low- level components like the IOLoop that use callbacks level to terminal color code • datefmt (str) – Datetime format. Used for formatting (asctime) placeholder in prefix_fmt. Changed in version 3.2: Added fmt and datefmt arguments. tornado.log.enable_p

before returning). There are many styles of asynchronous interfaces: Callback argument Return a placeholder (Future, Promise, Deferred) Deliver to a queue Callback registry (e.g. POSIX signals) Regardless do not actually make things asynchronous). Asynchronous operations in Tornado generally return placeholder objects (Futures), with the exception of some low-level components like the IOLoop that use callbacks [https://docs.python.org/3/library/stdtypes.html#str]) – Datetime format. Used for formatting (asctime) placeholder in prefix_fmt. Changed in version 3.2: Added fmt and datefmt arguments. tornado.log.enable_p

Button 结构体 在 Button 上的 width、height 和 label 字段会和其他组件不同；比如 TextField 可能有 width、height、label 以及 placeholder 字段。每一个我们希望能在屏幕上绘制的类型都会 使用不同的代码来实现 Draw trait 的 draw 方法来定义如何绘制特定的类型，像这里的 Button 类型（如上提到的并不包含任何实际的