Trait FocusMonitor 

pub trait FocusMonitor: Send + Sync {
    // Required methods
    fn register_callback(&self, id: &str, callback: FocusCallback);
    fn unregister_callback(&self, id: &str);
}

聚焦监控器 trait，定义平台无关的窗口焦点检测能力契约。 各平台实现此 trait，处理平台特定的窗口事件监听。 支持多个回调注册，窗口失去焦点时依次调用所有回调。 推送式（push-based）服务，与 InstallationMonitor / HotkeyManager 一致。

FocusMonitor 是强制启用的系统服务，构造即开始监控，不可停止。

Required Methods

fn register_callback(&self, id: &str, callback: FocusCallback)

注册焦点事件回调。

fn unregister_callback(&self, id: &str)

注销焦点事件回调。

Dyn Compatibility

This trait is dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety".

Implementors