Go WithValue

• WithValue有点层层封装，层层调用的意思，详见下述代码。

func WithValue(parent Context, key, val interface{}) Context {
    if key == nil {
        panic("nil key")
    }
    if !reflectlite.TypeOf(key).Comparable() {
        panic("key is not comparable")
    }
    return &valueCtx{parent, key, val}
}

type valueCtx struct {
    Context
    key, val interface{}
}


func (c *valueCtx) Value(key interface{}) interface{} {
    if c.key == key {
        return c.val
    }
    return c.Context.Value(key)
}