英文:
Go: How to use context value in a variable?
问题
我正在使用上下文将用户负载（特别是用户ID）附加到我的Go REST应用程序的中间件中。
// 中间件// 将负载附加到请求上下文claimsWithPayload, _ := token.Claims.(*handlers.Claims)ctx := context.WithValue(r.Context(), "userid", claimsWithPayload.Id)req := r.WithContext(ctx)h := http.HandlerFunc(handler)h.ServeHTTP(w, req)
然后在HTTP处理程序中，我需要提取该用户ID作为string/integer，但是context().Value()返回一个interface{}。
// 处理程序a := r.Context().Value("userid") // 这返回一个interface{}response := []byte("您的用户ID是" + a) // 如何将其用作字符串/整数？？w.Write(response)
请注意，上述代码片段中的注释已经被我翻译成了中文。
英文:
I am using context to attach user payload(userId specifically) in a middleware for my go rest application
// middleware// attaching payload to the request contextclaimsWithPayload, _ := token.Claims.(*handlers.Claims)ctx := context.WithValue(r.Context(), "userid", claimsWithPayload.Id)req := r.WithContext(ctx)h := http.HandlerFunc(handler)h.ServeHTTP(w, req)
And later in a http handler, I need to extract that userid as string/integer
but context().Value() returns an interface{}
// handlera := r.Context().Value("userid") // THIS returns an interface{}response := []byte("Your user ID is" + a) // 👈 how do I use it as a string/integer??w.Write(response)</details># 答案1**得分**: 7你可以使用类型断言将上下文值转换为其底层类型：```goa := r.Context().Value("userid").(string)
如果中间件存储的值不是字符串，或者其他内容将上下文键设置为非字符串值，这将引发恐慌。为了防止这种情况发生，你不应该使用内置类型作为上下文键，而是定义自己的类型并使用它：
type contextKey stringconst userIDKey contextKey = "userid"...ctx := context.WithValue(r.Context(), userIDKey, claimsWithPayload.Id)...a := r.Context().Value(userIDKey).(string)
由于contextKey和userIDKey是未导出的，只有你的包才能从上下文中读取或写入该值。
英文:
You can use a type assertion to get the context value as its underlying type:
a := r.Context().Value("userid").(string)
This will panic if the value stored by the middleware is not a string, or if something else sets the context key to something which is not a string. To guard against this you should never use builtin types as context keys, instead define your own type and use that:
type contextKey stringconst userIDKey contextKey = "userid"...ctx := context.WithValue(r.Context(), userIDKey, claimsWithPayload.Id)...a := r.Context().Value(userIDKey).(string)
Because contextKey and userIDKey are unexported, only your package can read or write this value from or to the context.
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