我正在尝试Exercise 22.3.3 from HtDP,但不知道如何检索点击按钮的标签。我收到此消息draw-message: expected <string> as second argument, given: (instantiate (class ...) ...)
这似乎暗示我需要一个字符串,但我得到一个类的实例。回调中的答案是什么?如果是这样,我该如何解构呢?
这是我到目前为止所做的:
(define pad1
'((1 2 3)
(4 5 6)
(7 8 9)
(\# 0 *)))
(define pad2
'((1 2 3 +)
(4 5 6 -)
(7 8 9 *)
(0 = \. /)))
(define (title t)
(make-message t))
(define display
(make-message ""))
(define (pad->gui p)
(cond
[(empty? p) empty]
[else (cons (button-maker (first p))
(pad->gui (rest p)))]))
;; make buttons out of a list
(define (button-maker a-list)
(cond
[(empty? a-list) empty]
[(number? (first a-list))(cons (make-button (number->string (first a-list)) call-back)
(button-maker (rest a-list)))]
[(symbol? (first a-list))(cons (make-button (symbol->string (first a-list)) call-back)
(button-maker (rest a-list)))]))
(define (call-back b)
(draw-message display ...))
(create-window
(append (list (list (title "Virtual Phone")))
(list (list display))
(pad->gui pad1)))
如果我理解正确,按下时每个按钮都会调用call-back
。这反过来应该调用display
来更新文本。但是,我不明白如何检索调用者的标签。例如如果按下“9”按钮,它将调用call-back
。但是如何检索值“9”?这是我不确定的。
答案 0 :(得分:3)
正确。 draw-message函数使用'window'和'string',这在同一节中的图62中有记录。您似乎将它应用于“按钮对象”。另请参见同一部分中的示例2,如下所示:
(define a-text-field
(make-text "Enter Text:"))
(define a-message
(make-message "`Hello World' is a silly program."))
(define (echo-message e)
(draw-message a-message (text-contents a-text-field)))
(define w (create-window
(list (list a-text-field a-message)
(list (make-button "Copy Now" echo-message)))))
当您单击“立即复制”按钮时,查看echo-messsage如何更改显示。
提示:由于每个按钮有一个回调,因此您确切地知道要从哪个按钮回调发送到显示器的字符串。
更正:本书中的示例1已被破坏。使用
(define u
(create-window (list (list (make-button "Close" (lambda (x) (hide-window u)))))))
代替。