function - AppleScript 处理程序中的可选参数

Question

Applescript documentation说从优胜美地开始，处理程序的参数可以是可选的。

从“参数规范”部分:

Labeled parameters may be declared with a default value by following the formal parameter name with :literal. Doing so makes the parameter optional when called. For example, this declares a make handler with a default value for the with data parameter:

on make new theClass with data theData : missing value

This handler can now be called without supplying a with data parameter; the handler would see theData set to the specified default missing value, which it could then test for and handle appropriately.

因此，需要一个带有可选参数的处理程序，我尝试创建一个。我已经走了这么远:

set theResult to Create of me given the string:"stuff", info:"this"

on Create given info:missing value, thestring:"stuff"
    if info is missing value then
        set present to false
    else
        set present to true
    end if
    return {present, thestring}
end Create

它编译，但给了我错误“变量 thestring 未定义。”

如果我只用一个参数调用它:

set theResult to Create of me given thestring:"stuff"

我收到错误消息:“Create 缺少信息参数。”即参数毕竟不是可选的。

如何获得在 Applescript 处理程序中工作的可选参数？

Answer 1

为了利用可选的标记参数，处理程序定义必须为您希望成为可选的参数分配一个默认值。然后，当调用者不提供该标记参数时，将使用默认值。

这是一个使用用户定义标签的示例，我发现它比 AppleScript 定义的神奇标签(of、by、for 等)更清晰。

SayWhat given greeting:"Hola", farewell:"Adios"
SayWhat given greeting:"Aloha"
SayWhat given farewell:"Ciao"

on SayWhat given greeting:strGreeting : "Hello", farewell:strFarewell : "Goodbye"
    log "You say " & strGreeting & "; I say " & strFarewell
end SayWhat

(*You say Hola; I say Adios*)
(*You say Aloha; I say Goodbye*)
(*You say Hello; I say Ciao*)