Question

在尝试解决问题How to print a sqlite table content with genie programming language时，我发现我可以尝试将PrintSingleRecipe称为来自Database.exec的回调。但是，似乎回调不能是常规函数，它们有一些我似乎没有在互联网中找到的属性。

我这样称呼它：

else if response is "3" //Show a Recipe
    res:string = UserInterface.raw_input("Select a recipe -> ")
    sql:string = "SELECT * FROM Recipes WHERE pkID = %res"
    db.exec(sql, PrintSingleRecipe, null)

功能本身看起来像：

def PrintSingleRecipe(n_columns:int, values:array of string, column_names:array of string)
    print "~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~"
    for i:int = 0 to n_columns
        stdout.printf ("%s = %s\n", column_names[i], values[i])
    print "~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~"
    print "Ingredient list"
    print " "
    stdout.printf("%-5s", "%03i" )

但是，我在编译时遇到以下错误：

valac --pkg sqlite3 --pkg gee-0.8 cookbook.gs
cookbook.gs:42.26-42.42: error: Argument 2: Cannot convert from `PrintSingleRecipe' to `Sqlite.Callback?'
            db.exec(sql, PrintSingleRecipe, null)
                         ^^^^^^^^^^^^^^^^^
Compilation failed: 1 error(s), 0 warning(s)

如何在Genie中正确运行回调？

Answer 1

当函数作为参数传递时，Vala编译器会对函数进行类型检查。当以这种方式使用函数时，它被称为＆＃34; delegate＆＃34;。具体来说，Vala编译器将检查函数的签名是否与委托类型定义的签名匹配。函数的签名由其参数类型和返回类型组成。 Cannot convert from 'PrintSingleRecipe' to 'Sqlite.Callback?'表示PrintSingleRecipe功能的签名与Sqlite.Callback委托定义的签名不匹配。

此处显示了Sqlite.Callback委托定义： http://valadoc.org/#!api=sqlite3/Sqlite.Callback 您已正确识别所需参数int, array of string, array of string，但您还需要包含返回类型。在这种情况下，它是int。所以你的回调应该是这样的：

def PrintSingleRecipe(n_columns:int, 
        values:array of string, 
        column_names:array of string
        ):int
    print "~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~"
    for i:int = 0 to n_columns
        stdout.printf ("%s = %s\n", column_names[i], values[i])
    print "~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~"
    print "Ingredient list"
    print " "
    stdout.printf("%-5s", "%03i" )
    return 0

返回非零将中止查询。见https://www.sqlite.org/capi3ref.html#sqlite3_exec

如何在Genie中使用Sqlite.Callback

1 个答案: