看看这段代码，该代码模仿features应用程序中如何安装Ktor。
(
SELECT
0 AS marker
pAll.ProductID,
SUM(CASE WHEN oAll.OrderDate BETWEEN '1996-10-01' AND '1996-10-31'
THEN odAll.Quantity * pAll.Price ELSE 0 END) -
SUM(CASE WHEN oAll.OrderDate BETWEEN '1996-09-01' AND '1996-09-30'
THEN odAll.Quantity * pAll.Price ELSE 0 END) AS sales_diff
FROM OrderDetails odAll
INNER JOIN Orders oAll
ON oAll.OrderID = odAll.OrderID
INNER JOIN Products pAll
ON odAll.ProductID = pAll.ProductID
GROUP BY pAll.ProductID
ORDER BY sales DESC
LIMIT 3
)
UNION ALL
(
SELECT
1
pAll.ProductID,
SUM(CASE WHEN oAll.OrderDate BETWEEN '1996-10-01' AND '1996-10-31'
THEN odAll.Quantity * pAll.Price ELSE 0 END) -
SUM(CASE WHEN oAll.OrderDate BETWEEN '1996-09-01' AND '1996-09-30'
THEN odAll.Quantity * pAll.Price ELSE 0 END) AS sales_diff
FROM OrderDetails odAll
INNER JOIN Orders oAll
ON oAll.OrderID = odAll.OrderID
INNER JOIN Products pAll
ON odAll.ProductID = pAll.ProductID
GROUP BY pAll.ProductID
ORDER BY sales
LIMIT 3
)
ORDER BY
marker,
CASE WHEN marker = 0 THEN sales_diff ELSE -1*sales_diff END DESC;
上面的代码段对我来说没有意义的是这一行fun main(args: Array<String>) {
val app = App()
app.installFeature(Authentication)
}
interface AppFeature {
fun install()
}
class Authentication {
companion object Feature : AppFeature {
override fun install() = println("Authentication Installed")
}
}
class App {
fun installFeature(appFeature: AppFeature) {
println("Installing appFeature `${appFeature::class.simpleName}`")
appFeature.install()
}
}
任何人都可以向我解释为什么使用app.installFeature(Authentication)名称而不是class名称就像更明显的方法companion object