Why does Kotlin for Android Developers (the book)

问题:

I know Anko provides the functions parseSingle, parseOpt and parseList , I don't understand why the code of Android Developers (the book) need to design extensions parseList again.

Could you tell me? Thanks!

https://github.com/antoniolg/Kotlin-for-Android-Developers/blob/master/app/src/main/java/com/antonioleiva/weatherapp/data/db/ForecastDb.kt

override fun requestForecastByZipCode(zipCode: Long, date: Long) = forecastDbHelper.use {

        val dailyRequest = "${DayForecastTable.CITY_ID} = ? AND ${DayForecastTable.DATE} >= ?"
        val dailyForecast = select(DayForecastTable.NAME)
                .whereSimple(dailyRequest, zipCode.toString(), date.toString())
                .parseList { DayForecast(HashMap(it)) }

}

https://github.com/antoniolg/Kotlin-for-Android-Developers/blob/master/app/src/main/java/com/antonioleiva/weatherapp/extensions/DatabaseExtensions.kt

fun <T : Any> SelectQueryBuilder.parseList(parser: (Map<String, Any?>) -> T): List<T> =
        parseList(object : MapRowParser<T> {
            override fun parseRow(columns: Map<String, Any?>): T = parser(columns)
})

回答1:

Anko's parseList takes a MapRowParser, not a function. This simplifies usage. With Anko version you'd write

.parseList { mapRowParser { DayForecast(HashMap(it)) } }

instead. That's assuming there is a constructor function like mapRowParser which I can't find in their sources; otherwise, you could write it quite trivially.

Or rather, it's already written for you in the example code, just not as a separate function:

fun <T> mapRowParser(parser: (Map<String, Any?>) -> T): MapRowParser<T> = 
    object : MapRowParser<T> {
        override fun parseRow(columns: Map<String, Any?>): T = parser(columns)
    }

I am honestly really surprised if this function doesn't exist already (maybe called something else, but what?). OTOH, if it does exist, Leiva should have used it.

回答2:

If you expected simple single column rows result, you can use ready-made Anko parsers:

• ShortParser
• IntParser
• LongParser
• FloatParser
• DoubleParser
• StringParser
• BlobParser

Your code might be:

val dailyForecast = select(DayForecastTable.NAME)
                .whereSimple(dailyRequest, zipCode.toString(), date.toString())
                .parseList(StringParser)

Or you can implement your own parser. Here below sample for three columns result (Int, String, String):

val userParser = rowParser { id: Int, name: String, email: String ->
    Triple(id, name, email)
}