如何对作为切片的接口{}进行切片？

Question

The datastore.GetMulti(c appengine.Context, key []*Key, dst interface{}) API allows me to get 1000 entities at most. I want to get more.

An obvious way to solve this generically is to create a wrapper function mypkg.GetMulti() which sub slices (key[0:1000], key[1000:2000]...) the original arguments and calls datastore.GetMulti() several times with them.

It's pretty clear how to sub slice key []*Key, but how do I sub slice dst interface{} which could be:

// dst must be a []S, []*S, []I or []P, for some struct type S, some interface
// type I, or some non-interface non-pointer type P such that P or *P
// implements PropertyLoadSaver. If an []I, each element must be a valid dst
// for Get: it must be a struct pointer or implement PropertyLoadSaver.
//
// As a special case, PropertyList is an invalid type for dst, even though a
// PropertyList is a slice of structs. It is treated as invalid to avoid being
// mistakenly passed when []PropertyList was intended.

Answer 1

Since you are the caller of datastore.GetMulti which takes an interface{} argument, you can provide any concrete value as that argument; it doesn't need to be converted to the empty-interface type beforehand. In other words, anything and everything implements the empty interface, so just pass that thing.

func GetMulti() {
    mySlice := make([]Whatever, 3000, 3000)
    for i := 0; i < 3; i++ {
        subSlice := mySlice[i * 1000 : (i + 1) * 1000]
        datastore.GetMulti(c,k, subSlice) // 'c' and 'k' assumed to be defined
    }
}

In case mypkg.GetMulti should be a generic function, taking an interface{} value as well, then you'll have to use reflection as in the following example where instead of fmt.Println with the length of the subslice you'd call datastore.GetMulti with each subslice:

package main

import "fmt"
import "reflect"

func GetMulti(i interface{}) {
    v := reflect.ValueOf(i)
    if v.Kind() != reflect.Slice {
        panic("argument not a slice")
    }
    l := v.Len()
    p := (l / 1000)
    for i := 0; i < p; i++ {
        fmt.Println(v.Slice(i*1000, (i+1)*1000).Len())
    }
    fmt.Println(v.Slice(p*1000, l).Len())

}

func main() {
    s := make([]int, 3560, 3560)
    GetMulti(s)
}