Method build

gapil/bapi/encode.go:2164–2202

build calls cb to encode an object instance to a proto. The return proto of cb is cached into i so each instance is only built once. i is a pointer to the proto instances slice (&e.instances.Foo) m is a pointer to the encoder instances map (e.maps.Foo) n is the object to encode out is a pointer to

(i, m, n, out interface{}, cb interface{})

// out is a pointer to the output identifier
// cb is a func with the signature: func() *OutputType
func (e *encoder) build(i, m, n, out interface{}, cb interface{}) {
	iV := reflect.ValueOf(i)
	mV := reflect.ValueOf(m)
	nV := reflect.ValueOf(n)
	outV := reflect.ValueOf(out)
	cbV := reflect.ValueOf(cb)

	if !nV.IsValid() ||
		((nV.Kind() == reflect.Ptr || nV.Kind() == reflect.Interface) && nV.IsNil()) {
		// n is nil.
		// Don't assign to out to keep it at default 0 (representing nil)
		return
	}

	if existing := mV.MapIndex(nV); existing.IsValid() {
		// Already built this object.
		outV.Elem().Set(existing)
		return
	}

	idx := iV.Elem().Len()

	// The ID is the index + 1. And ID of 0 represents a nil.
	idV := reflect.ValueOf(uint64(idx + 1))
	outV.Elem().Set(idV)
	mV.SetMapIndex(nV, idV)

	// Grow the slice to hold the new entry. We do this before the call to cb
	// as this might call build() again, reordering the elements of the i slice.
	iV.Elem().Set(reflect.Append(iV.Elem(), reflect.Zero(iV.Type().Elem().Elem())))

	// Build
	pV := cbV.Call([]reflect.Value{})[0]

	// Assign the output proto value to the content instances for the given id.
	iV.Elem().Index(idx).Set(pV)

	checkMessageEncodes(pV.Interface())
}

// Help debug `proto: bapi.Content: illegal tag 0 (wire type 0)` errors
func checkMessageEncodes(v interface{}) {