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// Copyright 2016 Google Inc. All Rights Reserved.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
// Package iterator provides support for standard Google API iterators.
// See
package iterator
import (
// Done is returned by an iterator's Next method when the iteration is
// complete; when there are no more items to return.
var Done = errors.New("no more items in iterator")
// We don't support mixed calls to Next and NextPage because they play
// with the paging state in incompatible ways.
var errMixed = errors.New("iterator: Next and NextPage called on same iterator")
// PageInfo contains information about an iterator's paging state.
type PageInfo struct {
// Token is the token used to retrieve the next page of items from the
// API. You may set Token immediately after creating an iterator to
// begin iteration at a particular point. If Token is the empty string,
// the iterator will begin with the first eligible item.
// The result of setting Token after the first call to Next is undefined.
// After the underlying API method is called to retrieve a page of items,
// Token is set to the next-page token in the response.
Token string
// MaxSize is the maximum number of items returned by a call to the API.
// Set MaxSize as a hint to optimize the buffering behavior of the iterator.
// If zero, the page size is determined by the underlying service.
// Use Pager to retrieve a page of a specific, exact size.
MaxSize int
// The error state of the iterator. Manipulated by and Pager.
// This is a latch: it starts as nil, and once set should never change.
err error
// If true, no more calls to fetch should be made. Set to true when fetch
// returns an empty page token. The iterator is Done when this is true AND
// the buffer is empty.
atEnd bool
// Function that fetches a page from the underlying service. It should pass
// the pageSize and pageToken arguments to the service, fill the buffer
// with the results from the call, and return the next-page token returned
// by the service. The function must not remove any existing items from the
// buffer. If the underlying RPC takes an int32 page size, pageSize should
// be silently truncated.
fetch func(pageSize int, pageToken string) (nextPageToken string, err error)
// Function that clears the iterator's buffer, returning any currently buffered items.
bufLen func() int
// Function that returns the buffer, after setting the buffer variable to nil.
takeBuf func() interface{}
// Set to true on first call to or Pager.NextPage. Used to check
// for calls to both Next and NextPage with the same iterator.
nextCalled, nextPageCalled bool
// NewPageInfo exposes internals for iterator implementations.
// It is not a stable interface.
var NewPageInfo = newPageInfo
// If an iterator can support paging, its iterator-creating method should call
// this (via the NewPageInfo variable above).
// The fetch, bufLen and takeBuf arguments provide access to the
// iterator's internal slice of buffered items. They behave as described in
// PageInfo, above.
// The return value is the method bound to the returned PageInfo value.
// (Returning it avoids exporting
func newPageInfo(fetch func(int, string) (string, error), bufLen func() int, takeBuf func() interface{}) (*PageInfo, func() error) {
pi := &PageInfo{
fetch: fetch,
bufLen: bufLen,
takeBuf: takeBuf,
return pi,
// Remaining returns the number of items available before the iterator makes another API call.
func (pi *PageInfo) Remaining() int { return pi.bufLen() }
// next provides support for an iterator's Next function. An iterator's Next
// should return the error returned by next if non-nil; else it can assume
// there is at least one item in its buffer, and it should return that item and
// remove it from the buffer.
func (pi *PageInfo) next() error {
pi.nextCalled = true
if pi.err != nil { // Once we get an error, always return it.
// TODO(jba): fix so users can retry on transient errors? Probably not worth it.
return pi.err
if pi.nextPageCalled {
pi.err = errMixed
return pi.err
// Loop until we get some items or reach the end.
for pi.bufLen() == 0 && !pi.atEnd {
if err := pi.fill(pi.MaxSize); err != nil {
pi.err = err
return pi.err
if pi.Token == "" {
pi.atEnd = true
// Either the buffer is non-empty or pi.atEnd is true (or both).
if pi.bufLen() == 0 {
// The buffer is empty and pi.atEnd is true, i.e. the service has no
// more items.
pi.err = Done
return pi.err
// Call the service to fill the buffer, using size and pi.Token. Set pi.Token to the
// next-page token returned by the call.
// If fill returns a non-nil error, the buffer will be empty.
func (pi *PageInfo) fill(size int) error {
tok, err := pi.fetch(size, pi.Token)
if err != nil {
pi.takeBuf() // clear the buffer
return err
pi.Token = tok
return nil
// Pageable is implemented by iterators that support paging.
type Pageable interface {
// PageInfo returns paging information associated with the iterator.
PageInfo() *PageInfo
// Pager supports retrieving iterator items a page at a time.
type Pager struct {
pageInfo *PageInfo
pageSize int
// NewPager returns a pager that uses iter. Calls to its NextPage method will
// obtain exactly pageSize items, unless fewer remain. The pageToken argument
// indicates where to start the iteration. Pass the empty string to start at
// the beginning, or pass a token retrieved from a call to Pager.NextPage.
// If you use an iterator with a Pager, you must not call Next on the iterator.
func NewPager(iter Pageable, pageSize int, pageToken string) *Pager {
p := &Pager{
pageInfo: iter.PageInfo(),
pageSize: pageSize,
p.pageInfo.Token = pageToken
if pageSize <= 0 {
p.pageInfo.err = errors.New("iterator: page size must be positive")
return p
// NextPage retrieves a sequence of items from the iterator and appends them
// to slicep, which must be a pointer to a slice of the iterator's item type.
// Exactly p.pageSize items will be appended, unless fewer remain.
// The first return value is the page token to use for the next page of items.
// If empty, there are no more pages. Aside from checking for the end of the
// iteration, the returned page token is only needed if the iteration is to be
// resumed a later time, in another context (possibly another process).
// The second return value is non-nil if an error occurred. It will never be
// the special iterator sentinel value Done. To recognize the end of the
// iteration, compare nextPageToken to the empty string.
// It is possible for NextPage to return a single zero-length page along with
// an empty page token when there are no more items in the iteration.
func (p *Pager) NextPage(slicep interface{}) (nextPageToken string, err error) {
p.pageInfo.nextPageCalled = true
if p.pageInfo.err != nil {
return "", p.pageInfo.err
if p.pageInfo.nextCalled {
p.pageInfo.err = errMixed
return "", p.pageInfo.err
if p.pageInfo.bufLen() > 0 {
return "", errors.New("must call NextPage with an empty buffer")
// The buffer must be empty here, so takeBuf is a no-op. We call it just to get
// the buffer's type.
wantSliceType := reflect.PtrTo(reflect.ValueOf(p.pageInfo.takeBuf()).Type())
if slicep == nil {
return "", errors.New("nil passed to Pager.NextPage")
vslicep := reflect.ValueOf(slicep)
if vslicep.Type() != wantSliceType {
return "", fmt.Errorf("slicep should be of type %s, got %T", wantSliceType, slicep)
for p.pageInfo.bufLen() < p.pageSize {
if err := p.pageInfo.fill(p.pageSize - p.pageInfo.bufLen()); err != nil {
p.pageInfo.err = err
return "", p.pageInfo.err
if p.pageInfo.Token == "" {
e := vslicep.Elem()
e.Set(reflect.AppendSlice(e, reflect.ValueOf(p.pageInfo.takeBuf())))
return p.pageInfo.Token, nil