page.go

package spotify

import (
	"errors"
	"reflect"
)

// ErrNoMorePages is the error returned when you attempt to get the next
// (or previous) set of data but you've reached the end of the data set.
var ErrNoMorePages = errors.New("spotify: no more pages")

// This file contains the types that implement Spotify's paging object.
// See: https://developer.spotify.com/web-api/object-model/#paging-object

// basePage contains all of the fields in a Spotify paging object, except
// for the actual items.  This type is meant to be embedded in other types
// that add the Items field.
type basePage struct {
	// A link to the Web API Endpoint returning the full
	// result of this request.
	Endpoint string `json:"href"`
	// The maximum number of items in the response, as set
	// in the query (or default value if unset).
	Limit int `json:"limit"`
	// The offset of the items returned, as set in the query
	// (or default value if unset).
	Offset int `json:"offset"`
	// The total number of items available to return.
	Total int `json:"total"`
	// The URL to the next page of items (if available).
	Next string `json:"next"`
	// The URL to the previous page of items (if available).
	Previous string `json:"previous"`
}

// FullArtistPage contains FullArtists returned by the Web API.
type FullArtistPage struct {
	basePage
	Artists []FullArtist `json:"items"`
}

// SimpleAlbumPage contains SimpleAlbums returned by the Web API.
type SimpleAlbumPage struct {
	basePage
	Albums []SimpleAlbum `json:"items"`
}

// SavedAlbumPage contains SavedAlbums returned by the Web API.
type SavedAlbumPage struct {
	basePage
	Albums []SavedAlbum `json:"items"`
}

// SimplePlaylistPage contains SimplePlaylists returned by the Web API.
type SimplePlaylistPage struct {
	basePage
	Playlists []SimplePlaylist `json:"items"`
}

// SimpleTrackPage contains SimpleTracks returned by the Web API.
type SimpleTrackPage struct {
	basePage
	Tracks []SimpleTrack `json:"items"`
}

// FullTrackPage contains FullTracks returned by the Web API.
type FullTrackPage struct {
	basePage
	Tracks []FullTrack `json:"items"`
}

// SavedTrackPage contains SavedTracks return by the Web API.
type SavedTrackPage struct {
	basePage
	Tracks []SavedTrack `json:"items"`
}

// PlaylistTrackPage contains information about tracks in a playlist.
type PlaylistTrackPage struct {
	basePage
	Tracks []PlaylistTrack `json:"items"`
}

// CategoryPage contains Category objects returned by the Web API.
type CategoryPage struct {
	basePage
	Categories []Category `json:"items"`
}

// pageable is an internal interface for types that support paging
// by embedding basePage.
type pageable interface{ canPage() }

func (b basePage) canPage() {}

// NextPage fetches the next page of items and writes them into p.
// It returns ErrNoMorePages if p already contains the last page.
func (c *Client) NextPage(p pageable) error {
	val := reflect.ValueOf(p).Elem()
	field := val.FieldByName("Next")
	nextURL := field.Interface().(string)

	if len(nextURL) == 0 {
		return ErrNoMorePages
	}

	// Zero out the page so that we can overwrite it in the next
	// call to get. This is necessary because encoding/json does
	// not clear out existing values when unmarshaling JSON null.
	zero := reflect.Zero(val.Type())
	val.Set(zero)

	return c.get(nextURL, p)
}

// PreviousPage fetches the previous page of items and writes them into p.
// It returns ErrNoMorePages if p already contains the last page.
func (c *Client) PreviousPage(p pageable) error {
	val := reflect.ValueOf(p).Elem()
	field := val.FieldByName("Previous")
	prevURL := field.Interface().(string)

	if len(prevURL) == 0 {
		return ErrNoMorePages
	}

	// Zero out the page so that we can overwrite it in the next
	// call to get. This is necessary because encoding/json does
	// not clear out existing values when unmarshaling JSON null.
	zero := reflect.Zero(val.Type())
	val.Set(zero)

	return c.get(prevURL, p)
}