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pager.go
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pager.go
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package pagination
import (
"errors"
"github.com/rackspace/gophercloud"
)
var (
// ErrPageNotAvailable is returned from a Pager when a next or previous page is requested, but does not exist.
ErrPageNotAvailable = errors.New("The requested page does not exist.")
)
// Page must be satisfied by the result type of any resource collection.
// It allows clients to interact with the resource uniformly, regardless of whether or not or how it's paginated.
// Generally, rather than implementing this interface directly, implementors should embed one of the concrete PageBase structs,
// instead.
// Depending on the pagination strategy of a particular resource, there may be an additional subinterface that the result type
// will need to implement.
type Page interface {
// NextPageURL generates the URL for the page of data that follows this collection.
// Return "" if no such page exists.
NextPageURL() (string, error)
// IsEmpty returns true if this Page has no items in it.
IsEmpty() (bool, error)
}
// Pager knows how to advance through a specific resource collection, one page at a time.
type Pager struct {
client *gophercloud.ServiceClient
initialURL string
createPage func(r PageResult) Page
Err error
// Headers supplies additional HTTP headers to populate on each paged request.
Headers map[string]string
}
// NewPager constructs a manually-configured pager.
// Supply the URL for the first page, a function that requests a specific page given a URL, and a function that counts a page.
func NewPager(client *gophercloud.ServiceClient, initialURL string, createPage func(r PageResult) Page) Pager {
return Pager{
client: client,
initialURL: initialURL,
createPage: createPage,
}
}
// WithPageCreator returns a new Pager that substitutes a different page creation function. This is
// useful for overriding List functions in delegation.
func (p Pager) WithPageCreator(createPage func(r PageResult) Page) Pager {
return Pager{
client: p.client,
initialURL: p.initialURL,
createPage: createPage,
}
}
func (p Pager) fetchNextPage(url string) (Page, error) {
resp, err := Request(p.client, p.Headers, url)
if err != nil {
return nil, err
}
remembered, err := PageResultFrom(resp)
if err != nil {
return nil, err
}
return p.createPage(remembered), nil
}
// EachPage iterates over each page returned by a Pager, yielding one at a time to a handler function.
// Return "false" from the handler to prematurely stop iterating.
func (p Pager) EachPage(handler func(Page) (bool, error)) error {
if p.Err != nil {
return p.Err
}
currentURL := p.initialURL
for {
currentPage, err := p.fetchNextPage(currentURL)
if err != nil {
return err
}
empty, err := currentPage.IsEmpty()
if err != nil {
return err
}
if empty {
return nil
}
ok, err := handler(currentPage)
if err != nil {
return err
}
if !ok {
return nil
}
currentURL, err = currentPage.NextPageURL()
if err != nil {
return err
}
if currentURL == "" {
return nil
}
}
}