Add generic pager implementation

sdk/azcore/runtime/pager.go

@@ -0,0 +1,95 @@
+//go:build go1.18
+// +build go1.18
+
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+package runtime
+
+import (
+	"context"
+	"errors"
+	"net/http"
+	"reflect"
+
+	"github.com/Azure/azure-sdk-for-go/sdk/azcore/internal/shared"
+	"github.com/Azure/azure-sdk-for-go/sdk/azcore/policy"
+)
+
+// PageProcessor contains the required data for constructing a Pager.
+type PageProcessor[T any] struct {
+	// Do sends the request to fetch the next page.
+	Do func(*policy.Request) (*http.Response, error)
+
+	// More returns a boolean indicating if there are more pages to fetch.
+	// It uses the provided page to make the determination.
+	More func(T) bool
+
+	// Requester creates the request to fetch the first page.
+	Requester func(context.Context) (*policy.Request, error)
+
+	// Advancer creates the request to fetch subsequent pages.
+	Advancer func(context.Context, T) (*policy.Request, error)
+
+	// Responder handles the responses when fetching pages.
+	Responder func(*http.Response) (T, error)
+}
+
+// Pager provides operations for iterating over paged responses.
+type Pager[T any] struct {
+	current   T
+	processor PageProcessor[T]
+	firstPage bool
+}
+
+// NewPager creates an instance of Pager using the specified PageProcessor.
+func NewPager[T any](processor PageProcessor[T]) *Pager[T] {
+	return &Pager[T]{
+		processor: processor,
+		firstPage: true,
+	}
+}
+
+// More returns true if there are more pages to retrieve.
+func (p *Pager[T]) More() bool {
+	if !reflect.ValueOf(p.current).IsZero() {
+		return p.processor.More(p.current)
+	}
+	return true
+}
+
+// NextPage advances the pager to the next page.
+func (p *Pager[T]) NextPage(ctx context.Context) (T, error) {
+	var req *policy.Request
+	var err error
+	if !reflect.ValueOf(p.current).IsZero() && p.firstPage {
+		// we get here if it's an LRO-pager, we already have the first page
+		p.firstPage = false
+		return p.current, nil
+	} else if !reflect.ValueOf(p.current).IsZero() {
+		if !p.processor.More(p.current) {
+			return *new(T), errors.New("no more pages")
+		}
+		req, err = p.processor.Advancer(ctx, p.current)
+	} else {
+		// non-LRO case
+		p.firstPage = false
+		req, err = p.processor.Requester(ctx)
+	}
+	if err != nil {
+		return *new(T), err
+	}
+	resp, err := p.processor.Do(req)
+	if err != nil {
+		return *new(T), err
+	}
+	if !shared.HasStatusCode(resp, http.StatusOK) {
+		return *new(T), shared.NewResponseError(resp)
+	}
+	result, err := p.processor.Responder(resp)
+	if err != nil {
+		return *new(T), err
+	}
+	p.current = result
+	return p.current, nil
+}