appview/pagination: context based pagination

introduce helper methods: `IntoContext` and `FromContext`
these will help using pagination obj form context easier and make it
less error-prune by using private empty struct as a key instead of raw
string

Signed-off-by: Seongmin Lee <[e-mail]>

appview/issues/issues.go

···
 		isOpen = true
 	}
 
-	page, ok := r.Context().Value("page").(pagination.Page)
-	if !ok {
-		l.Error("failed to get page")
-		page = pagination.FirstPage()
-	}
+	page := pagination.FromContext(r.Context())
 
 	user := rp.oauth.GetUser(r)
 	f, err := rp.repoResolver.Resolve(r)

appview/middleware/middleware.go

···
 			}
 		}
 
-		ctx := context.WithValue(r.Context(), "page", page)
+		ctx := pagination.IntoContext(r.Context(), page)
 		next.ServeHTTP(w, r.WithContext(ctx))
 	})
 }

appview/notifications/notifications.go

···
 	l := n.logger.With("handler", "notificationsPage")
 	user := n.oauth.GetUser(r)
 
-	page, ok := r.Context().Value("page").(pagination.Page)
-	if !ok {
-		l.Error("failed to get page")
-		page = pagination.FirstPage()
-	}
+	page := pagination.FromContext(r.Context())
 
 	total, err := db.CountNotifications(
 		n.db,

appview/pagination/page.go

···
 package pagination
 
+import "context"
+
 type Page struct {
 	Offset int // where to start from
 	Limit  int // number of items in a page
···
 		Offset: 0,
 		Limit:  30,
 	}
+}
+
+type ctxKey struct{}
+
+func IntoContext(ctx context.Context, page Page) context.Context {
+	return context.WithValue(ctx, ctxKey{}, page)
+}
+
+func FromContext(ctx context.Context) Page {
+	if ctx == nil {
+		return FirstPage()
+	}
+	v := ctx.Value(ctxKey{})
+	if v == nil {
+		return FirstPage()
+	}
+	page, ok := v.(Page)
+	if !ok {
+		return FirstPage()
+	}
+	return page
 }
 
 func (p Page) Previous() Page {

appview/state/gfi.go

···
 func (s *State) GoodFirstIssues(w http.ResponseWriter, r *http.Request) {
 	user := s.oauth.GetUser(r)
 
-	page, ok := r.Context().Value("page").(pagination.Page)
-	if !ok {
-		page = pagination.FirstPage()
-	}
+	page := pagination.FromContext(r.Context())
 
 	goodFirstIssueLabel := fmt.Sprintf("at://%s/%s/%s", consts.TangledDid, tangled.LabelDefinitionNSID, "good-first-issue")