Fix link to Swagger spec in Swagger UI by reworking URL config.

This makes it more clear that this config option should only be set if
the Swagger spec document is not expected to be hosted by this
middleware but instead by an external website or service. It also fixes
the link to the JSON document under the base path on the Swagger UI.

Also set the swaggerFiles handler prefix again, as this is required.

swagger.go

@@ -1,20 +1,20 @@
 package echoSwagger
 
 import (
+	"github.com/labstack/echo/v4"
+	"github.com/swaggo/files"
+	"github.com/swaggo/swag"
 	"html/template"
 	"log"
 	"net/http"
 	"path"
-
-	"github.com/labstack/echo/v4"
-	"github.com/swaggo/files"
-	"github.com/swaggo/swag"
 )
 
 // Config stores echoSwagger configuration variables.
 type Config struct {
-	// The url pointing to API definition (normally swagger.json or swagger.yaml). Default is `doc.json`.
-	URL string
+	// If the Swagger specification JSON document is external to the web server, this should be set to the full URL
+	// to the specification document.
+	ExternalURL *string
 
 	// The information for OAuth2 integration, if any.
 	OAuth *OAuthConfig
@@ -33,10 +33,15 @@ type OAuthConfig struct {
 	AppName string
 }
 
+type templateData struct {
+	SwaggerSpecFullURL string
+	Config
+}
+
 // URL presents the url pointing to API definition (normally swagger.json or swagger.yaml).
 func URL(url string) func(c *Config) {
 	return func(c *Config) {
-		c.URL = url
+		c.ExternalURL = &url
 	}
 }
 
@@ -48,9 +53,7 @@ func EchoWrapHandler(confs ...func(c *Config)) echo.HandlerFunc {
 
 	handler := swaggerFiles.Handler
 
-	config := &Config{
-		URL: "doc.json",
-	}
+	config := &Config{}
 
 	for _, c := range confs {
 		c(config)
@@ -64,20 +67,35 @@ func EchoWrapHandler(confs ...func(c *Config)) echo.HandlerFunc {
 	}
 
 	return func(c echo.Context) error {
-		_, path := path.Split(c.Request().RequestURI)
+		prefix, pathFile := path.Split(c.Request().RequestURI)
+		handler.Prefix = prefix
+
+		templateData := templateData{
+			SwaggerSpecFullURL: path.Join(prefix, "doc.json"),
+			Config:             *config,
+		}
+
+		if config.ExternalURL != nil {
+			// Configuration specifies override for Swagger specification URL.
+			templateData.SwaggerSpecFullURL = *config.ExternalURL
+		}
 
-		switch path {
+		switch pathFile {
 		case "":
 			fallthrough
 		case "index.html":
-			index.Execute(c.Response().Writer, config)
+			if err := index.Execute(c.Response().Writer, templateData); err != nil {
+				return echo.NewHTTPError(http.StatusInternalServerError, err)
+			}
 		case "doc.json":
 			doc, err := swag.ReadDoc()
 			if err != nil {
 				return echo.NewHTTPError(http.StatusInternalServerError, err)
 			}
 			c.Response().Header().Set("Content-Type", "application/json")
-			c.Response().Write([]byte(doc))
+			if _, err := c.Response().Write([]byte(doc)); err != nil {
+				return echo.NewHTTPError(http.StatusInternalServerError, err)
+			}
 		default:
 			handler.ServeHTTP(c.Response().Writer, c.Request())
 		}
@@ -161,7 +179,7 @@ const indexTempl = `<!-- HTML for static distribution bundle build -->
 window.onload = function() {
   // Build a system
   const ui = SwaggerUIBundle({
-    url: "{{.URL}}",
+    url: "{{.SwaggerSpecFullURL}}",
     dom_id: '#swagger-ui',
     validatorUrl: null,
     presets: [

swagger_test.go

@@ -37,14 +37,12 @@ func TestWrapHandler(t *testing.T) {
 func TestConfig(t *testing.T) {
 	router := echo.New()
 
-	swaggerHandler := EchoWrapHandler(func(c *Config) {
-		c.URL = "example.org/swagger.json"
-	})
-	router.GET("/*", swaggerHandler)
+	swaggerHandler := URL("http://example.org/swagger.json")
+	router.GET("/*", EchoWrapHandler(swaggerHandler))
 
 	w := performRequest("GET", "/", router)
 	assert.Equal(t, 200, w.Code)
-	assert.Contains(t, w.Body.String(), "url: \"example.org/swagger.json\"")
+	assert.Contains(t, w.Body.String(), "url: \"http:\\/\\/example.org\\/swagger.json\"")
 }
 
 func TestConfigWithOAuth(t *testing.T) {