Add GetVarNames() (#676)

**Summary of Changes**

1. Added `r.GetVarNames()` function to list all vars a route might need
in order to call `r.URL()`

---------

Co-authored-by: Anonymous <anonymous@github.com>
Co-authored-by: Corey Daley <cdaley@redhat.com>

README.md

@@ -366,6 +366,19 @@ url, err := r.Get("article").URL("subdomain", "news",
                                  "id", "42")
 ```
 
+To find all the required variables for a given route when calling `URL()`, the method `GetVarNames()` is available:
+```go
+r := mux.NewRouter()
+r.Host("{domain}").
+    Path("/{group}/{item_id}").
+    Queries("some_data1", "{some_data1}").
+    Queries("some_data2", "{some_data2}").
+    Name("article")
+
+// Will print [domain group item_id some_data1 some_data2] <nil>
+fmt.Println(r.Get("article").GetVarNames())
+
+```
 ### Walking Routes
 
 The `Walk` function on `mux.Router` can be used to visit all of the routes that are registered on a router. For example,

example_route_vars_test.go

@@ -0,0 +1,35 @@
+package mux_test
+
+import (
+	"fmt"
+	"github.com/gorilla/mux"
+)
+
+// This example demonstrates building a dynamic URL using
+// required vars and values retrieve from another source
+func ExampleRoute_GetVarNames() {
+	r := mux.NewRouter()
+
+	route := r.Host("{domain}").
+		Path("/{group}/{item_id}").
+		Queries("some_data1", "{some_data1}").
+		Queries("some_data2_and_3", "{some_data2}.{some_data3}")
+
+	dataSource := func(key string) string {
+		return "my_value_for_" + key
+	}
+
+	varNames, _ := route.GetVarNames()
+
+	pairs := make([]string, 0, len(varNames)*2)
+
+	for _, varName := range varNames {
+		pairs = append(pairs, varName, dataSource(varName))
+	}
+
+	url, err := route.URL(pairs...)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println(url.String())
+}

mux_test.go

@@ -2879,6 +2879,40 @@ func TestContextMiddleware(t *testing.T) {
 	r.ServeHTTP(rec, req)
 }
 
+func TestGetVarNames(t *testing.T) {
+	r := NewRouter()
+
+	route := r.Host("{domain}").
+		Path("/{group}/{item_id}").
+		Queries("some_data1", "{some_data1}").
+		Queries("some_data2_and_3", "{some_data2}.{some_data3}")
+
+	// Order of vars in the slice is not guaranteed, so just check for existence
+	expected := map[string]bool{
+		"domain":     true,
+		"group":      true,
+		"item_id":    true,
+		"some_data1": true,
+		"some_data2": true,
+		"some_data3": true,
+	}
+
+	varNames, err := route.GetVarNames()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if len(varNames) != len(expected) {
+		t.Fatalf("expected %d names, got %d", len(expected), len(varNames))
+	}
+
+	for _, varName := range varNames {
+		if !expected[varName] {
+			t.Fatalf("got unexpected %s", varName)
+		}
+	}
+}
+
 // mapToPairs converts a string map to a slice of string pairs
 func mapToPairs(m map[string]string) []string {
 	var i int

route.go

@@ -728,6 +728,25 @@ func (r *Route) GetHostTemplate() (string, error) {
 	return r.regexp.host.template, nil
 }
 
+// GetVarNames returns the names of all variables added by regexp matchers
+// These can be used to know which route variables should be passed into r.URL()
+func (r *Route) GetVarNames() ([]string, error) {
+	if r.err != nil {
+		return nil, r.err
+	}
+	var varNames []string
+	if r.regexp.host != nil {
+		varNames = append(varNames, r.regexp.host.varsN...)
+	}
+	if r.regexp.path != nil {
+		varNames = append(varNames, r.regexp.path.varsN...)
+	}
+	for _, regx := range r.regexp.queries {
+		varNames = append(varNames, regx.varsN...)
+	}
+	return varNames, nil
+}
+
 // prepareVars converts the route variable pairs into a map. If the route has a
 // BuildVarsFunc, it is invoked.
 func (r *Route) prepareVars(pairs ...string) (map[string]string, error) {