Add variadic RequestOption to Query and Mutate

.travis.yml

@@ -12,5 +12,5 @@ install:
 script:
   - go get -t -v ./...
   - diff -u <(echo -n) <(gofmt -d -s .)
-  - go tool vet .
+  - go vet .
   - go test -v -race ./...

graphql.go

@@ -30,22 +30,34 @@ func NewClient(url string, httpClient *http.Client) *Client {
 	}
 }
 
+// RequestOption is a variadic option for modifying an underlying HTTP request
+// for GraphQL.
+type RequestOption func(req *http.Request) error
+
+// WithRequestHeader sets an explicit HTTP header for usage
+func WithRequestHeader(key, value string) RequestOption {
+	return func(req *http.Request) error {
+		req.Header.Set(key, value)
+		return nil
+	}
+}
+
 // Query executes a single GraphQL query request,
 // with a query derived from q, populating the response into it.
 // q should be a pointer to struct that corresponds to the GraphQL schema.
-func (c *Client) Query(ctx context.Context, q interface{}, variables map[string]interface{}) error {
-	return c.do(ctx, queryOperation, q, variables)
+func (c *Client) Query(ctx context.Context, q interface{}, variables map[string]interface{}, opts ...RequestOption) error {
+	return c.do(ctx, queryOperation, q, variables, opts...)
 }
 
 // Mutate executes a single GraphQL mutation request,
 // with a mutation derived from m, populating the response into it.
 // m should be a pointer to struct that corresponds to the GraphQL schema.
-func (c *Client) Mutate(ctx context.Context, m interface{}, variables map[string]interface{}) error {
-	return c.do(ctx, mutationOperation, m, variables)
+func (c *Client) Mutate(ctx context.Context, m interface{}, variables map[string]interface{}, opts ...RequestOption) error {
+	return c.do(ctx, mutationOperation, m, variables, opts...)
 }
 
 // do executes a single GraphQL operation.
-func (c *Client) do(ctx context.Context, op operationType, v interface{}, variables map[string]interface{}) error {
+func (c *Client) do(ctx context.Context, op operationType, v interface{}, variables map[string]interface{}, opts ...RequestOption) error {
 	var query string
 	switch op {
 	case queryOperation:
@@ -65,7 +77,20 @@ func (c *Client) do(ctx context.Context, op operationType, v interface{}, variab
 	if err != nil {
 		return err
 	}
-	resp, err := ctxhttp.Post(ctx, c.httpClient, c.url, "application/json", &buf)
+	req, err := http.NewRequest(http.MethodPost, c.url, &buf)
+	if err != nil {
+		return err
+	}
+	req.Header.Set("Content-Type", "application/json")
+
+	for _, opt := range opts {
+		err := opt(req)
+		if err != nil {
+			return err
+		}
+	}
+
+	resp, err := ctxhttp.Do(ctx, c.httpClient, req)
 	if err != nil {
 		return err
 	}

graphql_test.go

@@ -153,6 +153,30 @@ func TestClient_Query_emptyVariables(t *testing.T) {
 	}
 }
 
+func TestClient_Query_requestOptions(t *testing.T) {
+	mux := http.NewServeMux()
+	mux.HandleFunc("/graphql", func(w http.ResponseWriter, req *http.Request) {
+		header := req.Header.Get("Custom-Request-Header")
+		if got, want := header, "test-custom-header"; got != want {
+			t.Errorf("got header: %v, want %v", got, want)
+		}
+
+		w.Header().Set("Content-Type", "application/json")
+		mustWrite(w, `{"data": {"user": {"name": "Gopher"}}}`)
+	})
+	client := graphql.NewClient("/graphql", &http.Client{Transport: localRoundTripper{handler: mux}})
+
+	var q struct {
+		User struct {
+			Name string
+		}
+	}
+	err := client.Query(context.Background(), &q, map[string]interface{}{}, graphql.WithRequestHeader("Custom-Request-Header", "test-custom-header"))
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
 // localRoundTripper is an http.RoundTripper that executes HTTP transactions
 // by using handler directly, instead of going over an HTTP connection.
 type localRoundTripper struct {