FileFromFS (gin-gonic#2112)

* Context.FileFromFS added

* Context File and FileFromFS examples at README

Co-authored-by: Bo-Yi Wu <appleboy.tw@gmail.com>

README.md

@@ -1182,6 +1182,24 @@ func main() {
 }
 ```
 
+### Serving data from file
+
+```go
+func main() {
+	router := gin.Default()
+
+	router.GET("/local/file", func(c *gin.Context) {
+		c.File("local/file.go")
+	})
+
+	var fs http.FileSystem = // ...
+	router.GET("/fs/file", func(c *gin.Context) {
+		c.FileFromFS("fs/file.go", fs)
+	})
+}
+
+```
+
 ### Serving data from reader
 
 ```go

context.go

@@ -925,6 +925,17 @@ func (c *Context) File(filepath string) {
 	http.ServeFile(c.Writer, c.Request, filepath)
 }
 
+// FileFromFS writes the specified file from http.FileSytem into the body stream in an efficient way.
+func (c *Context) FileFromFS(filepath string, fs http.FileSystem) {
+	defer func(old string) {
+		c.Request.URL.Path = old
+	}(c.Request.URL.Path)
+
+	c.Request.URL.Path = filepath
+
+	http.FileServer(fs).ServeHTTP(c.Writer, c.Request)
+}
+
 // FileAttachment writes the specified file into the body stream in an efficient way
 // On the client side, the file will typically be downloaded with the given filename
 func (c *Context) FileAttachment(filepath, filename string) {

context_test.go

@@ -992,6 +992,19 @@ func TestContextRenderFile(t *testing.T) {
 	assert.Equal(t, "text/plain; charset=utf-8", w.Header().Get("Content-Type"))
 }
 
+func TestContextRenderFileFromFS(t *testing.T) {
+	w := httptest.NewRecorder()
+	c, _ := CreateTestContext(w)
+
+	c.Request, _ = http.NewRequest("GET", "/some/path", nil)
+	c.FileFromFS("./gin.go", Dir(".", false))
+
+	assert.Equal(t, http.StatusOK, w.Code)
+	assert.Contains(t, w.Body.String(), "func New() *Engine {")
+	assert.Equal(t, "text/plain; charset=utf-8", w.Header().Get("Content-Type"))
+	assert.Equal(t, "/some/path", c.Request.URL.Path)
+}
+
 func TestContextRenderAttachment(t *testing.T) {
 	w := httptest.NewRecorder()
 	c, _ := CreateTestContext(w)