Docs for #1736 fluent binder (#172)

* Improve request binding documentation. See Echo pull request #1736 (labstack/echo#1736)

* Split binding to dedicated guide, some wording improvements

* Bind example more alike with other router examples.
Mention `UnixTimeNano()` as supported fluent binder function

Co-authored-by: Roland Lammel <[email protected]>

website/content/guide/binding.md

@@ -0,0 +1,205 @@
++++
+title = "Binding Request Data"
+description = "Binding request data"
+[menu.main]
+  name = "Binding"
+  parent = "guide"
++++
+
+## Bind using struct tags
+
+Echo provides following method to bind data from different sources (path params, query params, request body) to structure using 
+`Context#Bind(i interface{})` method.
+The default binder supports decoding application/json, application/xml and
+application/x-www-form-urlencoded data based on the Content-Type header.
+
+In the struct definitions each field can be tagged to restrict binding to specific source.
+
+* `query` - source is request query parameters.
+* `param` - source is route path parameter.
+* `form` - source is form. Values are taken from query and request body. Uses Go standard library form parsing.
+* `json` - source is request body. Uses Go [json](https://golang.org/pkg/encoding/json/) package fo unmarshalling.
+* `xml` - source is request body. Uses Go [xml](https://golang.org/pkg/encoding/xml/) package fo unmarshalling.
+
+```go
+type User struct {
+  ID string `path:"id" query:"id" form:"id" json:"id" xml:"id"`
+}
+```
+
+Request data is binded to the struct in given order:
+
+1. Path parameters
+2. Query parameters (only for GET/DELETE methods)
+3. Request body
+
+Notes:
+
+* For `query`, `param`, `form` **only** fields **with** tags are bound.
+* For `json` and `xml` can bind to *public* fields without tags but this is by their standard library implementation.
+* Each step can overwrite binded fields from the previous step. This means if your json request has query param
+  `&name=query` and body is `{"name": "body"}` then the result will be `User{Name: "body"}`.
+* To avoid security flaws try to avoid passing binded structs directly to other methods if
+  these structs contain fields that should not be bindable. It is advisable to have separate struct for binding and map it
+  explicitly to your business struct. Consider what will happen if your binded struct has public
+  field `IsAdmin bool` and request body would contain `{IsAdmin: true, Name: "hacker"}`.
+* When binding forms take note that Echo implementation uses standard library form parsing which parses form data 
+  from BOTH URL and BODY if content type is not MIMEMultipartForm. See documentation for [non-MIMEMultipartForm](https://golang.org/pkg/net/http/#Request.ParseForm)
+  and [MIMEMultipartForm](https://golang.org/pkg/net/http/#Request.ParseMultipartForm)
+* To bind data only from request body use following code
+  ```go
+  if err := (&DefaultBinder{}).BindBody(c, &payload); err != nil {
+    return err
+  }
+  ```
+* To bind data only from query parameters use following code
+  ```go
+  if err := (&DefaultBinder{}).BindQueryParams(c, &payload); err != nil {
+    return err
+  }
+  ```
+* To bind data only from path parameters use following code
+  ```go
+  if err := (&DefaultBinder{}).BindPathParams(c, &payload); err != nil {
+    return err
+  }
+  ```
+
+### Example
+
+Example below binds the request payload into `User` struct based on tags:
+
+```go
+// User
+type User struct {
+  Name  string `json:"name" form:"name" query:"name"`
+  Email string `json:"email" form:"email" query:"email"`
+}
+```
+
+```go
+e.POST("/users", func(c echo.Context) (err error) {
+  u := new(User)
+  if err = c.Bind(u); err != nil {
+    return
+  }
+  // To avoid security flaws try to avoid passing binded structs directly to other methods 
+  // if these structs contain fields that should not be bindable. 
+  user := UserDTO{
+    Name: u.Name,
+    Email: u.Email,
+    IsAdmin: false // because you could accidentally expose fields that should not be bind
+  }
+  executeSomeBusinessLogic(user)
+
+  return c.JSON(http.StatusOK, u)
+}
+```
+
+### JSON Data
+
+```sh
+curl -X POST http://localhost:1323/users \
+  -H 'Content-Type: application/json' \
+  -d '{"name":"Joe","email":"joe@labstack"}'
+```
+
+### Form Data
+
+```sh
+curl -X POST http://localhost:1323/users \
+  -d 'name=Joe' \
+  -d '[email protected]'
+```
+
+### Query Parameters
+
+```sh
+curl -X GET http://localhost:1323/users\?name\=Joe\&email\=[email protected]
+```
+
+## Fast binding with dedicated helpers
+
+For binding data found in a request a handful of helper functions are provided. This will allow binding of query parameters, path parameters or data found in the body like forms or JSON data.
+
+Following functions provide a handful of methods for binding to Go native types from request query or path parameters. These binders offer a fluent syntax and can be chained to configure, execute binding and handle errors. 
+
+* `echo.QueryParamsBinder(c)` - binds query parameters (source URL)
+* `echo.PathParamsBinder(c)` - binds path parameters (source URL)
+* `echo.FormFieldBinder(c)` - binds form fields (source URL + body). See also [Request.ParseForm](https://golang.org/pkg/net/http/#Request.ParseForm).
+
+A binder is usually completed by `BindError()` or `BindErrors()` which returns errors if binding fails.
+With `FailFast()` the binder can be configured stop binding on the first error or continue binding for 
+the binder call chain. Fail fast is enabled by default and should be disabled when using `BindErrors()`.
+
+`BindError()` returns the first bind error from binder and resets all errors in this binder.
+`BindErrors()` returns all bind errors from binder and resets errors in binder.
+
+```go
+// url =  "/api/search?active=true&id=1&id=2&id=3&length=25"
+var opts struct {
+  IDs []int64
+  Active bool
+}
+length := int64(50) // default length is 50
+
+// creates query params binder that stops binding at first error
+err := echo.QueryParamsBinder(c).
+  Int64("length", &length).
+  Int64s("ids", &opts.IDs).
+  Bool("active", &opts.Active).
+  BindError() // returns first binding error
+```
+
+### Supported types
+
+Types that are supported:
+
+* bool
+* float32
+* float64
+* int
+* int8
+* int16
+* int32
+* int64
+* uint
+* uint8/byte (does not support `bytes()`. Use BindUnmarshaler/CustomFunc to convert value from base64 etc to []byte{})
+* uint16
+* uint32
+* uint64
+* string
+* time
+* duration
+* BindUnmarshaler() interface
+* UnixTime() - converts unix time (integer) to time.Time
+* UnixTimeNano() - converts unix time with nano second precision (integer) to time.Time
+* CustomFunc() - callback function for your custom conversion logic
+
+For every supported type there are following methods:
+
+* `<Type>("param", &destination)` - if parameter value exists then binds it to given destination of that type i.e `Int64(...)`.
+* `Must<Type>("param", &destination)` - parameter value is required to exist, binds it to given destination of that type i.e `MustInt64(...)`.
+* `<Type>s("param", &destination)` - (for slices) if parameter values exists then binds it to given destination of that type i.e `Int64s(...)`.
+* `Must<Type>s("param", &destination)` - (for slices) parameter value is required to exist, binds it to given destination of that type i.e `MustInt64s(...)`.
+
+for some slice types `BindWithDelimiter("param", &dest, ",")` supports splitting parameter values before type conversion is done. For example URL `/api/search?id=1,2,3&id=1` can be bind to `[]int64{1,2,3,1}`
+
+## Custom Binder
+
+Custom binder can be registered using `Echo#Binder`.
+
+```go
+type CustomBinder struct {}
+
+func (cb *CustomBinder) Bind(i interface{}, c echo.Context) (err error) {
+  // You may use default binder
+  db := new(echo.DefaultBinder)
+  if err = db.Bind(i, c); err != echo.ErrUnsupportedMediaType {
+    return
+  }
+
+  // Define your custom implementation here
+  return
+}
+```