httpin/README.md

# httpin

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HTTP Input for Go - Decode an HTTP request into a custom struct

**Define the struct for your input and then fetch your data!**

## Quick View

<table>
<tr>
  <th>BEFORE (use net/http)</th>
  <th>AFTER (use httpin)</th>
</tr>
<tr>
  <td>

```go
func ListUsers(rw http.ResponseWriter, r *http.Request) {
	page, err := strconv.ParseInt(r.FormValue("page"), 10, 64)
	if err != nil {
		// ... invalid page
		return
	}

	perPage, err := strconv.ParseInt(r.FormValue("per_page"), 10, 64)
	if err != nil {
		// ... invalid per_page
		return
	}

	isVip, _ := strconv.ParseBool(r.FormValue("is_vip"))

	// do sth.
}
```

  </td>
  <td>

```go
type ListUsersInput struct {
	Page    int  `in:"form=page"`
	PerPage int  `in:"form=per_page"`
	IsVip   bool `in:"form=is_vip"`
}

func ListUsers(rw http.ResponseWriter, r *http.Request) {
	interfaceInput, err := httpin.New(ListUsersInput{}).Decode(r)
	if err != nil {
		// err can be *httpin.InvalidField
		return
	}

	input := interfaceInput.(*ListUsersInput)
	// do sth.
}
```

  </td>
</tr>
</table>

## Features

- [x] Builtin directive `form` to decode a field from HTTP query, i.e. `http.Request.Form`
- [x] Builtin directive `header` to decode a field from HTTP headers, e.g. `http.Request.Header`
- [x] Builtin decoders used by `form` and `header` directives for basic types, e.g. `bool`, `int`, `int64`, `float32`, `time.Time`, ... [full list](./decoders.go)
- [x] Decode a field by inspecting a set of keys from the same source
- [x] Decode a field from multiple sources, e.g. both query and headers
- [ ] Register or replace decoders for both builtin basic types and custom types
- [x] Define input struct with embedded struct fields
- [x] Builtin directive `required` to tag a field as **required**
- [ ] Builtin encoders for basic types
- [ ] Register or replace encoders for both builtin basic types and custom types
- [x] Register custom directive executors to extend the field resolving abilities, see directive [required](./required.go) as an example and think about implementing your own directives like `trim`, `to_lowercase`, `base58_to_int`, etc.

## Sample User Defined Input Structs

```go
type Authorization struct {
	// Decode from multiple sources, the former with higher priority
	Token string `in:"form=access_token;header=x-api-token;required"`
}

type Pagination struct {
	Page int `in:"form=page"`

	// Decode from multiple keys in the same source, the former with higher priority
	PerPage int `in:"form=per_page,page_size"`
}

type ListUsersInput struct {
	Gender   string `in:"form=gender"`
	AgeRange []int  `in:"form=age_range"`
	IsMember bool   `in:"form=is_member"`

	Pagination    // Embedded field works
	Authorization // Embedded field works
}
```

## Advanced

### Use middleware handlers to reduce much more trivial code

First, set up the middleware for your handlers. We recommend using [alice](https://github.com/justinas/alice) to chain your HTTP middleware functions.

```go
func init() {
	http.Handle("/users", alice.New(
		httpin.NewInput(ListUsersInput{}),
	).ThenFunc(ListUsers))
}
```

Second, fetch your input with **only one line** of code.

```go
func ListUsers(rw http.ResponseWriter, r *http.Request) {
	input := r.Context().Value(httpin.Input).(*UserQuery)
	// do sth.
}
```

### Extend `httpin` by adding custom directives

Know the concept of a `Directive`:

```go
type Authorization struct {
	Token string `in:"form=access_token,token;header=x-api-token;required"`
	                  ^---------------------^ ^----------------^ ^------^
	                            d1                    d2            d3
}
```

There are three directives above:

- d1: `form=access_token,token`
- d2: `header=x-api-token`
- d3: `required`

A directive consists of two parts separated by an equal sign (`=`). The left part is the name of an executor who was designed to run this directive. The right part is a list of arguments separated by commas (`,`) which will be passed to the corresponding executor at runtime.

For instance, `form=access_token,token`, here `form` is the name of the executor, and `access_token,token` is the argument list which will be parsed as `[]string{"access_token", "token"}`.

There are several builtin directive executors, e.g. `form`, `header`, `required`, ... [full list](./directives.go). And you can define your own by:

First, create a **directive executor** by implementing the `httpin.DirectiveExecutor` interface:

```go
func toLower(ctx *DirectiveContext) error {
	if ctx.ValueType.Kind() != reflect.String {
		return errors.New("not a string")
	}

	currentValue := ctx.Value.Elem().String()
	newValue := strings.ToLower(currentValue)
	ctx.Value.Elem().SetString(newValue)
	return nil
}

// Adapt toLower to httpin.DirectiveExecutor.
var MyLowercaseDirectiveExecutor = httpin.DirectiveExecutorFunc(toLower)
```

Seconds, register it to `httpin`:

```go
httpin.RegisterDirectiveExecutor("to_lowercase", MyLowercaseDirectiveExecutor)
```

Finally, you can use your own directives in the struct tags:

```go
type Authorization struct {
	Token string `in:"form=token;required;to_lowercase"`
}
```

The directives will run in the order as they defined in the struct tag.