tagscanner

This package is a helper library for creating a library that interacts with external data using structure tags.

It conceals complex reflection, Go's code analysis, type mapping codes from your code.

It contains three features:

• Extract values from struct (runtimescan.Encode())
• Inject values to struct (runtimescan.Decode())
• Generate code from struct code (staticscan.Scan())

runtimescan package dynamically analyses struct instances and works. staticscan package works for static analysis and code generation.

Basic Behavior

I define the word in this library (it is as same as encoding/json):

Encode: extract values from struct Decode: inject values to struct

For decoding, user implements type that satisfies Decoder interface. For encoding, user does Encoder interface.

runtimescan.Decode() and runtimescan.Encode will receive these instance. In both cases, this library calls ParseTag() method. User's logic analyses tag string and returns the information. This information will be passed to the next methods.

For decoding, ExtractValue() will be called. It receives tag information instance that ParseTag() returns. Return value ofExtractValue() will be passed to instance of struct.

For encoding, VisitField() will be called. It receives tag information instance that ParseTag() returns. In addition to this, VisitField() will be received the value that is extracted from struct instance.

Basic Usages

Write data from struct's instance to other container(runtimescan.Encode())

First, create a structure that satisfies the runtimescan.Encoder interface. Set the output destination to the field of the structure.

There are some helper functions. runtimescan.BasicParseTag() is a one of them. if you want to put only the field name in the tag, like encoding/json.

Then implement VisitField() that receives field value and stores to destination object (in this case, dest).

type encoder struct {
	dest map[string]any
}

func (m encoder) ParseTag(name, tagStr, pathStr string, elemType reflect.Type) (tag any, err error) {
	return runtimescan.BasicParseTag(name, tagStr, pathStr, elemType)
}

func (m *encoder) VisitField(tag, value any) (err error) {
	t := tag.(*runtimescan.BasicTag)
	m.dest[t.Tag] = value
	return nil
}

func (m encoder) EnterChild(tag any) (err error) {
	return nil
}

func (m encoder) LeaveChild(tag any) (err error) {
	return nil
}

At last, create an entry point function.

func Encode(dest map[string]any, src any) error {
	enc := &encoder{
		dest: dest,
	}
	return runtimescan.Encode(src, "map", enc)
}

Write data to struct(runtimescan/Decode())

First, create a structure that satisfies the runtimescan.Decoder interface.

Implement ExtractValue() method. The result value of this method will be passed to target struct.

type decoder struct {
	src map[string]any
}

func (m decoder) ParseTag(name, tagStr, pathStr string, elemType reflect.Type) (tag any, err error) {
	return runtimescan.BasicParseTag(name, tagStr, pathStr, elemType)
}

func (m *decoder) ExtractValue(tag any) (value any, err error) {
	t := tag.(*runtimescan.BasicTag)
	v, ok := m.src[t.Tag]
	if !ok {
		return nil, runtimescan.Skip
	}
	return v, nil
}

At last, create an entry point function.

func Decode(dest any, src map[string]any) error {
	dec := &decoder{
		src: src,
	}
	return runtimescan.Decode(dest, []string{"map"}, dec)
}

Generation code from structs' tag fields(staticscan.Scan())

This package provides functions to analyze and generate codes(``staticscan.Scan()`)

Utility functions

This package contains several utility functions.

• runtimescan.BasicParseTag(name, tagStr, pathStr string, elemType reflect.Type)

  This is the simplest implementation of ParseTag(). It returns tag value or lower field name if the field doesn't have tag.

• runtimescan.Str2PrimitiveValue(v str)

  It generates primitive from string like "1", "true". It is for creating primitive from string in tag.

• runtimescan.IsPointerOfStruct(i any), runtimescan.IsPointerOfSliceOfStruct(i any), runtimescan.IsPointerOfSliceOfPointerOfStruct(i any)

  Check the pointer passed as any is *struct or *[]struct or *[]*struct. This is useful to check type definition in Decode function.

• runtimescan.NewStructInstance(i any)

  Generate a new instance based on passed type. Whether the input is *struct or *[]struct or *[]*struct, it returns *struct.

Advanced usage samples

Compare two structure

Call runtimescan.Encode() for each instance and stores the structs' fields into map. Then compare the result in map.

Copy structure

Call runtimescan.Encode() for source instance and stores the struct's fields into map. Then call runtimescan.Decode().

Examples

examples/restmap

This maps HTTP request to struct instance by using runtimescan.Decode. Prepare struct like the following and pass its instance and http.Request to restmpa.Decode(). body tag detects body format by checking Content-Type header and accepts application/x-www-form-urlencoded, multipart/form-data, application/json format. It also support file uploading. rest:"path:param" can extract one of requested path when you uses chi router.

type Request struct {
	Method  string                `rest:"method"`
	Auth    string                `rest:"header:Authorization"`
	TraceID string                `rest:"cookie:trace-id"`
	Title   string                `rest:"body:title-field"`
	File    multipart.File        `rest:"body:file-field"`
	Header  *multipart.FileHeader `rest:"body:file-field"`
	Ctx     context.Context       `rest:"context"`
}

examples/binarypatternmatch

Loads binary data by using runtimescan.Decode. It assigns byte arrays into struct's instance.

• num specifies length (bits or bytes)
• << >> specifies literal. You can use string or number. The number can has length by using /.

type Image struct {
	Header     string `bytes:"<<HEAD>>"`
	Height     int32  `bytes:"4"`
    Width      int32  `bytes:"4"`
	ReadOnly   bool   `bits:"1"`
	_          byte   `bits:"3"`
	ColorType  byte   `bits:"4"'`
	CheckDigit byte   `bits:"<<0x5/6>>"`
}

func main() {
	var image Image
	f, _ := os.Open("imagefile")
    err := binarypatternmatch.Decode(&image, f)
}

Search tags statically

Extrude struct's information by parsing codes statically. This is for code generation.

staticscan.Scan(rootPath, tagName: string) ([]staticscan.Struct, error)

License

Apache2