go-roster

Go package provides interfaces and methods for defining internal lookup tables (or "rosters") for registering and instantiatinge custom interfaces with multiple implementations.

Documentation

Example

The following example is the body of the roster_test.go file:

package roster

import (
	"context"
	"fmt"
	"net/url"
	"testing"
)

// Create a toy interface that might have multiple implementations including a common
// method signature for creating instantiations of that interface.

type Example interface {
	String() string
}

type ExampleInitializationFunc func(context.Context, string) (Example, error)

func RegisterExample(ctx context.Context, scheme string, init_func ExampleInitializationFunc) error {
	return example_roster.Register(ctx, scheme, init_func)
}

func NewExample(ctx context.Context, uri string) (Example, error) {

	u, err := url.Parse(uri)

	if err != nil {
		return nil, fmt.Errorf("Failed to parse URI, %w", err)
	}

	scheme := u.Scheme

	i, err := example_roster.Driver(ctx, scheme)

	if err != nil {
		return nil, fmt.Errorf("Failed to find registeration for %s, %w", scheme, err)
	}

	init_func := i.(ExampleInitializationFunc)
	return init_func(ctx, uri)
}

// Something that implements the Example interface

type StringExample struct {
	Example
	value string
}

func NewStringExample(ctx context.Context, uri string) (Example, error) {

	u, err := url.Parse(uri)

	if err != nil {
		return nil, fmt.Errorf("Failed to parse URL, %w", err)
	}

	s := &StringExample{
		value: u.Path,
	}

	return s, nil
}

func (e *StringExample) String() string {
	return e.value
}

// Create a global "roster" of implementations of the Example interface

var example_roster Roster

// Ensure that there is a valid roster (for use by the code handling the Example interface)
// and register the StringExample implementation

func init() {

	ctx := context.Background()

	r, err := NewDefaultRoster()

	if err != nil {
		panic(err)
	}

	example_roster = r

	err = RegisterExample(ctx, "string", NewStringExample)

	if err != nil {
		panic(err)
	}
}

func TestRoster(t *testing.T) {

	ctx := context.Background()

	e, err := NewExample(ctx, "string:///helloworld")

	if err != nil {
		t.Fatalf("Failed to create new example, %v", err)
	}

	v := e.String()

	if v != "/helloworld" {
		t.Fatalf("Unexpected result: '%s'", v)
	}
}

Concrete examples

• https://github.com/whosonfirst/go-reader
• https://github.com/whosonfirst/go-writer