engine/exchange manager: Add support for custom exchanges (#749)

* engine: Setup exchange manager earlier

So it's available for applications before starting.

* engine: Add a custom exchange builder interface

Allows applications that import GCT as a lib to build new
custom exchanges without further modifications.

engine/engine.go

@@ -105,6 +105,8 @@ func NewFromSettings(settings *Settings, flagSet map[string]bool) (*Engine, erro
 		return nil, fmt.Errorf("failed to create script manager. Err: %s", err)
 	}
 
+	b.ExchangeManager = SetupExchangeManager()
+
 	validateSettings(&b, settings, flagSet)
 
 	return &b, nil
@@ -363,7 +365,6 @@ func (bot *Engine) Start() error {
 		bot.Config.PurgeExchangeAPICredentials()
 	}
 
-	bot.ExchangeManager = SetupExchangeManager()
 	gctlog.Debugln(gctlog.Global, "Setting up exchanges..")
 	err = bot.SetupExchanges()
 	if err != nil {

engine/exchange_manager.go

@@ -45,10 +45,17 @@ var (
 	ErrExchangeFailedToLoad  = errors.New("exchange failed to load")
 )
 
+// CustomExchangeBuilder interface allows external applications to create
+// custom/unsupported exchanges that satisfy the IBotExchange interface.
+type CustomExchangeBuilder interface {
+	NewExchangeByName(name string) (exchange.IBotExchange, error)
+}
+
 // ExchangeManager manages what exchanges are loaded
 type ExchangeManager struct {
 	m         sync.Mutex
 	exchanges map[string]exchange.IBotExchange
+	Builder   CustomExchangeBuilder
 }
 
 // SetupExchangeManager creates a new exchange manager
@@ -183,6 +190,9 @@ func (m *ExchangeManager) NewExchangeByName(name string) (exchange.IBotExchange,
 	case "zb":
 		exch = new(zb.ZB)
 	default:
+		if m.Builder != nil {
+			return m.Builder.NewExchangeByName(nameLower)
+		}
 		return nil, fmt.Errorf("%s, %w", nameLower, ErrExchangeNotFound)
 	}
 

engine/exchange_manager_test.go

@@ -1,10 +1,13 @@
 package engine
 
 import (
+	"fmt"
 	"strings"
 	"testing"
 
+	exchange "github.com/thrasher-corp/gocryptotrader/exchanges"
 	"github.com/thrasher-corp/gocryptotrader/exchanges/bitfinex"
+	"github.com/thrasher-corp/gocryptotrader/exchanges/sharedtestvalues"
 )
 
 func TestSetupExchangeManager(t *testing.T) {
@@ -79,3 +82,34 @@ func TestNewExchangeByName(t *testing.T) {
 		}
 	}
 }
+
+type ExchangeBuilder struct{}
+
+func (n ExchangeBuilder) NewExchangeByName(name string) (exchange.IBotExchange, error) {
+	var exch exchange.IBotExchange
+
+	switch name {
+	case "customex":
+		exch = new(sharedtestvalues.CustomEx)
+	default:
+		return nil, fmt.Errorf("%s, %w", name, ErrExchangeNotFound)
+	}
+
+	return exch, nil
+}
+
+func TestNewCustomExchangeByName(t *testing.T) {
+	m := SetupExchangeManager()
+	m.Builder = ExchangeBuilder{}
+	name := "customex"
+	exch, err := m.NewExchangeByName(name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err == nil {
+		exch.SetDefaults()
+		if !strings.EqualFold(exch.GetName(), name) {
+			t.Error("did not load expected exchange")
+		}
+	}
+}