Move initialization code closer to the appropriate structs. (#17)

Reviewed-on: #17
Co-authored-by: Anna Rose Wiggins <annabunches@gmail.com>
Co-committed-by: Anna Rose Wiggins <annabunches@gmail.com>

internal/mappingrules/rule_target_button.go

@@ -1,6 +1,12 @@
 package mappingrules
 
-import "github.com/holoplot/go-evdev"
+import (
+	"fmt"
+
+	"git.annabunches.net/annabunches/joyful/internal/configparser"
+	"git.annabunches.net/annabunches/joyful/internal/eventcodes"
+	"github.com/holoplot/go-evdev"
+)
 
 type RuleTargetButton struct {
 	DeviceName string
@@ -9,6 +15,25 @@ type RuleTargetButton struct {
 	Inverted   bool
 }
 
+func NewRuleTargetButtonFromConfig(targetConfig configparser.RuleTargetConfigButton, devs map[string]Device) (*RuleTargetButton, error) {
+	device, ok := devs[targetConfig.Device]
+	if !ok {
+		return nil, fmt.Errorf("non-existent device '%s'", targetConfig.Device)
+	}
+
+	eventCode, err := eventcodes.ParseCodeButton(targetConfig.Button)
+	if err != nil {
+		return nil, err
+	}
+
+	return NewRuleTargetButton(
+		targetConfig.Device,
+		device,
+		eventCode,
+		targetConfig.Inverted,
+	)
+}
+
 func NewRuleTargetButton(device_name string, device Device, code evdev.EvCode, inverted bool) (*RuleTargetButton, error) {
 	return &RuleTargetButton{
 		DeviceName: device_name,