feat: implement handler interface, add example emitter (stdout)

internal/handler/handler.go

@@ -0,0 +1,27 @@
+package handler
+
+import (
+	"context"
+
+	"github.com/celo-org/celo-blockchain/core/types"
+	"github.com/grassrootseconomics/w3-celo"
+)
+
+type (
+	EmitterEmitFunc func(context.Context, []byte) error
+
+	Handler interface {
+		Handle(context.Context, *types.Log, EmitterEmitFunc) error
+	}
+)
+
+func New() []Handler {
+	transferHandler := &TransferHandler{
+		topicHash: w3.H("0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef"),
+		event:     w3.MustNewEvent("Transfer(address indexed _from, address indexed _to, uint256 _value)"),
+	}
+
+	return []Handler{
+		transferHandler,
+	}
+}

internal/handler/transfer.go

@@ -0,0 +1,55 @@
+package handler
+
+import (
+	"context"
+	"encoding/json"
+	"math/big"
+
+	"github.com/celo-org/celo-blockchain/common"
+	"github.com/celo-org/celo-blockchain/core/types"
+	"github.com/grassrootseconomics/w3-celo"
+)
+
+type (
+	TransferHandler struct {
+		topicHash common.Hash
+		event     *w3.Event
+	}
+
+	TransferEvent struct {
+		Contract string
+		From     string
+		To       string
+		Value    uint64
+	}
+)
+
+func (h *TransferHandler) Handle(ctx context.Context, log *types.Log, emitFn EmitterEmitFunc) error {
+	if log.Topics[0] == h.topicHash {
+		var (
+			from  common.Address
+			to    common.Address
+			value big.Int
+		)
+
+		if err := h.event.DecodeArgs(log, &from, &to, &value); err != nil {
+			return err
+		}
+
+		transferEvent := &TransferEvent{
+			Contract: log.Address.Hex(),
+			From:     from.Hex(),
+			To:       to.Hex(),
+			Value:    value.Uint64(),
+		}
+
+		jsonData, err := json.Marshal(transferEvent)
+		if err != nil {
+			return err
+		}
+
+		return emitFn(ctx, jsonData)
+	}
+
+	return nil
+}