From 23ef0b98e99389735ac9c662a29d9961861afcb2 Mon Sep 17 00:00:00 2001
From: Hans van den Bogert <hvdb@worldstream.nl>
Date: Thu, 8 Jul 2021 15:06:52 +0200
Subject: [PATCH] feature: Allow command fields to implement IsZero()

which implements the IsZeroer interface
---
 command_check.go      | 16 +++++++++++++++-
 command_check_test.go | 41 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 56 insertions(+), 1 deletion(-)

diff --git a/command_check.go b/command_check.go
index 8d3672e4..6052fc72 100644
--- a/command_check.go
+++ b/command_check.go
@@ -21,6 +21,12 @@ import (
 	"github.com/looplab/eventhorizon/uuid"
 )
 
+// IsZeroer is used to check if a type is zero-valued, and in that case
+// is not allowed to be used in a command. See CheckCommand
+type IsZeroer interface {
+	IsZero() bool
+}
+
 // CommandFieldError is returned by Dispatch when a field is incorrect.
 type CommandFieldError struct {
 	Field string
@@ -47,7 +53,15 @@ func CheckCommand(cmd Command) error {
 			continue // Optional field.
 		}
 
-		if isZero(rv.Field(i)) {
+		var zero bool
+		switch foo := rv.Field(i).Interface().(type) {
+		case IsZeroer:
+			zero = foo.IsZero()
+		default:
+			zero = isZero(rv.Field(i))
+		}
+
+		if zero {
 			return CommandFieldError{field.Name}
 		}
 	}
diff --git a/command_check_test.go b/command_check_test.go
index d3a27708..d18cf6b8 100644
--- a/command_check_test.go
+++ b/command_check_test.go
@@ -111,6 +111,26 @@ func TestCheckCommand(t *testing.T) {
 	if err == nil || err.Error() != "missing field: StringArray" {
 		t.Error("there should be a missing field error:", err)
 	}
+
+	// IsZero, fail on zeroable int
+	err = CheckCommand(&TestCommandZeroableInt{
+		TestID: uuid.New(),
+		TestZeroableInt: 0,
+		TestInt:         0,
+	})
+	if err == nil || err.Error() != "missing field: TestZeroableInt"{
+		t.Error("there should be a missing field error:", err)
+	}
+
+	// IsZero, do not fail  on plain int
+	err = CheckCommand(&TestCommandZeroableInt{
+		TestID: uuid.New(),
+		TestZeroableInt: 1,
+		TestInt:         0,
+	})
+	if err != nil {
+		t.Error("there should not be an error:", err)
+	}
 }
 
 type TestCommandFields struct {
@@ -288,3 +308,24 @@ var _ = Command(TestCommandArray{})
 func (t TestCommandArray) AggregateID() uuid.UUID       { return t.TestID }
 func (t TestCommandArray) AggregateType() AggregateType { return AggregateType("Test") }
 func (t TestCommandArray) CommandType() CommandType     { return CommandType("TestCommandArray") }
+
+type ZeroableInt int
+
+type TestCommandZeroableInt struct {
+	TestID          uuid.UUID
+	TestZeroableInt ZeroableInt
+	TestInt         int
+}
+
+var _ = Command(TestCommandZeroableInt{})
+
+
+func (t TestCommandZeroableInt) AggregateID() uuid.UUID       { return t.TestID }
+func (t TestCommandZeroableInt) AggregateType() AggregateType { return TestAggregateType }
+func (t TestCommandZeroableInt) CommandType() CommandType {
+	return CommandType("TestCommandZeroableInt")
+}
+
+func (z ZeroableInt) IsZero () bool {
+	return z == 0
+}