Add Entity.With

component.go

@@ -69,6 +69,31 @@ func (entity Entity) AddComponent(component interface{}) {
 	w.modifiedEntities = append(w.modifiedEntities, entity)
 }
 
+// With accepts any function which takes one or more components as arguments.
+// This function will block until the provided function returns.
+func (entity Entity) With(f interface{}) {
+	components := w.components[entity]
+	if components == nil {
+		return
+	}
+
+	t := reflect.TypeOf(f)
+	v := reflect.ValueOf(f)
+	numIn := t.NumIn()
+
+	if t.Kind() != reflect.Func || v.IsNil() || numIn == 0 {
+		panic("component.With() must be provided with a function containing one or more components as arguments")
+	}
+
+	args := make([]reflect.Value, numIn)
+	for i := 0; i < numIn; i++ {
+		id := componentIDByName(t.In(i).String())
+		args[i] = reflect.ValueOf(components[id])
+	}
+
+	v.Call(args)
+}
+
 // getComponent gets a Component of an Entity.
 func (entity Entity) getComponent(componentID componentID) interface{} {
 	components := w.components[entity]

doc.go

@@ -34,8 +34,9 @@ each systems' set of required matching components.
 Component Design Guidelines
 
 Components are located in a separate package, typically named component. They
-should be public (start with an uppercase letter) and may have any number of
-publicly accessible data fields.  They should not have any logic (i.e. game code)
+should be public (start with an uppercase letter) and may be of any type.
+Using only struct types (with zero or more public fields) and accessing the
+structs via pointer is recommended. Components should not have any logic (i.e. game code)
 within them, as all logic should be implemented within a system.
 
 System Design Guidelines

system.go

@@ -8,6 +8,10 @@ import (
 
 // System represents a system that runs continuously.
 //
+// Systems may specify any number of required components by adding public
+// fields to their struct. When no required components are specified, the
+// system will run for every active entity.
+//
 // While systems must implement the Update and Draw methods, the special error
 // value ErrUnregister may be returned at any time by systems to indicate the
 // method returning the error should not be called again.
@@ -22,5 +26,6 @@ type System interface {
 	Draw(entity Entity, screen *ebiten.Image) error
 }
 
-// ErrUnregister is the error returned to unregister from Draw or Update events.
+// ErrUnregister is a special error value which may be used to unregister a
+// system from Draw or Update events.
 var ErrUnregister = errors.New("unregister system")