flyweight.go

// Package flyweight is an example of the Flyweight Pattern.
package flyweight

import "fmt"

// Flyweighter interface
type Flyweighter interface {
	Draw(width, height int, opacity float64) string
}

// FlyweightFactory implements a factory.
// If a suitable flyweighter is in pool, then returns it.
type FlyweightFactory struct {
	pool map[string]Flyweighter
}

// GetFlyweight creates or returns a suitable Flyweighter by state.
func (f *FlyweightFactory) GetFlyweight(filename string) Flyweighter {
	if f.pool == nil {
		f.pool = make(map[string]Flyweighter)
	}
	if _, ok := f.pool[filename]; !ok {
		f.pool[filename] = &ConcreteFlyweight{filename: filename}
	}
	return f.pool[filename]
}

// ConcreteFlyweight implements a Flyweighter interface.
type ConcreteFlyweight struct {
	filename string // internal state
}

// Draw draws image. Args width, height and opacity is external state.
func (f *ConcreteFlyweight) Draw(width, height int, opacity float64) string {
	return fmt.Sprintf("draw image: %s, width: %d, height: %d, opacity: %.2f", f.filename, width, height, opacity)
}