add a new 'overview' MVC example

Former-commit-id: f73cbf6010595c639f6c5b5119e2ec41bc9802a5

_examples/mvc/repository/services/README.md

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+# Service Layer
+
+The package which has access to call functions from the "repositories" and "models" ("datamodels" only in that simple example). It should contain the domain logic.

_examples/mvc/repository/services/movie_service.go

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+// file: services/movie_service.go
+
+package services
+
+import (
+	"github.com/kataras/iris/v12/_examples/mvc/repository/datamodels"
+	"github.com/kataras/iris/v12/_examples/mvc/repository/repositories"
+)
+
+// MovieService handles some of the CRUID operations of the movie datamodel.
+// It depends on a movie repository for its actions.
+// It's here to decouple the data source from the higher level compoments.
+// As a result a different repository type can be used with the same logic without any aditional changes.
+// It's an interface and it's used as interface everywhere
+// because we may need to change or try an experimental different domain logic at the future.
+type MovieService interface {
+	GetAll() []datamodels.Movie
+	GetByID(id int64) (datamodels.Movie, bool)
+	DeleteByID(id int64) bool
+	UpdatePosterAndGenreByID(id int64, poster string, genre string) (datamodels.Movie, error)
+}
+
+// NewMovieService returns the default movie service.
+func NewMovieService(repo repositories.MovieRepository) MovieService {
+	return &movieService{
+		repo: repo,
+	}
+}
+
+type movieService struct {
+	repo repositories.MovieRepository
+}
+
+// GetAll returns all movies.
+func (s *movieService) GetAll() []datamodels.Movie {
+	return s.repo.SelectMany(func(_ datamodels.Movie) bool {
+		return true
+	}, -1)
+}
+
+// GetByID returns a movie based on its id.
+func (s *movieService) GetByID(id int64) (datamodels.Movie, bool) {
+	return s.repo.Select(func(m datamodels.Movie) bool {
+		return m.ID == id
+	})
+}
+
+// UpdatePosterAndGenreByID updates a movie's poster and genre.
+func (s *movieService) UpdatePosterAndGenreByID(id int64, poster string, genre string) (datamodels.Movie, error) {
+	// update the movie and return it.
+	return s.repo.InsertOrUpdate(datamodels.Movie{
+		ID:     id,
+		Poster: poster,
+		Genre:  genre,
+	})
+}
+
+// DeleteByID deletes a movie by its id.
+//
+// Returns true if deleted otherwise false.
+func (s *movieService) DeleteByID(id int64) bool {
+	return s.repo.Delete(func(m datamodels.Movie) bool {
+		return m.ID == id
+	}, 1)
+}