350 lines
11 KiB
Go
350 lines
11 KiB
Go
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// Copyright (c) 2019 Uber Technologies, Inc.
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//
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// Permission is hereby granted, free of charge, to any person obtaining a copy
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// of this software and associated documentation files (the "Software"), to deal
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// in the Software without restriction, including without limitation the rights
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// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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// copies of the Software, and to permit persons to whom the Software is
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// furnished to do so, subject to the following conditions:
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//
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// The above copyright notice and this permission notice shall be included in
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// all copies or substantial portions of the Software.
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//
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// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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// THE SOFTWARE.
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// Package dig provides an opinionated way of resolving object dependencies.
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//
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// Status
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//
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// STABLE. No breaking changes will be made in this major version.
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//
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// Container
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//
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// Dig exposes type Container as an object capable of resolving a directed
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// acyclic dependency graph. Use the New function to create one.
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//
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// c := dig.New()
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//
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// Provide
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//
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// Constructors for different types are added to the container by using the
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// Provide method. A constructor can declare a dependency on another type by
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// simply adding it as a function parameter. Dependencies for a type can be
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// added to the graph both, before and after the type was added.
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//
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// err := c.Provide(func(conn *sql.DB) (*UserGateway, error) {
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// // ...
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// })
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// if err != nil {
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// // ...
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// }
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//
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// if err := c.Provide(newDBConnection); err != nil {
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// // ...
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// }
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//
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// Multiple constructors can rely on the same type. The container creates a
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// singleton for each retained type, instantiating it at most once when
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// requested directly or as a dependency of another type.
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//
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// err := c.Provide(func(conn *sql.DB) *CommentGateway {
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// // ...
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// })
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// if err != nil {
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// // ...
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// }
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//
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// Constructors can declare any number of dependencies as parameters and
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// optionally, return errors.
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//
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// err := c.Provide(func(u *UserGateway, c *CommentGateway) (*RequestHandler, error) {
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// // ...
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// })
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// if err != nil {
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// // ...
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// }
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//
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// if err := c.Provide(newHTTPServer); err != nil {
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// // ...
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// }
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//
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// Constructors can also return multiple results to add multiple types to the
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// container.
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//
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// err := c.Provide(func(conn *sql.DB) (*UserGateway, *CommentGateway, error) {
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// // ...
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// })
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// if err != nil {
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// // ...
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// }
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//
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// Constructors that accept a variadic number of arguments are treated as if
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// they don't have those arguments. That is,
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//
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// func NewVoteGateway(db *sql.DB, options ...Option) *VoteGateway
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//
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// Is treated the same as,
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//
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// func NewVoteGateway(db *sql.DB) *VoteGateway
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//
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// The constructor will be called with all other dependencies and no variadic
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// arguments.
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//
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// Invoke
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//
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// Types added to to the container may be consumed by using the Invoke method.
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// Invoke accepts any function that accepts one or more parameters and
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// optionally, returns an error. Dig calls the function with the requested
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// type, instantiating only those types that were requested by the function.
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// The call fails if any type or its dependencies (both direct and transitive)
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// were not available in the container.
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//
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// err := c.Invoke(func(l *log.Logger) {
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// // ...
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// })
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// if err != nil {
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// // ...
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// }
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//
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// err := c.Invoke(func(server *http.Server) error {
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// // ...
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// })
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// if err != nil {
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// // ...
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// }
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//
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// Any error returned by the invoked function is propagated back to the
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// caller.
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//
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// Parameter Objects
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//
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// Constructors declare their dependencies as function parameters. This can
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// very quickly become unreadable if the constructor has a lot of
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// dependencies.
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//
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// func NewHandler(users *UserGateway, comments *CommentGateway, posts *PostGateway, votes *VoteGateway, authz *AuthZGateway) *Handler {
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// // ...
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// }
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//
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// A pattern employed to improve readability in a situation like this is to
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// create a struct that lists all the parameters of the function as fields and
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// changing the function to accept that struct instead. This is referred to as
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// a parameter object.
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//
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// Dig has first class support for parameter objects: any struct embedding
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// dig.In gets treated as a parameter object. The following is equivalent to
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// the constructor above.
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//
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// type HandlerParams struct {
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// dig.In
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//
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// Users *UserGateway
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// Comments *CommentGateway
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// Posts *PostGateway
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// Votes *VoteGateway
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// AuthZ *AuthZGateway
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// }
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//
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// func NewHandler(p HandlerParams) *Handler {
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// // ...
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// }
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//
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// Handlers can receive any combination of parameter objects and parameters.
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//
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// func NewHandler(p HandlerParams, l *log.Logger) *Handler {
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// // ...
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// }
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//
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// Result Objects
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//
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// Result objects are the flip side of parameter objects. These are structs
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// that represent multiple outputs from a single function as fields in the
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// struct. Structs embedding dig.Out get treated as result objects.
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//
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// func SetupGateways(conn *sql.DB) (*UserGateway, *CommentGateway, *PostGateway, error) {
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// // ...
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// }
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//
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// The above is equivalent to,
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//
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// type Gateways struct {
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// dig.Out
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//
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// Users *UserGateway
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// Comments *CommentGateway
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// Posts *PostGateway
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// }
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//
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// func SetupGateways(conn *sql.DB) (Gateways, error) {
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// // ...
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// }
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//
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// Optional Dependencies
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//
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// Constructors often don't have a hard dependency on some types and
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// are able to operate in a degraded state when that dependency is missing.
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// Dig supports declaring dependencies as optional by adding an
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// `optional:"true"` tag to fields of a dig.In struct.
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//
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// Fields in a dig.In structs that have the `optional:"true"` tag are treated
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// as optional by Dig.
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//
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// type UserGatewayParams struct {
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// dig.In
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//
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// Conn *sql.DB
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// Cache *redis.Client `optional:"true"`
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// }
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//
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// If an optional field is not available in the container, the constructor
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// will receive a zero value for the field.
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//
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// func NewUserGateway(p UserGatewayParams, log *log.Logger) (*UserGateway, error) {
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// if p.Cache == nil {
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// log.Print("Logging disabled")
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// }
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// // ...
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// }
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//
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// Constructors that declare dependencies as optional MUST handle the case of
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// those dependencies being absent.
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//
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// The optional tag also allows adding new dependencies without breaking
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// existing consumers of the constructor.
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//
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// Named Values
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//
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// Some use cases call for multiple values of the same type. Dig allows adding
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// multiple values of the same type to the container with the use of Named
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// Values.
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//
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// Named Values can be produced by passing the dig.Name option when a
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// constructor is provided. All values produced by that constructor will have
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// the given name.
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//
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// Given the following constructors,
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//
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// func NewReadOnlyConnection(...) (*sql.DB, error)
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// func NewReadWriteConnection(...) (*sql.DB, error)
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//
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// You can provide *sql.DB into a Container under different names by passing
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// the dig.Name option.
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//
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// c.Provide(NewReadOnlyConnection, dig.Name("ro"))
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// c.Provide(NewReadWriteConnection, dig.Name("rw"))
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//
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// Alternatively, you can produce a dig.Out struct and tag its fields with
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// `name:".."` to have the corresponding value added to the graph under the
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// specified name.
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//
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// type ConnectionResult struct {
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// dig.Out
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//
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// ReadWrite *sql.DB `name:"rw"`
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// ReadOnly *sql.DB `name:"ro"`
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// }
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//
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// func ConnectToDatabase(...) (ConnectionResult, error) {
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// // ...
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// return ConnectionResult{ReadWrite: rw, ReadOnly: ro}, nil
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// }
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//
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// Regardless of how a Named Value was produced, it can be consumed by another
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// constructor by accepting a dig.In struct which has exported fields with the
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// same name AND type that you provided.
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//
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// type GatewayParams struct {
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// dig.In
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//
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// WriteToConn *sql.DB `name:"rw"`
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// ReadFromConn *sql.DB `name:"ro"`
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// }
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//
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// The name tag may be combined with the optional tag to declare the
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// dependency optional.
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//
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// type GatewayParams struct {
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// dig.In
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//
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// WriteToConn *sql.DB `name:"rw"`
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// ReadFromConn *sql.DB `name:"ro" optional:"true"`
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// }
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//
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// func NewCommentGateway(p GatewayParams, log *log.Logger) (*CommentGateway, error) {
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// if p.ReadFromConn == nil {
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// log.Print("Warning: Using RW connection for reads")
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// p.ReadFromConn = p.WriteToConn
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// }
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// // ...
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// }
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//
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// Value Groups
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//
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// Added in Dig 1.2.
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//
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// Dig provides value groups to allow producing and consuming many values of
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// the same type. Value groups allow constructors to send values to a named,
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// unordered collection in the container. Other constructors can request all
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// values in this collection as a slice.
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//
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// Constructors can send values into value groups by returning a dig.Out
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// struct tagged with `group:".."`.
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//
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// type HandlerResult struct {
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// dig.Out
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//
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// Handler Handler `group:"server"`
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// }
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//
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// func NewHelloHandler() HandlerResult {
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// ..
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// }
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//
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// func NewEchoHandler() HandlerResult {
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// ..
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// }
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//
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// Any number of constructors may provide values to this named collection.
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// Other constructors can request all values for this collection by requesting
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// a slice tagged with `group:".."`. This will execute all constructors that
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// provide a value to that group in an unspecified order.
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//
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// type ServerParams struct {
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// dig.In
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//
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// Handlers []Handler `group:"server"`
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// }
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//
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// func NewServer(p ServerParams) *Server {
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// server := newServer()
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// for _, h := range p.Handlers {
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// server.Register(h)
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// }
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// return server
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// }
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//
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// Note that values in a value group are unordered. Dig makes no guarantees
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// about the order in which these values will be produced.
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//
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// Value groups can be used to provide multiple values for a group from a
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// dig.Out using slices, however considering groups are retrieved by requesting
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// a slice this implies that the values must be retrieved using a slice of
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// slices. As of dig v1.9.0, if you want to provide individual elements to the
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// group instead of the slice itself, you can add the `flatten` modifier to the
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// group from a dig.Out.
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//
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// type IntResult struct {
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// dig.Out
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//
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// Handler []int `group:"server"` // [][]int from dig.In
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// Handler []int `group:"server,flatten"` // []int from dig.In
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// }
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//
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package dig // import "go.uber.org/dig"
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