365 lines
12 KiB
Go
365 lines
12 KiB
Go
// 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 fx
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import "go.uber.org/dig"
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// In can be embedded in a constructor's parameter struct to take advantage of
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// advanced dependency injection features.
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//
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// Modules should take a single parameter struct that embeds an In in order to
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// provide a forward-compatible API: since adding fields to a struct is
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// backward-compatible, modules can then add optional dependencies in minor
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// releases.
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//
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// # Parameter Structs
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//
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// Fx constructors declare their dependencies as function parameters. This can
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// quickly become unreadable if the constructor has a lot of 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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// To improve the readability of constructors like this, create a struct that
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// lists all the dependencies as fields and change the function to accept that
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// struct instead. The new struct is called a parameter struct.
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//
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// Fx has first class support for parameter structs: any struct embedding
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// fx.In gets treated as a parameter struct, so the individual fields in the
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// struct are supplied via dependency injection. Using a parameter struct, we
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// can make the constructor above much more readable:
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//
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// type HandlerParams struct {
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// fx.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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// Though it's rarely a good idea, constructors can receive any combination of
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// parameter structs 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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// # Optional Dependencies
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//
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// Constructors often have optional dependencies on some types: if those types are
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// missing, they can operate in a degraded state. Fx supports optional
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// dependencies via the `optional:"true"` tag to fields on parameter structs.
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//
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// type UserGatewayParams struct {
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// fx.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 isn't available in the container, the constructor
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// receives the field's zero value.
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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("Caching disabled")
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// }
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// // ...
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// }
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//
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// Constructors that declare optional dependencies MUST gracefully handle
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// situations in which those dependencies are 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 require the application container to hold multiple values of
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// the same type. For details on producing named values, see the documentation
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// for the Out type.
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//
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// Fx allows functions to consume named values via the `name:".."` tag on
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// parameter structs. Note that both the name AND type of the fields on the
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// parameter struct must match the corresponding result struct.
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//
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// type GatewayParams struct {
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// fx.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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// fx.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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// To make it easier to produce and consume many values of the same type, Fx
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// supports named, unordered collections called value groups. For details on
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// producing value groups, see the documentation for the Out type.
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//
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// Functions can depend on a value group by requesting a slice tagged with
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// `group:".."`. This will execute all constructors that provide a value to
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// that group in an unspecified order, then collect all the results into a
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// single slice. Keep in mind that this makes the types of the parameter and
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// result struct fields different: if a group of constructors each returns
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// type T, parameter structs consuming the group must use a field of type []T.
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//
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// type ServerParams struct {
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// fx.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. Fx 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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// # Soft Value Groups
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//
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// A soft value group can be thought of as a best-attempt at populating the
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// group with values from constructors that have already run. In other words,
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// if a constructor's output type is only consumed by a soft value group,
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// it will not be run.
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//
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// Note that Fx does not guarantee precise execution order of constructors
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// or invokers, which means that the change in code that affects execution
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// ordering of other constructors or functions will affect the values
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// populated in this group.
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//
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// To declare a soft relationship between a group and its constructors, use
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// the `soft` option on the group tag (`group:"[groupname],soft"`).
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// This option is only valid for input parameters.
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//
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// type Params struct {
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// fx.In
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//
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// Handlers []Handler `group:"server,soft"`
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// Logger *zap.Logger
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// }
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//
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// NewHandlerAndLogger := func() (Handler, *zap.Logger) { ... }
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// NewHandler := func() Handler { ... }
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// Foo := func(Params) { ... }
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//
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// app := fx.New(
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// fx.Provide(fx.Annotate(NewHandlerAndLogger, fx.ResultTags(`group:"server"`))),
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// fx.Provide(fx.Annotate(NewHandler, fx.ResultTags(`group::"server"`))),
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// fx.Invoke(Foo),
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// )
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//
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// The only constructor called is `NewHandlerAndLogger`, because this also provides
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// `*zap.Logger` needed in the `Params` struct received by `Foo`. The Handlers
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// group will be populated with a single Handler returned by `NewHandlerAndLogger`.
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//
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// In the next example, the slice `s` isn't populated as the provider would be
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// called only because `strings` soft group value is its only consumer.
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//
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// app := fx.New(
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// fx.Provide(
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// fx.Annotate(
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// func() (string, int) { return "hello", 42 },
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// fx.ResultTags(`group:"strings"`),
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// ),
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// ),
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// fx.Invoke(
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// fx.Annotate(func(s []string) {
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// // s will be an empty slice
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// }, fx.ParamTags(`group:"strings,soft"`)),
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// ),
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// )
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//
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// In the next example, the slice `s` will be populated because there is a
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// consumer for the same type which is not a `soft` dependency.
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//
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// app := fx.New(
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// fx.Provide(
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// fx.Annotate(
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// func() string { "hello" },
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// fx.ResultTags(`group:"strings"`),
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// ),
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// ),
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// fx.Invoke(
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// fx.Annotate(func(b []string) {
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// // b is []string{"hello"}
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// }, fx.ParamTags(`group:"strings"`)),
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// ),
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// fx.Invoke(
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// fx.Annotate(func(s []string) {
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// // s is []string{"hello"}
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// }, fx.ParamTags(`group:"strings,soft"`)),
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// ),
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// )
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//
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// # Unexported fields
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//
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// By default, a type that embeds fx.In may not have any unexported fields. The
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// following will return an error if used with Fx.
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//
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// type Params struct {
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// fx.In
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//
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// Logger *zap.Logger
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// mu sync.Mutex
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// }
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//
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// If you have need of unexported fields on such a type, you may opt-into
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// ignoring unexported fields by adding the ignore-unexported struct tag to the
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// fx.In. For example,
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//
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// type Params struct {
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// fx.In `ignore-unexported:"true"`
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//
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// Logger *zap.Logger
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// mu sync.Mutex
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// }
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type In = dig.In
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// Out is the inverse of In: it can be embedded in result structs to take
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// advantage of advanced features.
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//
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// Modules should return a single result struct that embeds an Out in order to
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// provide a forward-compatible API: since adding fields to a struct is
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// backward-compatible, minor releases can provide additional types.
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//
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// # Result Structs
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//
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// Result structs are the inverse of parameter structs (discussed in the In
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// documentation). These structs represent multiple outputs from a
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// single function as fields. Fx treats all structs embedding fx.Out as result
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// structs, so other constructors can rely on the result struct's fields
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// directly.
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//
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// Without result structs, we sometimes have function definitions like this:
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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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// With result structs, we can make this both more readable and easier to
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// modify in the future:
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//
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// type Gateways struct {
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// fx.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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// # Named Values
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//
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// Some use cases require the application container to hold multiple values of
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// the same type. For details on consuming named values, see the documentation
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// for the In type.
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//
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// A constructor that produces a result struct can tag any field with
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// `name:".."` to have the corresponding value added to the graph under the
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// specified name. An application may contain at most one unnamed value of a
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// given type, but may contain any number of named values of the same type.
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//
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// type ConnectionResult struct {
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// fx.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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// # Value Groups
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//
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// To make it easier to produce and consume many values of the same type, Fx
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// supports named, unordered collections called value groups. For details on
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// consuming value groups, see the documentation for the In type.
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//
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// Constructors can send values into value groups by returning a result struct
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// tagged with `group:".."`.
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//
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// type HandlerResult struct {
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// fx.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, but
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// the ordering of the final collection is unspecified. Keep in mind that
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// value groups require parameter and result structs to use fields with
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// different types: if a group of constructors each returns type T, parameter
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// structs consuming the group must use a field of type []T.
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//
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// To provide multiple values for a group from a result struct, produce a
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// slice and use the `,flatten` option on the group tag. This indicates that
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// each element in the slice should be injected into the group individually.
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//
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// type IntResult struct {
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// fx.Out
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//
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// Handler []int `group:"server"` // Consume as [][]int
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// Handler []int `group:"server,flatten"` // Consume as []int
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// }
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type Out = dig.Out
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