One 、 What is an order (Command) Pattern

Command pattern is a kind of behavioral design pattern , The purpose is to encapsulate a request as an object , This allows you to parameterize clients with different requests . Compared to another way of combining each command with the object calling the command to form a proprietary class , The advantage of command mode is that it decouples the object that calls the operation from the object that knows how to implement it , Adding new commands does not require modifying existing classes .

The structure of the command mode is as follows :



The participants are :

1.Invoker Requestor

Ask the command to execute the request , The caller of the command

2.Command Interface

3.ConcreteCommand Specific interface

4.Receiver The receiver

The actual implementer of the related operation of the command

5.Client

Collaborative process :

1.Client Create a ConcreteCommand Object and specify its Receiver object

2. some Invoker Object to store the ConcreteCommand object

3. The Invoker By calling Command Object's Excute Operation to submit a request . If the command is revocable ,ConcreteCommand Right now Excute The current state is stored before the operation to cancel the command

4.ConcreteCommand Object to call its Receiver To perform the request

Two 、go-redis command Related codes

// commands.go
// Invoker Requester interface 
type Cmdable interface {
Pipeline() Pipeliner
Pipelined(ctx context.Context, fn func(Pipeliner) error) ([]Cmder, error)
TxPipelined(ctx context.Context, fn func(Pipeliner) error) ([]Cmder, error)
TxPipeline() Pipeliner
Command(ctx context.Context) *CommandsInfoCmd
ClientGetName(ctx context.Context) *StringCmd
// ...
// And all Redis Command related methods
} // cmdable Realized Cmdable Interface
type cmdable func(ctx context.Context, cmd Cmder) error
func (c cmdable) Echo(ctx context.Context, message interface{}) *StringCmd {
cmd := NewStringCmd(ctx, "echo", message)
_ = c(ctx, cmd)
return cmd
}

It's worth mentioning here cmdable Is a function type ,func(ctx context.Context, cmd Cmder) error

And each cmdable There will be in every way _ = c(ctx, cmd), That is how to call cmd It's not clearly written here

Look back again redis.go, You'll find this piece of code

type Client struct {
*baseClient
cmdable
hooks
ctx context.Context
} func NewClient(opt *Options) *Client {
opt.init() c := Client{
baseClient: newBaseClient(opt, newConnPool(opt)),
ctx: context.Background(),
}
c.cmdable = c.Process // Underline return &c
}

c.cmdable = c.Process This line specifies how the request calls Command Of

stay ctrl+ Left click to track several layers , Will be in redis.go Find the specific procedure of the call in

// redis.go
func (c *baseClient) process(ctx context.Context, cmd Cmder) error {
......
err := cn.WithWriter(ctx, c.opt.WriteTimeout, func(wr *proto.Writer) error {
eturn writeCmd(wr, cmd)
}) err = cn.WithReader(ctx, c.cmdTimeout(cmd), cmd.readReply)
......

Then look for it Command, This is clearer , All in command.go in

// command.go
// Command Interface 
type Cmder interface {
Name() string
FullName() string
Args() []interface{}
String() string
stringArg(int) string
firstKeyPos() int8
setFirstKeyPos(int8) readTimeout() *time.Duration
readReply(rd *proto.Reader) error SetErr(error)
Err() error
} // Many more Cmder The concrete realization of , Part of the code for one of the implementations is as follows
type XPendingExtCmd struct {
baseCmd
val []XPendingExt
}
func (cmd *XPendingExtCmd) Val() []XPendingExt {
return cmd.val
}

I don't see it here Receiver, It's because of each Cmder All the functions are realized by themselves , There is no need for additional recipient objects .

3、 ... and 、 summary

Sometimes you have to submit a request to an object , But you don't know anything about the requested operation or the recipient of the request . You can use command mode at this time , By turning the request itself into an object, toolbox objects can make requests to unspecified application objects .

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