Detailed explanation of select in golang

Be careful

Monitoring case in , If the conditions are not met, it will block

If multiple conditions are met, choose one to execute

select It doesn't have a loop , Requiring an outer layer fo

default Usually not , Busy polling will occur

break Can only jump out of select One of them case

Added default branch , So whether the expression involving channel operation is blocked or not ,select Statements are not blocked . If those expressions are blocked , Or it doesn't meet the evaluation conditions , Then the default branch will be selected and executed .

If no default branch is added , So once all case None of the expressions satisfy the evaluation condition , that select The statement will be blocked . Until at least one of them case Until the expression satisfies the condition .

If select Statement finds that multiple candidate branches meet the selection criteria at the same time , Then it will use a pseudo-random algorithm to select one of these branches and execute . Be careful , Even if select The statement is found when awakened , It will do the same

effect

go It provides a keyword select, adopt select Can monitor channel Data flow on

select The usage and switch The language is very similar , from select Start a new selection block , Each selection block condition is defined by case To describe

And switch Statement can select any condition that can use equality comparison ,select There are more restrictions , One of the biggest restrictions is that each case There must be a IO operation

for {
    select {
        case <-chan1:
            //.....
        case chan2<-1:
            //....
        default:
            // It didn't work , Get into ......
    }
}

Copy

In a select In the sentence ,go The language evaluates each statement sent and received from beginning to end in sequence

If any of these statements can continue ( That is, it is not blocked ), Then select any one of the executable statements to use

If no statement can be executed ( That is, all channels are blocked ), So there are two possibilities :

If given default sentence , Then it will carry out default sentence , At the same time, the execution of the program will start from select Restore... In the statement after the statement

without default sentence , that select The statement will be blocked , Until at least one communication can go on

prevent channel Timeout mechanism

Sometimes there will be co process blocking , So how can we avoid this situation ? We can use select To set the timeout

func main() {
   c := make(chan int)
   o := make(chan bool)
   go func() {
      for {
         select {
         case v:= <-c:
            fmt.Println(v)
         //5 Seconds automatically turn off , Avoid long timeouts 
         case <-time.After(5 * time.Second):
            fmt.Println("timeout")
            o<-true
            break
         }
      }
   }()
  // If it's worth it, let's go , When the main cooperation process is finished, it's gone 
   <-o
   fmt.Println(" Program end ")
}

Copy

Timeout is often used to set the timing of the execution of certain functions

go func(){
  select {
  case <-time.After(5 * time.Second):
          dosometing()
  }
}

Copy