Event_ Loop event loop mechanism

One , What is? Event_loop

Event Loop It's an event cycle , Refers to the browser or Node A solution to javaScript A mechanism by which the single thread runtime does not block , That is, we often use asynchronous Principle .

Two , Process model of event loop

1, Select the current task queue to execute , Select the first task in the task queue , If the task queue is empty , namely null, Then execute jump to micro task (MicroTaks) The implementation steps of

2, Set the task in the event loop to the selected task

3, Perform tasks

4, Set the currently running task in the event loop to null

5, Delete the completed task from the task queue

6,microtasks step , Get into microtask checkpoint

7, Update interface rendering

8, Step one back

Execute entry microtask At the checkpoint , The user agent performs the following steps ：

Set up microtask The checkpoint is marked as true.
When events loop microtask When the execution is not empty ： Choose the one that enters first microtask Queued microtask, Will be a circular event microtask Set to selected microtask, function microtask, Will execute the completed microtask by null, Removed from the microtask Medium microtask.
clear IndexDB Business
Set to enter microtask The checkpoint is marked as false.
Link to the original text ：https://blog.csdn.net/weixin_53504991/article/details/115455041

 

  3、 ... and ,NodeJS Of Event Loop

Node Medium Event Loop Is based on libuv Realized , and libuv yes Node The new cross platform abstraction layer ,libuv Use asynchronous , Event driven programming , The core is to provide i/o Event loops and asynchronous callbacks .libuv Of API Including time , Non blocking network , Asynchronous file operations , Subprocesses and so on . Event Loop Is in the libuv Implemented in .

What is? libuv?

Libuv yes Node.js Multi platform support library , Mainly used for asynchronous I/O. It is mainly for Node.js Developed , as time goes on , It is widely used in other systems , Such as Luvit、pyuv、Julia etc. .Libuv Basically for platform dependent libev/IOCP The abstraction of , Provide users with information based on libev Of API.

libuv Some important features of are :

1. Support full function event loop
2. File system events
3. Asynchronous file and file system operations
4. asynchronous TCP and UDP Socket
5. Subprocesses

understand libuv The basic principle of

 Node Of Event loop Divided into 6 Stages , The details are as follows ：

1. timers: perform setTimeout and setInterval Medium maturity callback.
2. pending callback: A few in the last cycle callback It will be implemented at this stage .
3. idle, prepare: For internal use only .
4. poll: The most important stage , perform pending callback, Where appropriate, back blocking at this stage .
5. check: perform setImmediate(setImmediate() Is to insert an event at the end of the event queue , The functions of the main thread and the event queue are executed immediately after execution setImmediate The specified callback function ) Of callback.
6. close callbacks: perform close The event callback, for example socket.on('close'[,fn]) perhaps http.server.on('close, fn).
7. Node browser-specific Event Loop differences
8. In the browser environment ,microtask The task queue is each macrotask Execute after execution . And in the Node.js in ,microtask It is executed between the stages of the event loop , That is, a phase is completed , They will execute microtask Queue tasks .

  summary

The browser and Node In the environment ,microtask Task queues are executed at different times

Node End ,microtask Execute between phases of the event loop
Browser side ,microtask In the event loop macrotask Execute after execution