问题
I'm having trouble visualizing how Javascript can be both single-threaded but non-blocking on the client. I've always envisioned something like an assembly line:
At the start of your code execution, you've got a single assembly line putting together different parts to a car.
We get to a point at 20%-to-completion where an engine needs to be added, but an engine hasn't been assembled yet.
Instead of waiting for the engine to be assembled, the assembly line gets broken up into two assembly lines - two threads, right?
Line 1 continues to assemble the other parts of the car.
Line 2 starts to assemble the engine.
When Line 2 finishes the assembly of the engine, it goes back into Line 1 and the engine is added.
Line 1 could be at 30%-to-completion, 99%-to-completion, etc at this point depending on how fast the engine was assembled.
This is how I envisioned non-blocking, async code. The main thread of Line 1 gets to continue to chug along without having to wait for Line 2 to finish first. But this assembly line metaphor requires two assembly lines, or two threads, but JS is single-threaded.
So now I'm confused.
回答1:
In short, the runtime has an event loop that kinda simulates async in 1 thread. In long, I found this video and text to be a good explanation: http://2014.jsconf.eu/speakers/philip-roberts-what-the-heck-is-the-event-loop-anyway.html
回答2:
No, a thread is not an assembly line. To keep your metaphor:
- an assembly line is sequence of things that need to be done to your automobile (or whatever things the line is producing)
- a thread is like a workman who is working on these assembly lines
Yes, in a single-threaded environment (one-man factory) the worker can only work in one assembly line at a time. But that doesn't mean the assembly lines cannot be run concurrently. The worker can start a machine that paints the car, then go over to the other line and install a part of the motor, then go back to the now painted car and start drying it…
That's how JS works. The worker has to finish his current step before he can work on the next task, but then he can choose another assembly line as he sees it fitting. And this even allows parallel processing when all he needs to do is start a machine that does some work in the background, as he then is available to do other things while waiting for a signal that the machine has finished.
来源：https://stackoverflow.com/questions/31580437/how-is-js-both-non-blocking-asynchronous-but-single-threaded