Understanding the flex property

问题

Why, in this example, is the .main element (blue) dividing space only with .aside-1 (yellow) and .aside-2 (pink), and not with all elements?

We have a wrapper that is putting all elements occupying one line.

In .main we say flex: 3 0px, which I think says, this element will be 3x bigger than the other four elements and will occupy 3/(3+1+1+1+1).

However, I've noticed that with a nowrap wrapper the smallest item is .main.

And with wrap, it divides with the two closest elements.

Can't understand this.

.wrapper {
  display: flex;
  flex-flow: row wrap;
  font-weight: bold;
  text-align: center;
}

.wrapper>* {
  padding: 10px;
  flex: 1 100%;
}

.header {
  background: tomato;
}

.footer {
  background: lightgreen;
}

.main {
  text-align: left;
  background: deepskyblue;
  height: 50vh;
  flex: 3 0px;
}

.aside-1 {
  background: gold;
}

.aside-2 {
  background: hotpink;
}

.aside {
  flex: 1 auto;
}

.aside-1 {
  order: 1;
}

.main {
  order: 2;
}

.aside-2 {
  order: 3;
}

.footer {
  order: 4;
}

<div class="wrapper">
  <header class="header">Header</header>
  <article class="main">
    <p>Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Vestibulum tortor quam, feugiat vitae, ultricies eget, tempor sit amet, ante. Donec eu libero sit amet quam egestas semper. Aenean ultricies mi vitae est.
      Mauris placerat eleifend leo.</p>
  </article>
  <aside class="aside aside-1">Aside 1</aside>
  <aside class="aside aside-2">Aside 2</aside>
  <footer class="footer">Footer</footer>
</div>

回答1:

First take a look at this rule in your code:

.wrapper > * {
  padding: 10px;
  flex: 1 100%;
}

The selector above is targeting all five flex items:

header
article
aside
aside
footer

The flex component breaks down to this:

flex-grow: 1
flex-shrink: 1 (by default)
flex-basis: 100%

You wrote:

Why, in this example, is the .main element (blue) dividing space only with .aside-1 (yellow) and .aside-2 (pink), and not with all elements?

This is why:

The container is set to flex-flow: row wrap, meaning flex items are allowed to wrap.
As noted above, all flex items are set to flex-basis: 100% (i.e. width: 100%), meaning there can only be one flex item per row, except...
flex-basis: 100% only gets applied to the header and footer because...
it is being overridden by other rules later in the cascade sequence¹:
```
.main { flex: 3 0px; }

.aside { flex: 1 auto; }
```

However, I've noticed that with a nowrap wrapper the smallest item is .main.

Yes, because, as mentioned above, it has flex-basis: 0 and flex-shrink: 1.

In .main we say flex: 3 0px, which I think says, this element will be 3x bigger than the other four elements and will occupy 3/(3+1+1+1+1).

Not quite. flex-grow: 3 means that the element will consume 3x the amount of free space than other flex items with flex-grow: 1. It doesn't necessarily mean it will be 3x the size. More details here: flex-grow not sizing flex items as expected

¹ It may appear that specificity should win over the cascade, and all items should get flex-basis: 100%: