最简单、最可靠的解决方案是在正确的位置插入flex项.如果它们足够宽(width: 100%)，它们将强制换行.
.container {
background: tomato;
display: flex;
flex-flow: row wrap;
align-content: space-between;
justify-content: space-between;
}
.item {
width: 100px;
background: gold;
height: 100px;
border: 1px solid black;
font-size: 30px;
line-height: 100px;
text-align: center;
margin: 10px
}
.item:nth-child(4n - 1) {
background: silver;
}
.line-break {
width: 100%;
}
<div class="container">
<div class="item">1</div>
<div class="item">2</div>
<div class="item">3</div>
<div class="line-break"></div>
<div class="item">4</div>
<div class="item">5</div>
<div class="item">6</div>
<div class="line-break"></div>
<div class="item">7</div>
<div class="item">8</div>
<div class="item">9</div>
<div class="line-break"></div>
<div class="item">10</div>
</div>
但这是丑陋的，而且不符合语义.相反，我们可以在flex容器中生成伪元素，并使用order将它们移动到正确的位置.
.container {
background: tomato;
display: flex;
flex-flow: row wrap;
align-content: space-between;
justify-content: space-between;
}
.item {
width: 100px;
background: gold;
height: 100px;
border: 1px solid black;
font-size: 30px;
line-height: 100px;
text-align: center;
margin: 10px
}
.item:nth-child(3n) {
background: silver;
}
.container::before, .container::after {
content: '';
width: 100%;
order: 1;
}
.item:nth-child(n + 4) {
order: 1;
}
.item:nth-child(n + 7) {
order: 2;
}
<div class="container">
<div class="item">1</div>
<div class="item">2</div>
<div class="item">3</div>
<div class="item">4</div>
<div class="item">5</div>
<div class="item">6</div>
<div class="item">7</div>
<div class="item">8</div>
<div class="item">9</div>
</div>
但是有一个限制:flex容器只能有一个::before和一个::after伪元素.这意味着你只能强制2次换行.
要解决这个问题，您可以在flex项中生成伪元素，而不是在flex容器中.这样您就不会被限制为2个，但是那些伪元素将不是弹性项，因此它们将不能强制换行.
但幸运的是，CSS Display L3引入了display: contents(目前只支持Firefox 37):
元素本身不会生成任何框，但它的子级和
伪元素仍然照常生成方框.为了…的目的
框生成和布局时，必须将元素视为具有
已在文档中替换为其子元素和伪元素
树.
因此，您可以将display: contents应用于flex容器的子容器，并将每个容器的内容包装在一个额外的包装器中.然后，flex项将是那些附加的包装和子元素的伪元素.
.container {
background: tomato;
display: flex;
flex-flow: row wrap;
align-content: space-between;
justify-content: space-between;
}
.item {
display: contents;
}
.item > div {
width: 100px;
background: gold;
height: 100px;
border: 1px solid black;
font-size: 30px;
line-height: 100px;
text-align: center;
margin: 10px;
}
.item:nth-child(3n) > div {
background: silver;
}
.item:nth-child(3n)::after {
content: '';
width: 100%;
}
<div class="container">
<div class="item"><div>1</div></div>
<div class="item"><div>2</div></div>
<div class="item"><div>3</div></div>
<div class="item"><div>4</div></div>
<div class="item"><div>5</div></div>
<div class="item"><div>6</div></div>
<div class="item"><div>7</div></div>
<div class="item"><div>8</div></div>
<div class="item"><div>9</div></div>
<div class="item"><div>10</div></div>
</div>
或者，根据an old version of the spec的说法，Flexbox允许通过使用break-before、break-after或其旧的CSS 2.1别名来强制中断:
.item:nth-child(3n) {
page-break-after: always; /* CSS 2.1 syntax */
break-after: always; /* CSS 3 syntax */
}
但是这些强制换行符只在Firefox上有效，我认为它们不应该按照当前的规范工作.新提议的方式(没有在任何地方实施)是wrap-before or wrap-after:
.item:nth-child(3n) {
wrap-after: flex; /* New proposed syntax */
}
.container {
background: tomato;
display: flex;
flex-flow: row wrap;
align-content: space-between;
justify-content: space-between;
}
.item {
width: 100px;
background: gold;
height: 100px;
border: 1px solid black;
font-size: 30px;
line-height: 100px;
text-align: center;
margin: 10px
}
.item:nth-child(3n) {
page-break-after: always;
break-after: always;
wrap-after: flex;
background: silver;
}
<div class="container">
<div class="item">1</div>
<div class="item">2</div>
<div class="item">3</div>
<div class="item">4</div>
<div class="item">5</div>
<div class="item">6</div>
<div class="item">7</div>
<div class="item">8</div>
<div class="item">9</div>
<div class="item">10</div>
</div>