多选与单选（Checks & radios）
使用我们重构过的勾选组件，建立跨浏览器、跨设备皆一致的复选框（checkbox）及单选按钮（radio）。
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方法
浏览器默认的复选框和单选按钮被.form-check所取代，这是一系列用于两种输入类型的类，它们改进了HTML元素的布局和行为，提供了更好的自定义和跨浏览器一致性。复选框用于在列表中选择一个或多个选项，而单选按钮用于从多个选项中选择一个选项。
在结构上，我们的input和label是兄弟元素，而不是label中的input。由于必须指定id和属性才能将input和label关联起来，因此这会稍微详细一些。我们对所有input状态使用同级选择器（~），例如：checked或：disabled。当与.form-check-label类结合使用时，我们可以根据input的状态轻松地为每个项设置文本样式。
复选框
<div class="form-check">
<input class="form-check-input" type="checkbox" value="" id="flexCheckDefault">
<label class="form-check-label" for="flexCheckDefault">
Default checkbox
</label>
</div>
<div class="form-check">
<input class="form-check-input" type="checkbox" value="" id="flexCheckChecked" checked>
<label class="form-check-label" for="flexCheckChecked">
Checked checkbox
</label>
</div>不确定
当通过JavaScript手动设置时，复选框可以利用不确定伪类 :indeterminate（没有可用的HTML属性来指定它）。
<div class="form-check">
<input class="form-check-input" type="checkbox" value="" id="flexCheckIndeterminate">
<label class="form-check-label" for="flexCheckIndeterminate">
Indeterminate checkbox
</label>
</div>禁用
添加disabled属性，相关的label将自动设置为与较浅颜色匹配的样式，以帮助指示输入的状态。
<div class="form-check">
<input class="form-check-input" type="checkbox" value="" id="flexCheckDisabled" disabled>
<label class="form-check-label" for="flexCheckDisabled">
Disabled checkbox
</label>
</div>
<div class="form-check">
<input class="form-check-input" type="checkbox" value="" id="flexCheckCheckedDisabled" checked disabled>
<label class="form-check-label" for="flexCheckCheckedDisabled">
Disabled checked checkbox
</label>
</div>单选按钮
<div class="form-check">
<input class="form-check-input" type="radio" name="flexRadioDefault" id="flexRadioDefault1">
<label class="form-check-label" for="flexRadioDefault1">
Default radio
</label>
</div>
<div class="form-check">
<input class="form-check-input" type="radio" name="flexRadioDefault" id="flexRadioDefault2" checked>
<label class="form-check-label" for="flexRadioDefault2">
Default checked radio
</label>
</div>禁用
添加disabled属性，相关的label将自动设置为与较浅颜色匹配的样式，以帮助指示输入的状态。
<div class="form-check">
<input class="form-check-input" type="radio" name="flexRadioDisabled" id="flexRadioDisabled" disabled>
<label class="form-check-label" for="flexRadioDisabled">
Disabled radio
</label>
</div>
<div class="form-check">
<input class="form-check-input" type="radio" name="flexRadioDisabled" id="flexRadioCheckedDisabled" checked disabled>
<label class="form-check-label" for="flexRadioCheckedDisabled">
Disabled checked radio
</label>
</div>开关
开关具有自定义复选框的标记，但使用.form-switch类来呈现切换开关。开关还支持disabled属性。
<div class="form-check form-switch">
<input class="form-check-input" type="checkbox" id="flexSwitchCheckDefault">
<label class="form-check-label" for="flexSwitchCheckDefault">Default switch checkbox input</label>
</div>
<div class="form-check form-switch">
<input class="form-check-input" type="checkbox" id="flexSwitchCheckChecked" checked>
<label class="form-check-label" for="flexSwitchCheckChecked">Checked switch checkbox input</label>
</div>
<div class="form-check form-switch">
<input class="form-check-input" type="checkbox" id="flexSwitchCheckDisabled" disabled>
<label class="form-check-label" for="flexSwitchCheckDisabled">Disabled switch checkbox input</label>
</div>
<div class="form-check form-switch">
<input class="form-check-input" type="checkbox" id="flexSwitchCheckCheckedDisabled" checked disabled>
<label class="form-check-label" for="flexSwitchCheckCheckedDisabled">Disabled checked switch checkbox input</label>
</div>默认 (叠放)
默认情况下，任何数量的直接同级复选框和单选按钮都将垂直堆叠，并使用 .form-check适当隔开。
<div class="form-check">
<input class="form-check-input" type="checkbox" value="" id="defaultCheck1">
<label class="form-check-label" for="defaultCheck1">
Default checkbox
</label>
</div>
<div class="form-check">
<input class="form-check-input" type="checkbox" value="" id="defaultCheck2" disabled>
<label class="form-check-label" for="defaultCheck2">
Disabled checkbox
</label>
</div><div class="form-check">
<input class="form-check-input" type="radio" name="exampleRadios" id="exampleRadios1" value="option1" checked>
<label class="form-check-label" for="exampleRadios1">
Default radio
</label>
</div>
<div class="form-check">
<input class="form-check-input" type="radio" name="exampleRadios" id="exampleRadios2" value="option2">
<label class="form-check-label" for="exampleRadios2">
Second default radio
</label>
</div>
<div class="form-check">
<input class="form-check-input" type="radio" name="exampleRadios" id="exampleRadios3" value="option3" disabled>
<label class="form-check-label" for="exampleRadios3">
Disabled radio
</label>
</div>内联
通过将.form-check-inline添加到.form-check，将复选框或单选框在同一水平行上分组显示。
<div class="form-check form-check-inline">
<input class="form-check-input" type="checkbox" id="inlineCheckbox1" value="option1">
<label class="form-check-label" for="inlineCheckbox1">1</label>
</div>
<div class="form-check form-check-inline">
<input class="form-check-input" type="checkbox" id="inlineCheckbox2" value="option2">
<label class="form-check-label" for="inlineCheckbox2">2</label>
</div>
<div class="form-check form-check-inline">
<input class="form-check-input" type="checkbox" id="inlineCheckbox3" value="option3" disabled>
<label class="form-check-label" for="inlineCheckbox3">3 (disabled)</label>
</div><div class="form-check form-check-inline">
<input class="form-check-input" type="radio" name="inlineRadioOptions" id="inlineRadio1" value="option1">
<label class="form-check-label" for="inlineRadio1">1</label>
</div>
<div class="form-check form-check-inline">
<input class="form-check-input" type="radio" name="inlineRadioOptions" id="inlineRadio2" value="option2">
<label class="form-check-label" for="inlineRadio2">2</label>
</div>
<div class="form-check form-check-inline">
<input class="form-check-input" type="radio" name="inlineRadioOptions" id="inlineRadio3" value="option3" disabled>
<label class="form-check-label" for="inlineRadio3">3 (disabled)</label>
</div>无标签
对于没有标签文本的复选框和收音机，省略wrapping.form复选框。记住，仍然要为辅助技术提供某种形式的可访问名称（例如，使用aria-label标签）。有关详细信息，请参见表单概述辅助功能部分。
<div>
<input class="form-check-input" type="checkbox" id="checkboxNoLabel" value="" aria-label="...">
</div>
<div>
<input class="form-check-input" type="radio" name="radioNoLabel" id="radioNoLabel1" value="" aria-label="...">
</div>切换按钮
通过使用.btn样式而不是
复选切换按钮
<input type="checkbox" class="btn-check" id="btn-check" autocomplete="off">
<label class="btn btn-primary" for="btn-check">Single toggle</label><input type="checkbox" class="btn-check" id="btn-check-2" checked autocomplete="off">
<label class="btn btn-primary" for="btn-check-2">Checked</label><input type="checkbox" class="btn-check" id="btn-check-3" autocomplete="off" disabled>
<label class="btn btn-primary" for="btn-check-3">Disabled</label>
从外观上看，这些复选框切换按钮与按钮插件切换按钮相同。然而，辅助技术传达的方式不同：屏幕阅读器会将复选框切换宣布为“选中”/“未选中”（因为外观如此，但它们本质上仍然是复选框），而按钮插件切换按钮将宣布为“按钮”/“按下按钮”。这两种方法之间的选择将取决于您正在创建的切换类型，以及当作为复选框或实际按钮发布时，切换对用户是否有意义。
单选切换按钮
<input type="radio" class="btn-check" name="options" id="option1" autocomplete="off" checked>
<label class="btn btn-secondary" for="option1">Checked</label>
<input type="radio" class="btn-check" name="options" id="option2" autocomplete="off">
<label class="btn btn-secondary" for="option2">Radio</label>
<input type="radio" class="btn-check" name="options" id="option3" autocomplete="off" disabled>
<label class="btn btn-secondary" for="option3">Disabled</label>
<input type="radio" class="btn-check" name="options" id="option4" autocomplete="off">
<label class="btn btn-secondary" for="option4">Radio</label>轮廓样式
支持.btn的不同变量，例如各种outline样式是被支持的。
<input type="checkbox" class="btn-check" id="btn-check-outlined" autocomplete="off">
<label class="btn btn-outline-primary" for="btn-check-outlined">Single toggle</label><br>
<input type="checkbox" class="btn-check" id="btn-check-2-outlined" checked autocomplete="off">
<label class="btn btn-outline-secondary" for="btn-check-2-outlined">Checked</label><br>
<input type="radio" class="btn-check" name="options-outlined" id="success-outlined" autocomplete="off" checked>
<label class="btn btn-outline-success" for="success-outlined">Checked success radio</label>
<input type="radio" class="btn-check" name="options-outlined" id="danger-outlined" autocomplete="off">
<label class="btn btn-outline-danger" for="danger-outlined">Danger radio</label>Sass
Variables
$form-check-input-width: 1em;
$form-check-min-height: $font-size-base * $line-height-base;
$form-check-padding-start: $form-check-input-width + .5em;
$form-check-margin-bottom: .125rem;
$form-check-label-color: null;
$form-check-label-cursor: null;
$form-check-transition: null;
$form-check-input-active-filter: brightness(90%);
$form-check-input-bg: $input-bg;
$form-check-input-border: 1px solid rgba(0, 0, 0, .25);
$form-check-input-border-radius: .25em;
$form-check-radio-border-radius: 50%;
$form-check-input-focus-border: $input-focus-border-color;
$form-check-input-focus-box-shadow: $input-btn-focus-box-shadow;
$form-check-input-checked-color: $component-active-color;
$form-check-input-checked-bg-color: $component-active-bg;
$form-check-input-checked-border-color: $form-check-input-checked-bg-color;
$form-check-input-checked-bg-image: url("data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 20 20'><path fill='none' stroke='#{$form-check-input-checked-color}' stroke-linecap='round' stroke-linejoin='round' stroke-width='3' d='M6 10l3 3l6-6'/></svg>");
$form-check-radio-checked-bg-image: url("data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='-4 -4 8 8'><circle r='2' fill='#{$form-check-input-checked-color}'/></svg>");
$form-check-input-indeterminate-color: $component-active-color;
$form-check-input-indeterminate-bg-color: $component-active-bg;
$form-check-input-indeterminate-border-color: $form-check-input-indeterminate-bg-color;
$form-check-input-indeterminate-bg-image: url("data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 20 20'><path fill='none' stroke='#{$form-check-input-indeterminate-color}' stroke-linecap='round' stroke-linejoin='round' stroke-width='3' d='M6 10h8'/></svg>");
$form-check-input-disabled-opacity: .5;
$form-check-label-disabled-opacity: $form-check-input-disabled-opacity;
$form-check-btn-check-disabled-opacity: $btn-disabled-opacity;
$form-check-inline-margin-end: 1rem;