Introdução ao Bootstrap 4 - Código Simples .NET

O Bootstrap 4 é uma grande reescrita do tão famoso Bootstrap. Depois de 2 anos na versão alfa, o Bootstrap 4 agora é beta e entra em fase final para o release final. O Bootstrap é a o framework mais popular do mundo para criar sites e aplicativos móveis e na sua versão 4 apresenta várias novidades, onde iremos analisar alguma delas e mostrar nossa “primeira impressão” da nova versão do Boostrap:

À medida que a web evolui cada vez mais para o design responsivo, pode ser um verdadeiro desafio para os desenvolvedores web manterem-se. O Bootstrap pode tornar as coisas muito mais fáceis. O Bootstrap permite que você crie sites responsivos sem que você precise fazer adaptações, o Bootstrap cuida disso.

Começo rápido

Para adicionar rapidamente o Bootstrap ao seu projeto, use o Bootstrap CDN, fornecido gratuitamente pelo pessoal da MaxCDN.

Veja este exemplo de quickstart:

<!DOCTYPE html>
<html lang="en">
  <head>
    <!-- Required meta tags -->
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">

    <!-- Bootstrap CSS -->
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/css/bootstrap.min.css" integrity="sha384-/Y6pD6FV/Vv2HJnA6t+vslU6fwYXjCFtcEpHbNJ0lyAFsXTsjBbfaDjzALeQsN6M" crossorigin="anonymous">
  </head>
  <body>
    <h1>Hello, world!</h1>

    <!-- Optional JavaScript -->
    <!-- jQuery first, then Popper.js, then Bootstrap JS -->
    <script src="https://code.jquery.com/jquery-3.2.1.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.11.0/umd/popper.min.js" integrity="sha384-b/U6ypiBEHpOf/4+1nzFpr53nxSS+GLCkfwBdFNTxtclqqenISfwAzpKaMNFNmj4" crossorigin="anonymous"></script>
    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/js/bootstrap.min.js" integrity="sha384-h0AbiXch4ZDo7tp9hKZ4TsHbi047NrKGLO3SEJAg45jXxnGIfYzk4Si90RDIqNm1" crossorigin="anonymous"></script>
  </body>
</html>

Isso é tudo o que você precisa para obter os requisitos gerais da página básica para uma aplicação com Boostrap 4.

Veja uma tabela comparativa do site Quackit que mostra um comparativo entre as versões 3 e 4.

Component	Bootstrap 3	Bootstrap 4
Global
Source CSS Files	LESS	SCSS
Primary CSS Unit	`px`	`rem`
Media Queries Unit	`px`	`px`
Global Font Size	`14px`	`16px`
Default Fonts	Helvetica Neue, Helvetica, Arial, sans-serif	Uses a “native font stack” (user’s system fonts), with a fallback to Helvetica Neue, Arial, and sans-serif
Grids
Grid Tiers	4 tier grid system (xs, sm, md, lg)	5 tier grid system (xs, sm, md, lg, xl). Bootstrap 4 has removed the `xs` from the lowest break point. Therefore, `col-*` covers all devices (no need to specify the size in this case).
Offsetting Columns	Uses `col--offset-` classes to offset columns. For example, `col-md-offset-4`.	Uses `offset--` classes to offset columns. For example, `offset-md-4`.
Tables
Inverse Tables	Not supported.	Added inverse tables with the `.table-inverse` class.
Table Head Styles	Not supported.	Added table head styles with the `.thead-default` and `.thead-inverse` classes.
Condensed Tables	`.table-condensed`	`.table-sm`
Contextual Classes	Bootstrap 3 doesn’t use the `.table-` prefix for its contextual classes. For example, Bootstrap 3 uses `.active` whereas Bootstrap 4 uses `.table-active`. Other than that, both versions use the same 5 contextual keywords (active, success, info, warning, danger).	Added the `.table-` prefix for its contextual classes.
Responsive Tables	The `.table-responsive` class must be added to a parent `div` element.	Can add `.table-responsive` to the actual `table` element.
Reflow Tables	Not supported.	~~Added reflow tables with the `.table-reflow` class.~~ Reflow tables have now been dropped.
Forms
Horizontal Forms	Horizontal forms require the `.form-horizontal` class. Forms don’t require `.row` when using grids (although this class is still a requirement on Bootstrap 3 grids in general).	Bootstrap 4 dropped the `.form-horizontal` class — it is no longer needed to display horizontal forms. Forms require the `.row` class when using grids.
Horizontal Forms	Use `.control-label` when using grids for form layout.	Bootstrap 4 uses `.col-form-label` * when using grids for form layout. * Note that Bootstrap 4 initially used `.form-control-label` but subesquently changed this to `.col-form-label`.
Checkboxes and Radio Buttons	Uses `.radio`, `.radio-inline`, `.checkbox`, or `.checkbox-inline` to display checkboxes and radio buttons.	Uses `.form-check`, `.form-check-label`, `.form-check-input`, and `.form-check-inline`.
Form Control Size	Use `.input-lg` and `.input-sm` to increase or decrease the size of an input control.	Use `.form-control-lg` and `.form-control-sm` to increase or decrease the size of an input control.
Help Text	Use the `.help-block` class to display help text.	Bootstrap 4 uses the `.form-text` class to display help text. ~~Bootstrap 4 forms don’t have an official class for help text. Instead, the `.text-muted` class was introduced for more flexibility.~~
Validation and Feedback Icons	Includes validation styles for error, warning, and success states on form controls (for example, `.has-warning`). To present icons on the input fields using Bootstrap 3, use glyphicons.	Validation styles are not available for Bootstrap 4 forms. Use custom Bootstrap form validation messages instead.
Legends	No classes for styling the form legends.	Provides the option of using `.col-form-legend` on `legend` elements to style it more like a label.
Custom Forms	Not supported.	Bootstrap 4 introduced custom forms — completely custom form elements that replace the browser defaults.
Buttons
Styles	Includes the `.btn-default` and `.btn-info` classes. The `.btn-secondary` class isn’t available in Bootstrap 3.	Introduced the `.btn-secondary`, `.btn-light`, and `.btn-dark` classes. Dropped the `.btn-default` class. Note that the `.btn-info` class was initially dropped in Bootstrap 4 but then reappeared again.
Outline Buttons	Not supported.	Introduced the `.btn-outline-` classes for styling buttons with an outline color. ~~Introduced the `.btn--outline` classes for styling buttons with an outline color.~~ (The `.btn--outline` classes became `.btn-outline-` in Alpha 3).
Button Sizes	The `.btn-xs` class is available.	Dropped the `.btn-xs` class (only `.btn-sm` and `.btn-lg` are available now).
Images
Responsive Images	Use `.img-responsive` class.	Use `.img-fluid` class.
Image Alignment	Use `.pull-right`, `.pull-left`, and `.center-block` helper classes.	Uses `.m-x-auto` instead of `.center-block` to align block-level images. Can also use the various `.pull--right` and `.pull--left` responsive helper classes, as well as the `.text--left`, `.text--center`, and `.text--right` helper classes on the image’s parent. Can use the various `.pull--none` classes to disable floating.
Media Objects
Classes	Includes many different classes for media objects, including `.media`, `.media-body` `.media-object`, `.media-heading`, `.media-right`, `.media-left`, and `.media-list` and `.media-body`.	Uses just `.media` class. Margins can be applied using spacer utilities. Media objects are flexbox enabled in Bootstrap 4, so the various flexbox classes can also be applied (such as reordering, etc).
Dropdowns
Structure	Apply dropdowns to lists (i.e. using `ul` and `li`).	Dropdowns can be built with `ul` or `div`s. Apply the `.dropdown-item` to a `a` or `button` element and wrap them all in a `div` (or `ul`) with a `.dropdown-menu` class applied.
Menu Headers	Apply `.dropdown-header` to the `li` tag.	Apply `.dropdown-header` to `h1` – `h2` tags (as Bootstrap no longer uses `li` tags to build dropdowns).
Dividers	Apply the `.divider` class to the `li` element (because it used lists to build dropdowns).	Apply the `.dropdown-divider` to the `div` element.
Disabled Menu Items	Apply the `.disabled` class to the `li` element.	Apply the `.disabled` class to the `a` element.
Button Groups
Justified?	Supports justified button groups (via the `.btn-group-justified` class).	Not supported.
Extra Small?	Supports extra small button groups (via the `.btn-group-xs` class).	Not supported (dropped the `.btn-group-xs` class).
Navs
Inline Navs	There is no `.nav-inline` class.	Can use the `.nav-inline` class to explicitly specify navs to be displayed inline.
Navbars
Colors	Limited (preset) color options. Supports inverse navbars but not the other classes.	New (preset) color options. Introduced the `.navbar-light` and the `.navbar-dark` classes, and allows the `.bg-*` classes to be used on navbars.
Navbar Alignment	Use `.navbar-right`, `.navbar-left` to align components within the navbar.	~~Use the various `.pull--right` and `.pull--left` responsive helper classes.~~ ~~Can also use the various `.pull-*-none` classes to disable floating.~~ Can either use spacing utilities such as `.mr-auto`, or any of the flexbox alignment utilities.
Navbar Forms	Add the `.navbar-form` class to forms within navbars.	Bootstrap 4 dropped the `.navbar-form` class. It’s no longer necessary.
Fixed Navbars	Uses `.navbar-fixed-top` and `.navbar-fixed-bottom` to fix navbars to the top or bottom.	Uses `.fixed-top` and `.fixed-bottom`.
Pagination
Default Pagination	Only requires `.pagination` to be added (to the `ul` element that represents the list of pages).	Must also add `.page-item` to each `li` element and `.page-link` to each `a` element.
Pagers	Uses `.previous` and `.next` for aligning pagers.	Pagers have been dropped in Bootstrap 4 (Alpha 3). ~~Uses `.pager-prev` and `.pager-next` for aligning pagers.~~
Labels
Pill Labels	The `.label-pill` class is not available. However, Bootstrap 3 does have badges (which achieves a similar visual effect).	Labels have been replaced by badges in Bootstrap 4. Badges can use the `.badge-pill` for rounded corners.
Tags
Supported?	No. “Tags” are called “Labels” in Bootstrap 3 (i.e. they use the `.label` class).	~~Yes.~~ ~~The `.tag` class was introduced in Bootstrap 4 (Alpha 3) to replace the `.label` class.~~ ~~This was done to disambiguate from the `label` element.~~ Tags have now been renamed to “badges”. These replace labels from Bootstrap 3.
Jumbotron
Full-Width	The `.jumbotron-fluid` class is not required on full-width jumbotrons.	Introduced the `.jumbotron-fluid` class for full-width jumbotrons.
Progress Bars
Uses `progress`?	Doesn’t use the `progress` for progress bars. Instead, applies progress bar classes to nested `div` elements.	~~Uses the HTML5 `progress` element when working with progress bars.~~ Using the `progress` element was abandoned in Alpha 6. Bootstrap 4 now uses the `div` element again.
Glyphicons
Supported?	Supported.	Not supported.
Typography
Blockquotes	Bootstrap styles are applied to the `blockquote` element by default.	Introduced the `.blockquote` class for styling the `blockquote` element (i.e. styling this element is now opt-in).
Blockquote Alignment	Use `.blockquote-reverse` to align a blockquote to the right.	Use the text utilities to align blockquotes (`.text-center` and `.text-right`).
Page Headers	The `.page-header` class is supported.	The `.page-header` class is not supported.
Description Lists	The `.dl-horizontal` class was used to declare a horizontal list.	Horizontal lists are now declared with the `.row` class on the `dl` tag, then any of the grid system’s predefined classes to the `dt` and `dd` tags.
Non-Responsive Usage
Supported?	Supported. You can specify a layout to be non-responsive.	Not supported.
List Groups
Linked List Items / Button List Items	Apply `.list-group-item` to the `a` element.	Apply `.list-group-item-action` to the `a` element.
Collapse
Show content	Uses `.in` to have content expanded upon page load.	Uses `.show` to have content expanded upon page load.
Cards
Supported?	Not supported.	Introduced in Bootstrap 4. Cards replace functionality that was previously provided by panels, wells, and thumbnails.
Panels
Supported?	Supported.	Not supported. Use cards instead.
Wells
Supported?	Supported.	Not supported. Use cards instead.
Thumbnails
Supported?	Supported.	Not supported. Use cards instead.
Breadcrumbs
Classes	Uses the `.breadcrumb` class against the `ul` tag.	Also requires `.breadcrumb-item` to be applied against all `li` elements that make up the breadcrumb. Breadcrumbs can also be used outside of lists. For example, the `.breadcrumb-item` could be applied to a bunch of `a` elements that are wrapped in a `.breadcrumb` element.
Carousels
Carousel Item	Use `.item` class.	Use `.carousel-item` class.
Affix
Supported?	Yes.	No.

O Bootstrap 4 alpha foi lançado em 19 de agosto de 2015. Após dois anos em vários lançamentos alfa, o Bootstrap 4 finalmente chegou à versão beta em 10 de agosto de 2017. Esta versão envolve uma grande revisão do Bootstrap com algumas grandes mudanças na forma como funciona quando em comparação com o Bootstrap 3. Houve muitas alterações na maioria dos componentes, incluindo tabelas, formulários, grades, dropdowns, navbars e muito mais.

Para fazer download do bootstrap você pode clicar na página oficial e acessar os arquivos completos para utilizar offline em sua aplicação.

Você também pode acessar este tutorial completo sobre boostrap (em inglês) para mais detalhes.

Um grande abraço!