History: Bootstrap 3 to 4 transition
Preview of version: 45
Bootstrap 4.0.0 has now been released. Tiki, since version 13, has been integrating Bootstrap 3 for a layout and styling framework. On April 21, the Bootstrap 4 experimental branch (of the Tiki development files) was merged into trunk; branching of branch 19 is scheduled for September, with Tiki 19 set for release in October).
At TikiFestJapan2017, a Bootstrap 4 experimental branch was branched from trunk: https://sourceforge.net/p/tikiwiki/code/HEAD/tree/branches/experimental/bootstrap4/.
Bootstrap 4 experimental branch
I've started going through the themes directory making the necessary changes. The first goal was to get a completed SCSS compile, for the base files (tiki_base.css) initially. Of course even after updating many variable names, etc., there were still errors due to the many differences between the Bootstrap versions. Update: All of the themes in the Tiki package now compile without errors and most styling details are implemented; in other words, the themes are mostly updated from Bootstrap 3 to Bootstrap 4 compatibility.
Scout-App
The PHP SCSS compiler that can be accessed from the terminal in PhpStorm, etc. doesn't give much information about compile errors — sometimes just a line number, but we don't know in what file or the nature of the error. Maybe the author can be approached to improve this. For now, I downloaded a free application called Scout-App (http://scout-app.io, available for all OSs) that gives explicit information about compile errors; it watches for file changes and runs a compile process after each change. So I use this to identify places to fix and then use the compiler in PhpStorm for the compiling.
Browser Support
As Bootstrap 4 states "Dropped IE8, IE9, and iOS 6 support. v4 is now only IE10+ and iOS 7+. For sites needing either of those, use v3." since Tiki 19.x we are dropping the support for these too.
See https://getbootstrap.com/docs/4.0/migration/#browser-support
Updating a Tiki theme from Bootstrap 3 to 4
Please see Updating a Tiki theme from Bootstrap 3 to 4 for a description of the process that I () have used to update the themes in the Tiki package, as well as the Zukathemes files.
Tiki code migration
Generic Bootstrap changes
For a summary of Bootstrap changes between 3 and 4, please read https://getbootstrap.com/docs/4.0/migration/ .
Some main points for Tiki (my quick check):
- Dropped panels, thumbnails, and wells for a new all-encompassing component, cards. So all instances of these in templates, etc. and references in CSS will need to be updated.
- Dropped the Glyphicons icon font. So the related files should be removed from themes/icons.
- Dropped
.page-header- class. The use of this in Tiki needs to be checked. - Renamed .img-responsive to .img-fluid. - This needs to be updated in the img plugin and other locations.
- Forms - multiple changes that'll need updating in Tiki tpls, etc.
- Buttons: Removed .btn-default and added .btn-secondary. - Instances of btn-default in tpls, etc. will need to be updated so (in this order):
- Change instances of btn-primary to btn-secondary.
- Change instances of btn-default to btn-primary.
- Dropdowns: Dropdown items now require the
.dropdown-itemclass, so this needs updating. - Modals - multiple changes to deal with. Update: thanks to Jonny for making modals work in the experimental branch.
- Navs and navbars - Totally rewritten using flexbox. Also, .navbar-inverse and .navbar-default are renamed to .*-dark and .*-light, matching color schemes used elsewhere. Also .navbar-primary is supported, so I think Tiki's Admin Look & Feel should have a checkbox to choose which navbar color scheme to use per site. Themes should support all three variants.
- Pagination - Totally rewritten using flexbox.
- Carousel: "Overhauled the entire component to simplify design and styling. We have fewer styles for you to override, new indicators, and new icons."
Specific Tiki code tasks/bugs
- Side column show/hide anchors don't work properly.
- Card (formerly "panel") background and foreground colors aren't set by card classes, but by separate text and background color classes, like
bg-primaryandtext-dark. Discussion is needed about how to replace our previously usedpanel-defaultcoloring for former panels coded directly in .tpl files. For modules, the color classes can be input as parameters. - Navbars instead of
*-defaultand*-inverseare now*-lightand*-dark, with*-primaryalso an option now. So themes should support all three, and there should be an admin L&F checkbox to select which the site uses. - (CMS) Article display will need revising to accommodate FB link image specs. The BS4 equivalent of what is being used now for Tiki's CMS articles is very simple and has no CSS opportunities for modifying the displayed image size.
- Hopefully the article layout revising can also enable positioning the article image / featured image either to the side or above the article text (https://dev.tiki.org/item6596-Wish-enable-post-article-layout-that-has-large-image-above).
- The new drag-and-drop menu admin doesn't seem to work.
- Popovers containing links disappear too quickly on hover so the links can't be clicked.
- Tour doesn't work, in a quick test of an existing tour. The first popup appears but the second doesn't. But according to the Bootstrap Tour site it should work so we need to be sure we're linking to the latest master and that our implementation is ok.
Other possible related changes
- Consolidate layout templates to reduce redundancy. Maybe the two fixed-navbar layouts could be reduced to one, and maybe the "classic" and "wide side column" layouts could be reduced to one. Or maybe all could be reduced to one if there is a checkbox in L&F admin for "Fixed-top navbar".
- Consolidate menu types ("Bootstrap" and "CSS/suckerfish" and Tiki legacy)
- Pluses:
- Simpler menu backend construction
- Simpler CSS to maintain (and learn, for new theme authors)
- Minuses:
- Bootstrap menus natively have only parent and child levels (but this probably can be overcome with a script)
- Bootstrap parent link only opens dropdown and isn't for navigation (but https://stackoverflow.com/questions/25692514/bootstrap-how-do-i-make-dropdown-navigation-parent-links-an-active-link, etc.).
- Pluses:
- Related to these, maybe a good time to implement Off-canvas menus and sidebars.
- Put wiki page elements (like wiki topline, pagebar, action icons for print, etc.) in modules for more flexibility in page layout.
[-] Commits to branch outside of the themes directory:
At Jonny's suggestion, the commits to the experimental branch that aren't in the themes directory are being recorded here. This should smooth the merge of the branch back to trunk.
http://sourceforge.net/p/tikiwiki/code/65126
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Revision: 65126 http://sourceforge.net/p/tikiwiki/code/65126 Author: chibaguy Date: 2018-01-08 06:14:25 +0000 (Mon, 08 Jan 2018) Log Message: ----------- [UPD] Update Classic layout's module.tpl (has shadow div) relative to default modoule.tpl, and to Bootstrap 4 beta 3. Modified Paths: -------------- branches/experimental/bootstrap4/templates/layouts/classic/module.tpl
http://sourceforge.net/p/tikiwiki/code/65112
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Revision: 65112 http://sourceforge.net/p/tikiwiki/code/65112 Author: chibaguy Date: 2018-01-07 12:26:15 +0000 (Sun, 07 Jan 2018) Log Message: ----------- [FIX] Update navbar class for fixed-top position (.navbar-fixed-top -> .fixed-top). Modified Paths: -------------- branches/experimental/bootstrap4/templates/layouts/fixed_top_modules/layout_view.tpl
http://sourceforge.net/p/tikiwiki/code/65093
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Revision: 65093 http://sourceforge.net/p/tikiwiki/code/65093 Author: chibaguy Date: 2018-01-05 17:53:05 +0000 (Fri, 05 Jan 2018) Log Message: ----------- [FIX] Adjust grid classes on center, left and right columns for correct size and order in various display sizes. Modified Paths: -------------- branches/experimental/bootstrap4/templates/layouts/fixed_top_modules/layout_view.tpl
http://sourceforge.net/p/tikiwiki/code/65092
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Revision: 65092 http://sourceforge.net/p/tikiwiki/code/65092 Author: chibaguy Date: 2018-01-05 17:47:55 +0000 (Fri, 05 Jan 2018) Log Message: ----------- [FIX] Adjust grid classes on center, left and right columns for correct size and order in various display sizes. Modified Paths: -------------- branches/experimental/bootstrap4/templates/layouts/header_middle_footer_containers_3-6-3/layout_view.tpl
http://sourceforge.net/p/tikiwiki/code/65091
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Revision: 65091 http://sourceforge.net/p/tikiwiki/code/65091 Author: chibaguy Date: 2018-01-05 17:03:14 +0000 (Fri, 05 Jan 2018) Log Message: ----------- [FIX] Adjust grid classes on center, left and right columns for correct size and order in various display sizes. Modified Paths: -------------- branches/experimental/bootstrap4/templates/layouts/basic/layout_view.tpl branches/experimental/bootstrap4/templates/layouts/classic/layout_view.tpl branches/experimental/bootstrap4/templates/layouts/social/layout_view.tpl
http://sourceforge.net/p/tikiwiki/code/65089
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Revision: 65089 http://sourceforge.net/p/tikiwiki/code/65089 Author: chibaguy Date: 2018-01-05 14:06:02 +0000 (Fri, 05 Jan 2018) Log Message: ----------- [FIX] Put side-col-toggle divs in a row div to prevent interference with content columns. (Todo: find out why flex widths aren't working.) Modified Paths: -------------- branches/experimental/bootstrap4/templates/layouts/social/layout_view.tpl
http://sourceforge.net/p/tikiwiki/code/65088
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Revision: 65088 http://sourceforge.net/p/tikiwiki/code/65088 Author: chibaguy Date: 2018-01-05 12:48:15 +0000 (Fri, 05 Jan 2018) Log Message: ----------- [FIX] Adjust grid classes on center, left and right columns for correct size and order in various display sizes. Modified Paths: -------------- branches/experimental/bootstrap4/templates/layouts/social/layout_view.tpl
http://sourceforge.net/p/tikiwiki/code/65033
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Revision: 65033 http://sourceforge.net/p/tikiwiki/code/65033 Author: chibaguy Date: 2017-12-31 09:36:39 +0000 (Sun, 31 Dec 2017) Log Message: ----------- [UX] Don't display side-column toggles in narrow screens; adjust grid classes. Modified Paths: -------------- branches/experimental/bootstrap4/templates/layouts/basic/layout_view.tpl
http://sourceforge.net/p/tikiwiki/code/64411
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Revision: 64411 http://sourceforge.net/p/tikiwiki/code/64411 Author: chibaguy Date: 2017-10-28 10:07:43 +0000 (Sat, 28 Oct 2017) Log Message: ----------- [UPD] Add dropdown-link classes in Page actions dropdown. Modified Paths: -------------- branches/experimental/bootstrap4/templates/tiki-wiki_topline.tpl Modified: branches/experimental/bootstrap4/templates/tiki-wiki_topline.tpl
http://sourceforge.net/p/tikiwiki/code/64410
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Revision: 64410 http://sourceforge.net/p/tikiwiki/code/64410 Author: chibaguy Date: 2017-10-28 08:48:26 +0000 (Sat, 28 Oct 2017) Log Message: ----------- [UPD] Add Bootstrap 4 classes to navbar ul item and a . Modified Paths: -------------- branches/experimental/bootstrap4/lib/smarty_tiki/block.tabset.php Modified: branches/experimental/bootstrap4/lib/smarty_tiki/block.tabset.php
http://sourceforge.net/p/tikiwiki/code/64409
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Revision: 64409 http://sourceforge.net/p/tikiwiki/code/64409 Author: chibaguy Date: 2017-10-28 08:22:36 +0000 (Sat, 28 Oct 2017) Log Message: ----------- [UPD] Increase thumbnail size. Modified Paths: -------------- branches/experimental/bootstrap4/templates/admin/include_look.tpl Modified: branches/experimental/bootstrap4/templates/admin/include_look.tpl
http://sourceforge.net/p/tikiwiki/code/64408
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Revision: 64408 http://sourceforge.net/p/tikiwiki/code/64408 Author: chibaguy Date: 2017-10-28 08:17:16 +0000 (Sat, 28 Oct 2017) Log Message: ----------- [UPD] Update theme selector and thumbnail to Flexbox. Modified Paths: -------------- branches/experimental/bootstrap4/templates/admin/include_look.tpl Modified: branches/experimental/bootstrap4/templates/admin/include_look.tpl
http://sourceforge.net/p/tikiwiki/code/64407
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Revision: 64407 http://sourceforge.net/p/tikiwiki/code/64407 Author: chibaguy Date: 2017-10-28 07:50:33 +0000 (Sat, 28 Oct 2017) Log Message: ----------- [UPD] Update remaining layout files to Flexbox. Modified Paths: -------------- branches/experimental/bootstrap4/templates/layouts/basic/layout_view.tpl branches/experimental/bootstrap4/templates/layouts/classic/layout_view.tpl branches/experimental/bootstrap4/templates/layouts/fixed_top_modules/layout_view.tpl branches/experimental/bootstrap4/templates/layouts/header_middle_footer_containers_3-6-3/layout_view.tpl
http://sourceforge.net/p/tikiwiki/code/64383
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Revision: 64383 http://sourceforge.net/p/tikiwiki/code/64383 Author: chibaguy Date: 2017-10-26 16:09:30 +0000 (Thu, 26 Oct 2017) Log Message: ----------- [FIX] Fix content column order. Modified Paths: -------------- branches/experimental/bootstrap4/templates/layouts/basic/layout_view.tpl Modified: branches/experimental/bootstrap4/templates/layouts/basic/layout_view.tpl
http://sourceforge.net/p/tikiwiki/code/64381
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Revision: 64381 http://sourceforge.net/p/tikiwiki/code/64381 Author: chibaguy Date: 2017-10-26 12:48:52 +0000 (Thu, 26 Oct 2017) Log Message: ----------- [UPD] Update following upgrade to Bootstrap 4 beta 2. Amelia compiles ok now also, but still working out a good arrangement of files, etc. Modified Paths: -------------- branches/experimental/bootstrap4/lib/smarty_tiki/block.accordion_group.php (Replaced CSS selector "panel" with "card", only.) ...
http://sourceforge.net/p/tikiwiki/code/64379
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Revision: 64379 http://sourceforge.net/p/tikiwiki/code/64379 Author: chibaguy Date: 2017-10-26 05:45:26 +0000 (Thu, 26 Oct 2017) Log Message: ----------- [UPD] Further update social layout navbar construction and styling. Modified Paths: -------------- branches/experimental/bootstrap4/templates/layouts/social/layout_view.tpl
http://sourceforge.net/p/tikiwiki/code/64316
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Revision: 64316 http://sourceforge.net/p/tikiwiki/code/64316 Author: chibaguy Date: 2017-10-18 16:58:00 +0000 (Wed, 18 Oct 2017) Log Message: ----------- [FIX] Typo (heading should be header). Modified Paths: -------------- branches/experimental/bootstrap4/templates/module.tpl
http://sourceforge.net/p/tikiwiki/code/64308
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Revision: 64308 http://sourceforge.net/p/tikiwiki/code/64308 Author: chibaguy Date: 2017-10-18 10:20:21 +0000 (Wed, 18 Oct 2017) Log Message: ----------- [UPD] Plugin warning updated to BS4 (more class changes, etc.). Modified Paths: -------------- branches/experimental/bootstrap4/templates/tiki-plugin_blocked.tpl
http://sourceforge.net/p/tikiwiki/code/64307
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Revision: 64307 http://sourceforge.net/p/tikiwiki/code/64307 Author: chibaguy Date: 2017-10-18 10:18:43 +0000 (Wed, 18 Oct 2017) Log Message: ----------- [UPD] Updated button types (classes) for Bootstrap 4 (no more "default"; now "primary, secondary, etc.") Modified Paths: -------------- branches/experimental/bootstrap4/lib/smarty_tiki/function.button.php
http://sourceforge.net/p/tikiwiki/code/64306
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Revision: 64306 http://sourceforge.net/p/tikiwiki/code/64306 Author: chibaguy Date: 2017-10-18 09:41:05 +0000 (Wed, 18 Oct 2017) Log Message: ----------- [UPD] Plugin warning updated to BS4. Todo: find (or wait for from Bootstrap) method for auto-inverse text color against dark background. Modified Paths: -------------- branches/experimental/bootstrap4/templates/tiki-plugin_blocked.tpl
http://sourceforge.net/p/tikiwiki/code/64302
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Revision: 64302 http://sourceforge.net/p/tikiwiki/code/64302 Author: chibaguy Date: 2017-10-17 15:55:08 +0000 (Tue, 17 Oct 2017) Log Message: ----------- [UPD] Update basic and social layout files to flexbox (social navbar not right yet). Modified Paths: -------------- branches/experimental/bootstrap4/templates/layouts/basic/layout_view.tpl branches/experimental/bootstrap4/templates/layouts/social/layout_view.tpl
http://sourceforge.net/p/tikiwiki/code/64301
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Revision: 64301 http://sourceforge.net/p/tikiwiki/code/64301 Author: chibaguy Date: 2017-10-17 11:55:22 +0000 (Tue, 17 Oct 2017) Log Message: ----------- [UPD] Update to flexbox layout classes. Modified Paths: -------------- branches/experimental/bootstrap4/templates/layouts/basic/layout_view.tpl
http://sourceforge.net/p/tikiwiki/code/64297
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Revision: 64297 http://sourceforge.net/p/tikiwiki/code/64297 Author: jonnybradley Date: 2017-10-17 10:37:26 +0000 (Tue, 17 Oct 2017) Log Message: ----------- [FIX] bootstrap4: popper.js was removed from BS4, replace temporarily with the CDN version for a quick fix (thanks chibaguy) Modified Paths: -------------- branches/experimental/bootstrap4/lib/setup/theme.php
http://sourceforge.net/p/tikiwiki/code/64296
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Revision: 64296 http://sourceforge.net/p/tikiwiki/code/64296 Author: chibaguy Date: 2017-10-17 10:02:07 +0000 (Tue, 17 Oct 2017) Log Message: ----------- [UPD] Less to SCSS transition (update file names, variables, etc.). Also (so far) made a simple replacement of "panel" with "card" in module.tpl. Modified Paths: -------------- branches/experimental/bootstrap4/templates/module.tpl (plus theme files)
http://sourceforge.net/p/tikiwiki/code/64190
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Revision: 64190 http://sourceforge.net/p/tikiwiki/code/64190 Author: chibaguy Date: 2017-10-10 04:10:27 +0000 (Tue, 10 Oct 2017) Log Message: ----------- [UPD] Beginning Less to SCSS transition (updating variable name syntax). Modified Paths: -------------- branches/experimental/bootstrap4/templates/styleguide/sections/navbars.tpl (plus theme files)
http://sourceforge.net/p/tikiwiki/code/64184
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Revision: 64184 http://sourceforge.net/p/tikiwiki/code/64184 Author: jonnybradley Date: 2017-10-09 09:13:47 +0000 (Mon, 09 Oct 2017) Log Message: ----------- [FIX] bootstrap4: Catch all scss exceptions Modified Paths: -------------- branches/experimental/bootstrap4/lib/core/Tiki/Command/ScssCompileCommand.php
http://sourceforge.net/p/tikiwiki/code/64183
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Revision: 64183 http://sourceforge.net/p/tikiwiki/code/64183 Author: jonnybradley Date: 2017-10-09 08:40:01 +0000 (Mon, 09 Oct 2017) Log Message: ----------- [FIX] bootstrap4: Removed exceptions display by mistake - oops, back now Modified Paths: -------------- branches/experimental/bootstrap4/lib/core/Tiki/Command/ScssCompileCommand.php
http://sourceforge.net/p/tikiwiki/code/64182
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Revision: 64182 http://sourceforge.net/p/tikiwiki/code/64182 Author: jonnybradley Date: 2017-10-09 08:28:55 +0000 (Mon, 09 Oct 2017) Log Message: ----------- [ENH] bootstrap4: Use scss compile directly in the ScssPhp\Compiler, so we can handle exceptions and display the problem rather than outputting them to the css file Modified Paths: -------------- branches/experimental/bootstrap4/lib/core/Tiki/Command/ScssCompileCommand.php
http://sourceforge.net/p/tikiwiki/code/64178
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Revision: 64178 http://sourceforge.net/p/tikiwiki/code/64178 Author: jonnybradley Date: 2017-10-09 05:24:33 +0000 (Mon, 09 Oct 2017) Log Message: ----------- [ENH] bootstrap4: Add scss:compile command and remove less (untested, scss files coming soon Modified Paths: -------------- branches/experimental/bootstrap4/lib/core/Tiki/Command/ConsoleApplicationBuilder.php Added Paths: ----------- branches/experimental/bootstrap4/lib/core/Tiki/Command/ScssCompileCommand.php Removed Paths: ------------- branches/experimental/bootstrap4/lib/core/Tiki/Command/LessCompileCommand.php
http://sourceforge.net/p/tikiwiki/code/64177
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Revision: 64177 http://sourceforge.net/p/tikiwiki/code/64177 Author: jonnybradley Date: 2017-10-09 04:43:29 +0000 (Mon, 09 Oct 2017) Log Message: ----------- [UPD] bootstrap4: Add leafo/scssphp and remove less.php Modified Paths: -------------- branches/experimental/bootstrap4/vendor_bundled/composer.json branches/experimental/bootstrap4/vendor_bundled/composer.lock
http://sourceforge.net/p/tikiwiki/code/64176
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Revision: 64176 http://sourceforge.net/p/tikiwiki/code/64176 Author: jonnybradley Date: 2017-10-09 04:09:40 +0000 (Mon, 09 Oct 2017) Log Message: ----------- [FIX] bootstrap4: Add include for bundled popper.js now required for bootstrap4 dropdowns Modified Paths: -------------- branches/experimental/bootstrap4/lib/setup/theme.php
http://sourceforge.net/p/tikiwiki/code/64175
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Revision: 64175 http://sourceforge.net/p/tikiwiki/code/64175 Author: chibaguy Date: 2017-10-09 03:44:03 +0000 (Mon, 09 Oct 2017) Log Message: ----------- [FIX] Use correct Bootstrap 4 beta information. Modified Paths: -------------- branches/experimental/bootstrap4/vendor_bundled/composer.json
http://sourceforge.net/p/tikiwiki/code/64174
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Revision: 64174 http://sourceforge.net/p/tikiwiki/code/64174 Author: chibaguy Date: 2017-10-09 03:40:27 +0000 (Mon, 09 Oct 2017) Log Message: ----------- [FIX] Use correct Bootstrap 4 beta information. Modified Paths: -------------- branches/experimental/bootstrap4/vendor_bundled/composer.lock
http://sourceforge.net/p/tikiwiki/code/64173
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Revision: 64173 http://sourceforge.net/p/tikiwiki/code/64173 Author: chibaguy Date: 2017-10-09 02:10:06 +0000 (Mon, 09 Oct 2017) Log Message: ----------- [FIX] Use better Bootstrap 4 version number. Modified Paths: -------------- branches/experimental/bootstrap4/vendor_bundled/composer.json
http://sourceforge.net/p/tikiwiki/code/64172
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Revision: 64172 http://sourceforge.net/p/tikiwiki/code/64172 Author: chibaguy Date: 2017-10-08 16:40:00 +0000 (Sun, 08 Oct 2017) Log Message: ----------- [UPD] Switch to Bootstrap 4 beta. Modified Paths: -------------- branches/experimental/bootstrap4/vendor_bundled/composer.json
http://sourceforge.net/p/tikiwiki/code/65853
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Revision: 65853 http://sourceforge.net/p/tikiwiki/code/65853 Author: jonnybradley Date: 2018-04-01 07:47:12 +0000 (Sun, 01 Apr 2018) Log Message: ----------- [FIX] bootstrap4: Fix three of the ways bs modals are opened for AJAX content use, buttons with data attributes, openModal and clickModal (i.e. tracker fields) and plugin edit. Some refactoring and convergence would be nice after we merge back to trunk... Modified Paths: -------------- branches/experimental/bootstrap4/lib/jquery_tiki/pluginedit.js branches/experimental/bootstrap4/lib/jquery_tiki/tiki-jquery.js
[+] Testing a quick and dirty BS4 integration (older information)
Two kinds of changes to deal with
Looks like there are basically two kinds of changes we'll need to cope with. One is the switch from Less to Sass. That's pretty straightforward as it mostly involves file names and variable names in those files (@variable to $variable, etc.). Of course, there are some method differences but probably not many that will cause trouble for us (one I came across is a color function - Less uses "spin" and Sass uses "adjust_color"). Also, paths like the image path variable have slightly different syntax. But I adjusted these as I came across them and will post some docs.
The second kind of change is with the Bootstrap bits themselves. In my "trunk - bootstrap 4 alpha", I found the grid layout and typography worked nicely already (BS4 switches to rem for the grid and for the typography), but panels and wells have been replaced by the card class, so in the templates we're going to have to do those replacements. Forms, especially horizontal forms, have changed quite a bit and will need to be adjusted. Also, the navbar is being simplified, as apparently we're not alone in having trouble wrangling content into that area. Menus and tabs weren't working yet either in my installation or at the Bootstrap 4 demo so apparently, they're still being worked out.
Looks like there are also Sass-variable files for Chosen; not sure about Table-sorter, but with Sass being more popular actually than Less, chances are good we can find support.
Interestingly, for Bootstrap 5, they're planning to move from Sass to PostCSS. ("Bootstrap 6" written originally in the previous sentence was a typo; thanks to Marc for correcting that.) PostCSS has a lot of momentum. Apparently, its power comes from the modules you add to it; it can do the standard Sass thing but can be configured to do custom things so you're not limited to the abilities of Sass or Less or whatever.
Useful links
Bootstrap 4
http://blog.getbootstrap.com/2018/01/18/bootstrap-4/ - Bootstrap 4.0.0 release announcement
https://getbootstrap.com - Overview - Currently v4.0.0
https://getbootstrap.com/docs/4.0/getting-started/introduction/ - Bootstrap 4 documentation
https://github.com/twbs/bootstrap/milestones
https://www.quackit.com/bootstrap/bootstrap_4/examples/
https://scotch.io/bar-talk/whats-new-in-bootstrap-4
http://bootstrapbay.com/bootstrap4/ (Bootstrap 4 Cheatsheet)
https://medium.com/wdstack/bootstrap-4-whats-new-visual-guide-c84dd81d8387#.b2sxbfsf6
http://www.hongkiat.com/blog/boostrap-4-best-features/
https://hackerthemes.com/bootstrap-cheatsheet/
https://bootstrapcreative.com/resources/bootstrap-4-css-classes-index/
https://www.udemy.com/bootstrap-4/
http://codepen.io/search?q=bootstrap+cards&limit=all&depth=everything&show_forks=false
SCSS (Sass)/Less
http://scout-app.io - Scout-App - Easy/informative SCSS compiling
https://gist.github.com/chriseppstein/674726 - Sass/Less Comparison
https://css-tricks.com/sass-vs-less/
Flexbox (Bootstrap 4 default layout method)
https://css-tricks.com/snippets/css/a-guide-to-flexbox/