How do ligature icons work in Material Icons?

问题

Using Material Icons, a plus icon can be added as follows:

<i class="material-icons">add</i>

The text add is no longer visible. Why does this happen and where does the plus icon come from? I know it's defined in the font file, but how?

If it's due to the word add linked with the plus icon in the font file, then why doesn't the following work in Bootstrap, with its Glyphicons?

<span style="font-family: 'Glyphicons Halflings'">\20ac</span>

回答1:

EXPLANATION

When you strip all the technical information, the answer is really quite straightforward, the font file incorporates a few tables amongst which:

[MANDATORY] the list of characters
[MANDATORY] the hexadecimal codes of those characters
[OPTIONAL] one or more aliases/alternative names for those characters

The one or more aliases/alternative names are the 'ligatures' you are referring to and reside in the font file.

Essentially, when using a character/icon from a font file with ligatures, we have the option to use

the 'regular' hexadecimal code: &#xxxx;
or the alternative/alias/ligature name: ligature-name or alias

That is probably all the important info for a web designer to know.

EXTRAS

Go to CSS-Tricks: How do ligature icons work... to see usage examples and a brief explanation.

And if you want to mess around with your own icon font files I suggest you start using the IcoMoon APP:

start the APP, select an icon and select 'generate font' (bottom right)
Enable display of ligatures with the 'show ligatures'-button (top left 3rd button)

回答2:

Material Icons. It is possible in a font to define special glyphs for combinations of characters. An example in English is the glyph æ, which is a combination of a and e. This is called a ligature. Other examples are special renderings of ff, ft and tt. Instead of drawing an f followed by another one, the two glyphs are drawn as a single connected glyph: f f versus ff. What the designers of the Material Icons set did is (ab)using this system to make it easy to use icons.

Let's take a step back for a moment. You'll notice in the usage of the add icon that it is possible to include it by directly using a character code that is mapped, in the font, to the correct icon.

<i class="material-icons">&#xE145;</i>

This refers to Unicode character U+E145, which falls in one of the Private Use Area blocks of the Unicode specification. This means that no character is usually assigned to this position and every font designer is free to put any glyph they want at that position. Google chose to put the add icon at that spot. Thus, this character, with font family Material Icons, will render as a nice icon.

In addition to that, they created a ligature in their font family that says that the combination of characters add should be rendered as the same glyph. When browsers support ligatures in their font rendering engine, this will result in the same output as using &#xE145 would.

Google documents this very briefly as well.

In a nutshell: both  (U+E145) and the string add will render as when using Material Icons.

<link href="https://fonts.googleapis.com/icon?family=Material+Icons"
      rel="stylesheet">
As character: <span class="material-icons">&#xE145;</span>.<br>
As ligature: <span class="material-icons">add</span>.

Boostrap and Glyphicons. The Glyphicons font does not define ligatures, but referencing the correct characters definitely does work. This is exactly what Bootstrap does, by setting (for the plus icon from Glyphicons) content: "\002b";. This sets the content of the span it is applied on to the character represented by the escaped code point U+002B, which is the plus sign. The Glyphicons Halflings font family renders this as some sort of icon, just like Material Icons. The only difference is that the icon is represented by a different character.

Why does using \002B in a span not work, you ask? That's because escaping a Unicode character in CSS is different than in HTML. In HTML, you'd use + instead (or € to get the example you have in your question). You can read more about escaping here.

Thus, + (U+002B) renders as and € (U+20AC) renders as when using the Glyphicons Halflings font family. You'll notice that for the Glyphicons, they chose to use characters resembling the icons, whereas Material Icons use special, reserved characters.

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet"/>
<span style="font-family: 'Glyphicons Halflings'">&#x002B; &#x20AC;</span>

回答3:

Here it states that "To use these icons, use the material-icons class on an element and provide the ligature as the text content." The add in the add is what creates the plus sign. The classes on the  element tells it to apply the css and the size that you want the icon. If you inspect the element of the icons on this page then you can see that the words below the icon come from a  that they have put beside it.

Bootstrap glyphicons are attached to their own classes. Here it states "Icon classes should only be used on elements that contain no text content and have no child elements." So bootstrap glyphicons require the elements to be empty. Materialize and Bootstrap have different methods for accessing their icons. Bootstrap uses .glyphicon-asterisk{ &:before { content: "\002a"; } } and applies the content :before, whereas materialize does not. It replaces the current content with the right icon.

回答4:

yes, you can with material<link rel="stylesheet" href="https://fonts.googleapis.com/icon?family=Material+Icons"> then in the span you place the class 

then add CSS with the names of the icons

.depth-1 > li.sub > a > span::after {
    content: 'arrow_drop_down'; 
}

.depth-1 > li.active.sub > a > span::after {
    content: 'arrow_drop_up'; 
}

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https://codepen.io/gusanofunx/pen/vpaqbP