问题
Is it possible to detect if the client supports a particular Unicode character or if it will be rendered as a missing glyph box?
Important: Support in as many browsers as possible
Not important: Efficiency, speed, or elegance
The only method I can think of trying is using a canvas, so I figured I'd ask before I start going down that road.
Thanks!
Edit: This is not intended for use on a public web site; I am just trying to compile a list of characters supported by each browser.
回答1:
This is more of a wild idea than a real answer:
If you could find a character which you knew would always render as a missing glyph box, you could use the same technique as this javascript font detector--render the character and the missing glyph box offscreen and compare their widths. If they're different, then you know the character is not rendering as a missing glyph box. Of course, this won't work at all for fixed-width fonts, and it could have a lot of fixed negatives for other fonts where a lot of the characters are the same width.
回答2:
Not sure whether it can be relied upon going forward (browsers might change what is shown for unsupported characters), nor am I sure that this is optimized (as I don't have a good understanding of the ideal boundaries to measure here), but the following approach (of drawing text in canvas and inspecting the result as an image) may, if reviewed, provide a more reliable and accurate check than could checking the width. All of the code in the beginning is just browser detection which we must use since feature detection is not possible.
(function () {
// http://www.quirksmode.org/js/detect.html
var BrowserDetect = {
init: function () {
this.browser = this.searchString(this.dataBrowser) || "An unknown browser";
this.version = this.searchVersion(navigator.userAgent)
|| this.searchVersion(navigator.appVersion)
|| "an unknown version";
this.OS = this.searchString(this.dataOS) || "an unknown OS";
},
searchString: function (data) {
for (var i=0;i<data.length;i++) {
var dataString = data[i].string;
var dataProp = data[i].prop;
this.versionSearchString = data[i].versionSearch || data[i].identity;
if (dataString) {
if (dataString.indexOf(data[i].subString) != -1)
return data[i].identity;
}
else if (dataProp)
return data[i].identity;
}
},
searchVersion: function (dataString) {
var index = dataString.indexOf(this.versionSearchString);
if (index == -1) return;
return parseFloat(dataString.substring(index+this.versionSearchString.length+1));
},
dataBrowser: [
{
string: navigator.userAgent,
subString: "Chrome",
identity: "Chrome"
},
{ string: navigator.userAgent,
subString: "OmniWeb",
versionSearch: "OmniWeb/",
identity: "OmniWeb"
},
{
string: navigator.vendor,
subString: "Apple",
identity: "Safari",
versionSearch: "Version"
},
{
prop: window.opera,
identity: "Opera",
versionSearch: "Version"
},
{
string: navigator.vendor,
subString: "iCab",
identity: "iCab"
},
{
string: navigator.vendor,
subString: "KDE",
identity: "Konqueror"
},
{
string: navigator.userAgent,
subString: "Firefox",
identity: "Firefox"
},
{
string: navigator.vendor,
subString: "Camino",
identity: "Camino"
},
{ // for newer Netscapes (6+)
string: navigator.userAgent,
subString: "Netscape",
identity: "Netscape"
},
{
string: navigator.userAgent,
subString: "MSIE",
identity: "Explorer",
versionSearch: "MSIE"
},
{
string: navigator.userAgent,
subString: "Gecko",
identity: "Mozilla",
versionSearch: "rv"
},
{ // for older Netscapes (4-)
string: navigator.userAgent,
subString: "Mozilla",
identity: "Netscape",
versionSearch: "Mozilla"
}
],
dataOS : [
{
string: navigator.platform,
subString: "Win",
identity: "Windows"
},
{
string: navigator.platform,
subString: "Mac",
identity: "Mac"
},
{
string: navigator.userAgent,
subString: "iPhone",
identity: "iPhone/iPod"
},
{
string: navigator.platform,
subString: "Linux",
identity: "Linux"
}
]
};
BrowserDetect.init();
/**
* Checks whether a given character is supported in the specified font. If the
* font argument is not provided, it will default to sans-serif, the default
* of the canvas element
* @param {String} chr Character to check for support
* @param {String} [font] Font Defaults to sans-serif
* @returns {Boolean} Whether or not the character is visually distinct from characters that are not supported
*/
function characterInFont (chr, font) {
var data,
size = 10, // We use 10 to confine results (could do further?) and minimum required for 10px
x = 0,
y = size,
canvas = document.createElement('canvas'),
ctx = canvas.getContext('2d');
// Necessary?
canvas.width = size;
canvas.height = size;
if (font) { // Default of canvas is 10px sans-serif
font = size + 'px ' + font; // Fix size so we can test consistently
/**
// Is there use to confining by this height?
var d = document.createElement("span");
d.font = font;
d.textContent = chr;
document.body.appendChild(d);
var emHeight = d.offsetHeight;
document.body.removeChild(d);
alert(emHeight); // 19 after page load on Firefox and Chrome regardless of canvas height
//*/
}
ctx.fillText(chr, x, y);
data = ctx.getImageData(0, 0, ctx.measureText(chr).width, canvas.height).data; // canvas.width
data = Array.prototype.slice.apply(data);
function compareDataToBox (data, box, filter) {
if (filter) { // We can stop making this conditional if we confirm the exact arrays will continue to work, or otherwise remove and rely on safer full arrays
data = data.filter(function (item) {
return item != 0;
});
}
return data.toString() !== box;
}
var missingCharBox;
switch (BrowserDetect.browser) {
case 'Firefox': // Draws nothing
missingCharBox = '';
break;
case 'Opera':
//missingCharBox = '0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,197,0,0,0,255,0,0,0,255,0,0,0,255,0,0,0,255,0,0,0,73,0,0,0,0,0,0,0,0,0,0,0,36,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,36,0,0,0,0,0,0,0,0,0,0,0,36,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,36,0,0,0,0,0,0,0,0,0,0,0,36,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,36,0,0,0,0,0,0,0,0,0,0,0,36,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,36,0,0,0,0,0,0,0,0,0,0,0,197,0,0,0,255,0,0,0,255,0,0,0,255,0,0,0,255,0,0,0,73,0,0,0,0';
missingCharBox = '197,255,255,255,255,73,36,36,36,36,36,36,36,36,197,255,255,255,255,73';
break;
case 'Chrome':
missingCharBox = '2,151,255,255,255,255,67,2,26,2,26,2,26,2,26,2,26,2,26,2,26,2,26,2,151,255,255,255,255,67';
break;
case 'Safari':
missingCharBox = '17,23,23,23,23,5,52,21,21,21,21,41,39,39,39,39,39,39,39,39,63,40,40,40,40,43';
break;
default:
throw 'characterInFont() not tested successfully for this browser';
}
return compareDataToBox(data, missingCharBox, true);
}
// EXPORTS
((typeof exports !== 'undefined') ? exports : this).characterInFont = characterInFont;
}());
var r1 = characterInFont('a', 'Arial'); // true
var r2 = characterInFont('\uFAAA', 'Arial'); // false
alert(r1);
alert(r2);
UPDATE 1
I tried to update for modern Firefox (to try to check for the expected hex digits within the canvas), and checking to ensure that, unlike my code above, the canvas (and pattern to match it) was just large enough to accommodate the widest character per context.measureText() (U+0BCC from my testing, though presumably dependent on font, in my case "Arial Unicode MS"). Per https://bugzilla.mozilla.org/show_bug.cgi?id=442133#c9 , however, measureText currently mistakenly responds to the zoom for only the unknown characters. Now, if only one could simulate the zoom in JavaScript canvas so as to affect these measurements (and only those measurements)...
Code available for reference at https://gist.github.com/brettz9/1f061bb2ce06368db3e5
回答3:
You can always evaluate each character using the charCodeAt() method. This will return the unicode character value. Depending on what you are doing, you can restrict the range of which you want to accept as "Valid" characters... If you copy the character that's in the "box", you can use a character translator on the web to see what the corresponding unicode value is.
Here's one that I googled and found: enter link description here
回答4:
If you want to maximize browser support, you probably don't want to rely on javascript for anything. Many mobile browsers don't even support it.
If the browser does not support a char set, what is the fall back? Displaying the content in another language? Perhaps links one the site that switch languages on demand would be more robust.
来源：https://stackoverflow.com/questions/1911000/detecting-individual-unicode-character-support-with-javascript