Add missing metadata lib

src/libs/metadata.js

@@ -0,0 +1,32 @@
+import request from 'request'
+
+function loadJSON(url, defaultValue, cb) {
+  request({
+    url: url,
+    withCredentials: false,
+  }, (error, response, body) => {
+    if (!error && body && response.statusCode == 200) {
+      try {
+        cb(JSON.parse(body))
+      } catch(err) {
+        console.error(err)
+        cb(defaultValue)
+      }
+    } else {
+      console.warn('Can not metadata for ' + url)
+      cb(defaultValue)
+    }
+  })
+}
+
+export function downloadGlyphsMetadata(urlTemplate, cb) {
+  if(!urlTemplate) return cb([])
+  const url = urlTemplate.replace('{fontstack}/{range}.pbf', 'fontstacks.json')
+  loadJSON(url, [], cb)
+}
+
+export function downloadSpriteMetadata(baseUrl, cb) {
+  if(!baseUrl) return cb([])
+  const url = baseUrl + '.json'
+  loadJSON(url, {}, glyphs => cb(Object.keys(glyphs)))
+}