feat(downloading): save download session support

src/background/torrentClient.js

@@ -1,3 +1,44 @@
 const WebTorrent = require('webtorrent')
-let torrentClient = new WebTorrent({webSeeds: false})
+const fs = require('fs')
+
+const torrentClient = new WebTorrent({webSeeds: false})
+torrentClient.saveSession = (sessionFile) => {
+	fs.writeFileSync(sessionFile, JSON.stringify({
+        torrents: torrentClient.torrents.map(torrent => ({
+            infoHash: torrent.infoHash,
+            path: torrent.path,
+            torrent: torrent.torrentObject
+        }))
+    }, null, 4), 'utf8');
+}
+torrentClient.loadSession = (sessionFile) => {
+    if(!fs.existsSync(sessionFile))
+	{
+        console.log('no download sessions - ignore')
+        return
+    }
+
+	const data = fs.readFileSync(sessionFile, 'utf8')
+    const obj = JSON.parse(data);
+    if(!obj.torrents)
+    {
+        console.log('no torrents list for loading session')
+        return
+    }
+    if(!torrentClient._add)
+    {
+        console.log('no overriden _add() method')
+        return
+    }
+    const {torrents} = obj
+    torrents.forEach(({torrent, infoHash, path}) => {
+        if(!torrent || !infoHash || !path)
+        {
+            console.log('no info for starting download this torrent')
+            return
+        }
+        console.log('restore download session:', torrent.name)
+        torrentClient._add(torrent, path)
+    })
+}
 module.exports = torrentClient