var before = null; requestAnimationFrame(function animate(now) {            var c = earth.getPosition();             elapsed = before? now - before: 0;            before = now;             earth.setCenter([c[0], c[1] + 0.1*(elapsed/30)]);             requestAnimationFrame(animate);                                 });before變量是開(kāi)關(guān)變量，怎么通過(guò)開(kāi)關(guān)去判斷requestAnimationFrame(animate)方法是否去執(zhí)行？現(xiàn)在需要停止這個(gè)動(dòng)畫(huà)方法，該怎么做？