assetsManager:setDelegate(onProgress, cc.ASSETSMANAGER_PROTOCOL_PROGRESS) 后，
下载进度不连续 ， 打印是 onProgress回调函数的，downloading是c++ 那边打印出来的
downloading… 1%
downloading… 2%
downloading… 3%
downloading… 4%
downloading… 5%
downloading… 6%
downloading… 7%
downloading… 8%
downloading… 9%
downloading… 10%
downloading… 11%
downloading… 12%
downloading… 13%
downloading… 14%
downloading… 15%
downloading… 16%
downloading… 17%
downloading… 18%
已下载:18%
已下载:18%
已下载:18%
已下载:18%
已下载:18%
已下载:18%
已下载:18%
已下载:18%
已下载:18%
已下载:18%
已下载:18%
已下载:18%
已下载:18%
已下载:18%
已下载:18%
已下载:18%
已下载:18%
已下载:18%
downloading… 19%
downloading… 20%
downloading… 21%
downloading… 22%
downloading… 23%
downloading… 24%
downloading… 25%
downloading… 26%
downloading… 27%
downloading… 28%
downloading… 29%
downloading… 30%
downloading… 31%
downloading… 32%
downloading… 33%
downloading… 34%
downloading… 35%
downloading… 36%
downloading… 37%
downloading… 38%
downloading… 39%
downloading… 40%
downloading… 41%
downloading… 42%
downloading… 43%
downloading… 44%
downloading… 45%
downloading… 46%
downloading… 47%
downloading… 48%
downloading… 49%
downloading… 50%
downloading… 51%
downloading… 52%
downloading… 53%
downloading… 54%
downloading… 55%
downloading… 56%
downloading… 57%
downloading… 58%
downloading… 59%
downloading… 60%
downloading… 61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
已下载:61%
downloading… 62%
downloading… 63%
downloading… 64%
downloading… 65%
downloading… 66%
downloading… 67%
downloading… 68%
downloading… 69%
downloading… 70%
downloading… 71%
downloading… 72%
downloading… 73%
downloading… 74%
downloading… 75%
downloading… 76%
downloading… 77%
downloading… 78%
downloading… 79%
downloading… 80%
downloading… 81%
downloading… 82%
downloading… 83%
downloading… 84%
downloading… 85%
downloading… 86%
downloading… 87%
downloading… 88%
downloading… 89%
downloading… 90%
downloading… 91%
downloading… 92%
downloading… 93%
downloading… 94%
downloading… 95%
downloading… 96%
downloading… 97%
downloading… 98%
downloading… 99%
downloading… 100%
已下载:100%
已下载:100%
已下载:100%
已下载:100%
已下载:100%