近期发现部分机器 react-native一直loading问题，分析日志没有追踪到问题根源。

ReactNativeJS: Possible Unhandled Promise Rejection (id: 0):
                                                                      Network request Failed

经过一番折腾才查到是fetch没有回调

A fetch() promise will reject with a TypeError when a network error is encountered,although this usually means permission issues or similar — a 404 does not 
constitute a network error,for example. An accurate check for a successful 
fetch() would include checking that the promise resolved,then checking that the 
Response.ok property has a value of true. The code would look something like this:

fetch('flowers.jpg')
.then(function(response) {
 if(response.ok) {
  return response.blob();
 }
 throw new Error('Network response was not ok.');
})
.then(function(myBlob) {
 var objectURL = URL.createObjectURL(myBlob);
 myImage.src = objectURL;
})
.catch(function(error) {
 console.log('There has been a problem with your fetch operation: ' + error.message);
});

按照文章意思增加catch异常处理逻辑即可。