Question

我正试图让JavaScript承诺按预期工作。

我的代码：

var p = new Promise(function(resolve, reject) {
      for (var i = 0; i < pics_needed.length; i++) {
        download_pics(pics_needed[i])
      }
      for (var i = 0; i < partners_pics_needed.length; i++) {
        partners_download_pics(partners_pics_needed[i])
      }
      resolve('Success!');
    })
    p.then(function() { 
      AsyncStorage.setItem("database",responseText)
      AsyncStorage.removeItem("time")
      alert ("Success! \nYour update has been installed.")
      go()

    });

for循环中调用的两个函数都从服务器下载图片。问题是，在下载所有图片之前，函数的p.then部分正在运行。我怎样才能改变这一点，以便在完成所有下载后p.then部分发生？

功能的作用：

function download_pics (id){

 var path = RNFS.DocumentDirectoryPath + '/' + id + '.jpg';
 fetch('address of server ='+id)
  .then((response) => response.text())
   .then((responseText) => {
     var pic_object = JSON.parse(responseText)
     RNFS.writeFile(path, pic_object.response, 'base64')        
    });
 }

Answer 1

正如评论中所暗示的那样 - 如果你想要落后于他们，承诺并非毫无价值。

function download_pics (id){
 var path = RNFS.DocumentDirectoryPath + '/' + id + '.jpg';
 return fetch('address of server ='+id)  // <--- NOTE: return here
  .then((response) => response.text())
   .then((responseText) => {
     var pic_object = JSON.parse(responseText)
     return RNFS.writeFile(path, pic_object.response, 'base64') // <-- and here
    });
 }

然后

  var pics_promises = pics_needed.map(function(pic) {
    return download_pics(pic);
  });
  var partners_pics_promises = partners_pics_needed.map(function(pic) {
    return partners_download_pics(pic);
  });
  return Promise.all(pics_promises.concat(partners_pics_promises));

编辑：将RNFS.writeFile添加到@adeneo的承诺链中（我不熟悉RNFS）。

与承诺混淆

1 个答案: