我正在尝试使用FileReader使用promises顺序读取多个文件 . 问题是我需要将读取操作分成几个块来使它们可行 . 通过这样做,我失去了Promise链 . 这是在下面的代码中发生的,我得到控制台日志 here
,然后是 catched
(意思是我丢失了链),然后是 inside
然后是 finished
. 不知何故,上传中的承诺不受尊重 .
这是代码(请转到上一次编辑,我保留原始文本)
var reader = new FileReader();
reader.onloadend = function(e) {
if (e.target.readyState == 2) {
console.log('inside')
start = temp_end;
temp_end = start + BYTES_PER_CHUNK;
if (temp_end > end) temp_end = end;
upload();
}
};
Array.reduce(function(promise, item) {
start = 0;
temp_end = start + BYTES_PER_CHUNK;
end = parseInt(totalsize);
if (temp_end > end) temp_end = end;
uploading_file = item;
return upload()
.then(function(){
console.log('not shown');
})
.catch(console.log('catched'));
},0);
function upload() {
return new Promise(function(resolve,reject) {
if (start < end) {
console.log('here')
var chunk = uploading_file.slice(start, temp_end);
reader.readAsBinaryString(chunk);
} else {
console.log('finished')
uploading_file = null;
resolve();
}
}
}
EDIT1:我正在尝试的新代码,仍然是console.log stop 1
出现在 upload
和 here 2
之前
var start = 0;
var temp_end = start + BYTES_PER_CHUNK;
var end = parseInt(item.size);
if (temp_end > end) temp_end = end;
uploading_file = item;
console.log('here 1')
Promise.resolve().then(function() {
return upload();
})
.then(function(){
console.log('here 2')
})
.catch(console.log('stop 1'));
function upload() {
console.log('upload')
if (start < end) {
var chunk = uploading_file.slice(start, temp_end);
var reader = new FileReader();
reader.readAsArrayBuffer(chunk);
reader.onload = function(e) {
if (e.target.readyState == 2) {
start = temp_end;
temp_end = start + BYTES_PER_CHUNK;
if (temp_end > end) temp_end = end;
return upload();
}
}
} else {
console.log('finished')
uploading_file = null;
return Promise.resolve();
}
}
EDIT2:由于缺少函数(),因此存在catch日志 . 以下代码似乎有效,但我看不到我最后感兴趣的值 content
.
console.log说,问题是承诺确实在这里不起作用
here 1
here 2
here 4
here 3
代码:
var item; // item is a file
var BYTES_PER_CHUNK = 100000;
var start = 0;
var temp_end = start + BYTES_PER_CHUNK;
var end = parseInt(item.size);
if (temp_end > end) temp_end = end;
var content = ''; // to be filled by the content of the file
var uploading_file = item;
console.log('here 1');
Promise.resolve().then(function() {
console.log('here 2');
return upload();
})
.then(function(content){
console.log('here 4');
console.log(content); // this is empty (!!)
})
.catch(function(){
console.log('stop 1')
});
function upload() {
if (start < end) {
var chunk = uploading_file.slice(start, temp_end);
var reader = new FileReader();
reader.readAsArrayBuffer(chunk);
reader.onload = function(e) {
if (e.target.readyState == 2) {
content += new TextDecoder("utf-8").decode(e.target.result);
start = temp_end;
temp_end = start + BYTES_PER_CHUNK;
if (temp_end > end) temp_end = end;
return upload();
}
}
} else {
uploading_file = null;
console.log(content); // it shows the content of the file
console.log('here 3');
return Promise.resolve(content);
}
}
1 回答
您的
upload()
函数并不总是返回Promise . 它在else条件下执行,但if条件在回调内部没有't. You have a return statement, but it',因此upload
的调用者不会接收它 .尝试将其更改为:
现在,
upload
总是返回一个Promise而不是undefined
.