问题描述
我正在尝试使用FileReader来使用诺言顺序读取多个文件.问题是我需要将读取操作分成几部分以使其可行.这样,我失去了Promise链.这就是下面的代码中发生的情况,在该代码中,我获得了控制台日志here
,然后是catched
(这意味着我已经丢失了链),然后是inside
然后是finished
.不知何故不遵守上传承诺.
I'm trying to use FileReader to read several files sequentially using promises for that. The problem is that I need to divide the reading operations in several chunks to make them doable. By doing so, I lose the Promise chain. This is what happen in the following code, where I get the console log here
, then catched
(meaning I've lost the chain), then inside
and then finished
. Somehow the Promise in upload is not respected.
这是代码(请转到最后的EDIT,尽管如此,我仍保留原始文本)
Here it is the code (please go to the last EDIT, I keep the original text nonetheless)
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();
}
}
}
我正在尝试的新代码,console.log stop 1
仍然出现在upload
和here 2
new code I'm trying, still the console.log stop 1
appears before the upload
and the here 2
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();
}
}
由于缺少function(),捕获日志在那里.以下代码似乎有效,但最后看不到我感兴趣的值content
.
The catch logs were there due to a lack of function(). The following code seems to work, but I cannot see the value content
that I am interested at the end.
console.log说,问题是诺言确实在这里不起作用
The problem is that promises are indeed not working here, the console.log says
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);
}
}
推荐答案
您的upload()
函数并不总是返回Promise.它在 else 条件下运行,但是if条件没有.您有一个return语句,但是它在 callback 中,因此upload
的调用者不会收到它.
Your upload()
function does not always return a Promise. It does in the else condition, but the if condition doesn't. You have a return statement, but it's inside of a callback, so it won't be received by the caller of upload
.
尝试将其更改为此:
function upload() {
if (start < end) {
return new Promise(function (resolve, reject) {
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;
resolve(upload());
}
}
});
} else {
uploading_file = null;
console.log(content); // it shows the content of the file
return Promise.resolve(content);
}
}
现在,upload
始终返回Promise而不是undefined
.
Now, upload
always returns a Promise instead of undefined
.
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