Example code for writeStream ignores that stream.write() returns a promise?

[lll000111]

The example code at https://github.com/wkh237/react-native-fetch-blob#file-stream completely ignores the promise returned by the write function.

Now I wonder, if it does not matter, why return a promise? But if it matters, don't I have to wait for the promise? One, to be sure of success, two, for sequencing the writes correctly?

[wkh237]

Well, basically it does not matter because the fs I/O operations are queued and executed in the same thread.

[lll000111]

That leaves open the problem of a rejected writer promise though....

The example code is

RNFetchBlob.fs.writeStream(
    PATH_TO_FILE,
    // encoding, should be one of `base64`, `utf8`, `ascii`
    'utf8',
    // should data append to existing content ?
    true)
.then((ofstream) => {
    ofstream.write('foo')
    ofstream.write('bar')
    ofstream.close()
})

In the above code any rejected write() promise will be ignored.

It should be something like

async function write () {
    const ofstream = await RNFetchBlob.fs.writeStream(
        PATH_TO_FILE,
        // encoding, should be one of `base64`, `utf8`, `ascii`
        'utf8',
        // should data append to existing content ?
        true
    ;)

    // IF ANY OF THESE PROMISES REJECT THIS WHOLE FUNCTION WILL TOO
    await ofstream.write('foo');
    await ofstream.write('bar');
    await ofstream.close();
}

With pure promise notation it's more difficult because only the first one in the .then() chain gets the stream object - the write() command unfortunately resolves with undefined in case of success, if it would resolve with the stream object promise chaining would be simple.

[lll000111]

Suggestion (very short PR on the way): If write() resolves with the RNFetchBlobWriteStream instance instead of with undefined the write calls can be chained:

[return to calling function] RNFetchBlob.fs.writeStream(PATH_TO_FILE, 'utf8', true)
.then((ofstream) => ofstream.write('foo'))
.then((ofstream) => ofstream.write('bar'))
.then((ofstream) => ofstream.write('foobar'))
.then((ofstream) => ofstream.close())

No more issues with unhandled rejections, AND the returned promise now resolves when the last write actually happened, the original example code resolves when the stream object is created.

This is optional, since writes are in the order they are issued (Are they? Does the low-level iOS and Android code guarantee that?) an alternative is to issue all writes at once:

[return to calling function] RNFetchBlob.fs.writeStream(PATH_TO_FILE, 'utf8', true)
.then((ofstream) => Promise.all([
    ofstream.write('foo'),
    ofstream.write('bar'),
    ofstream.write('foobar')
]))
// Use array destructuring to get the stream object from the first item
.then(([ofstream]) => ofstream.close())

[lll000111]

@wkh237 I don't understand how Github and git work. When I try to create a pull request all the previous changes from the already merged pull request from yesterday are in it again! Even though I update my local repo!

Could you please insert this tiny(!) patch (change resolve()to resolve(this)) manually? I HATE git, it is WAY too complex (the correct word: not complicated but complex - complexity arises from interactions of by themselves very simple things)

How do I get new pull requests to only show NEW commits, and not stuff that has already been merged? Why won't let Github let me select the commits I want to put into a pull request?

I still don't know how it works, but I managed to create a pull request that shows only the relevant two lines in the diff (1 code line, 1 Flow type annotation for the function's return type).

[lll000111]

@lll000111 TODO for myself: PR to update the documentation for 0.10.9 too

Note to myself: Also fix the link to the hash() function Wiki page while editing the README.md file.

[lll000111]

I'm closing it here and create a link to this issue in my own folk of this repo.