CODEWITHSUNDEEP
node.jsstreamcassandraecmascript-6
daniyel
daniyel

Reputation: 682

Readable stream and callback function

I have a bit of a problem with reading the stream from cassandra (I do not even know is it possible what I am trying to achieve). So on the cassandra-driver git repository page there is an example how to use stream. I've tried it and it works.

But however I am trying to use so called classes from ES6 proposal with nodejs 5. I want to define model as a class and in one of the methods I am using stream (where I am fetching the data from cassandra). Problem is with the readable state, where you have this.read() in callback function and now when this is called in a class this becomes a class scope so it is always undefined. I've tried extending my class with the ResultStream from cassandra-driver module but no luck, maybe I am not calling it properly. I've tried with the data state (different class and method as a callback) and it is working since data state has a argument passed as a chunk.

So the question is, how i could encapsulate this stream call in a class method, so the readable state could be read?

Example code of what I would like to achieve:

class Foobar {
    constructor(client) {
        this.client = client;
        this.collection = [];
        this.error;
    }
    getByProductName(query, params) {
        this.client.stream(query, params, {prepare: true})
            .on('readable', () => {
                var row;
                while(row = this.read()) { // Problem with this scope
                    this.collection.push(row);
                }
            })
            .on('error', err => {
                if(err) {
                    this.error = err;
                }
            })
            .on('end', () => {
                console.log('end');
            });
    }
}

Thank you for any advices.

Upvotes: 2

Views: 1846

Answers (1)

jorgebg
jorgebg

Reputation: 6600

You could capture the stream instance in a closure:

class Foobar {
    constructor(client) {
        this.client = client;
        this.collection = [];
        this.error;
    }
    getByProductName(query, params) {
        const stream = this.client.stream(query, params, { prepare: true })
            .on('readable', () => {
                var row;
                while(row = stream.read()) { // <- use stream instance 
                    this.collection.push(row);
                }
            })
            .on('error', err => {
                if(err) {
                    this.error = err;
                }
            })
            .on('end', () => {
                console.log('end');
            });
    }
}

Upvotes: 3

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