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KeyPromiseQueue.js

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// KeyPromiseQueue is a simple promise queue

// used to queue operations per key basis.

// Once the tail promise in the key-queue fulfills,

// the chain on that key will be cleared.

export class KeyPromiseQueue {

constructor() {

this.queue = {};

}

enqueue(key, operation) {

const tuple = this.beforeOp(key);

const toAwait = tuple[1];

const nextOperation = toAwait.then(operation);

const wrappedOperation = nextOperation.then(result => {

this.afterOp(key);

return result;

});

tuple[1] = wrappedOperation;

return wrappedOperation;

}

beforeOp(key) {

let tuple = this.queue[key];

if (!tuple) {

tuple = [0, Promise.resolve()];

this.queue[key] = tuple;

}

tuple[0]++;

return tuple;

}

afterOp(key) {

const tuple = this.queue[key];

if (!tuple) {

return;

}

tuple[0]--;

if (tuple[0] <= 0) {

delete this.queue[key];

return;

}

}

}