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fetch_queue.js
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fetch_queue.js
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// Copyright 2020 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
class FetchQueue {
constructor(maxPending) {
this.maxPending = maxPending;
this.pending = {};
this.waiting = [];
this.seq = 0;
this.nPending = 0;
this.nWaiting = 0;
}
fetch(url) {
const promise = this.addToWaiting(url);
this.processOneWaitingIfPossible();
return promise;
}
addToWaiting(url) {
this.nWaiting += 1;
const requestDetails = {
url: url
};
const promise = new Promise((resolve,reject) => {
requestDetails.resolve = resolve;
requestDetails.reject = reject;
});
this.waiting.push(requestDetails);
return promise;
}
processOneWaitingIfPossible() {
if (this.nWaiting <= 0 || this.nPending >= this.maxPending) {
return;
}
const requestDetails = this.waiting.shift();
this.nWaiting -= 1;
this.nPending += 1;
this.pending[requestDetails.url] = requestDetails;
fetch(requestDetails.url)
.then(response => {
requestDetails.resolve(response);
})
.catch(error => {
requestDetails.reject(error);
})
.finally(() => {
this.nPending -= 1;
delete this.pending[requestDetails.url];
this.processOneWaitingIfPossible();
});
}
}
export {FetchQueue};