URL loader: allow overriding default Xhr based loader by a custom one

useful for P2P/testing
add a callback to deal with timeout
related to #2
related to #3

src/utils/xhr-loader.js

@@ -0,0 +1,89 @@
+ /*
+  * Xhr based Loader
+  *
+  */
+
+import {logger}             from '../utils/logger';
+
+ class XhrLoader {
+
+  constructor() {
+  }
+
+  destroy() {
+    this.abort();
+    this.loader = null;
+  }
+
+  abort() {
+    if(this.loader &&this.loader.readyState !== 4) {
+      this.loader.abort();
+    }
+    if(this.timeoutHandle) {
+      window.clearTimeout(this.timeoutHandle);
+    }
+  }
+
+  load(url,responseType,onSuccess,onError,onTimeout,timeout,maxRetry,retryDelay) {
+    this.url = url;
+    this.responseType = responseType;
+    this.onSuccess = onSuccess;
+    this.onTimeout = onTimeout;
+    this.onError = onError;
+    this.trequest = new Date();
+    this.timeout = timeout;
+    this.maxRetry = maxRetry;
+    this.retryDelay = retryDelay;
+    this.retry = 0;
+    this.timeoutHandle = window.setTimeout(this.loadtimeout.bind(this),timeout);
+    this.loadInternal();
+  }
+
+  loadInternal() {
+    var xhr = this.loader = new XMLHttpRequest();
+    xhr.onload =  this.loadsuccess.bind(this);
+    xhr.onerror = this.loaderror.bind(this);
+    xhr.onprogress = this.loadprogress.bind(this);
+    xhr.open('GET', this.url , true);
+    xhr.responseType = this.responseType;
+    this.tfirst = null;
+    this.loaded = 0;
+    xhr.send();
+  }
+
+  loadsuccess(event) {
+    window.clearTimeout(this.timeoutHandle);
+    this.onSuccess(event,{trequest : this.trequest, tfirst : this.tfirst, tload : new Date(), loaded : this.loaded});
+  }
+
+  loaderror(event) {
+    if(this.retry < this.maxRetry) {
+      logger.log(`${event.type} while loading ${this.url}, retrying in ${this.retryDelay}...`);
+      this.destroy();
+      window.setTimeout(this.loadInternal.bind(this),this.retryDelay);
+      // exponential backoff
+      this.retryDelay=Math.min(2*this.retryDelay,64000);
+      this.retry++;
+    } else {
+      window.clearTimeout(this.timeoutHandle);
+      logger.log(`${event.type} while loading ${this.url}` );
+      this.onError(event);
+    }
+  }
+
+  loadtimeout(event) {
+    logger.log(`timeout while loading ${this.url}` );
+    this.onTimeout(event,{trequest : this.trequest, tfirst : this.tfirst, loaded : this.loaded});
+  }
+
+  loadprogress(event) {
+    if(this.tfirst === null) {
+      this.tfirst = new Date();
+    }
+    if(event.lengthComputable) {
+      this.loaded = event.loaded;
+    }
+  }
+}
+
+export default XhrLoader;