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GWT Uploader is a freely available open source GWT library that encapsulates the file upload capabilities provided by the File and XMLHttpRequest Level 2 APIs as well as the SWFUpload library.

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/*
 * Copyright (c) 2016 WisePersist.org
 *
 * 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.
 */
package org.wisepersist.gwt.uploader.client.events;

/**
 * An interface that can be used as a callback handler when uploader start events are fired by
 * the Uploader component.  General usage is as follows:
 *  * uploader.setUploadStartHandler(new UploadStartHandler() {
 *    public boolean onUploadStart(UploadStartEvent event) {
 *       Window.alert("Beginning uploader of " + event.getFile().getName());
 *       return true;
 *    }
 * });
 * 
 * 
 * Note that the 'uploadStart' event is called immediately before the file is uploaded. This event
 * provides an opportunity to perform any last minute validation, add post params or do any other
 * work before the file is uploaded.
 * 

 * The uploader can be cancelled by returning 'false' from uploadStart in the handler. If you return
 * 'true' or do not return any value then the uploader proceeds. Returning 'false' will cause an
 * uploadError event to fired.
 * 
 * See the documentation on the {@link UploadStartEvent} class for more details on the data
 * available when an uploader start event occurs.
 *
 * @author [email protected]
 */
public interface UploadStartHandler {

  /**
   * This method is fired whenever a uploader start event occurs in the Uploader component.  See
   * the {@link UploadStartEvent} class for more details on the data available when this event is
   * fired.
   *
   * @param uploadStartEvent The details of the event that occurred.
   * @return The response to send back to the event handler function
   */
  public boolean onUploadStart(UploadStartEvent uploadStartEvent);

}