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SWT is an open source widget toolkit for Java designed to provide efficient, portable access to the user-interface facilities of the operating systems on which it is implemented.
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/*******************************************************************************
 * Copyright (c) 2000, 2011 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *******************************************************************************/
package org.eclipse.swt.dnd;

 
import org.eclipse.swt.internal.ole.win32.*;
import org.eclipse.swt.internal.win32.*;

/**
 * Transfer provides a mechanism for converting between a java 
 * representation of data and a platform specific representation of data and 
 * vice versa.  It is used in data transfer operations such as drag and drop and 
 * clipboard copy/paste.
 *
 * You should only need to become familiar with this class if you are 
 * implementing a Transfer subclass and you are unable to subclass the 
 * ByteArrayTransfer class.
 * 
 * @see ByteArrayTransfer
 * @see Drag and Drop snippets
 * @see SWT Example: DNDExample
 * @see Sample code and further information
 */
public abstract class Transfer {

private static final int RETRY_LIMIT = 10;
/* 
 * Feature in Windows. When another application has control
 * of the clipboard, the clipboard is locked and it's not
 * possible to retrieve data until the other application is
 * finished. To allow other applications to get the
 * data, use PeekMessage() to enable cross thread
 * message sends.
 */
int getData(IDataObject dataObject, FORMATETC pFormatetc, STGMEDIUM pmedium) {
	if (dataObject.GetData(pFormatetc, pmedium) == COM.S_OK) return COM.S_OK;
	try {Thread.sleep(50);} catch (Throwable t) {}
	int result = dataObject.GetData(pFormatetc, pmedium);
	int retryCount = 0;
	while (result != COM.S_OK && retryCount++ < RETRY_LIMIT) {
		MSG msg = new MSG();
		OS.PeekMessage(msg, 0, 0, 0, OS.PM_NOREMOVE | OS.PM_NOYIELD);
		try {Thread.sleep(50);} catch (Throwable t) {}
		result = dataObject.GetData(pFormatetc, pmedium);
	}
	return result;
}

/**
 * Returns a list of the platform specific data types that can be converted using 
 * this transfer agent.
 *
 * Only the data type fields of the TransferData objects are filled 
 * in.
 *
 * @return a list of the data types that can be converted using this transfer agent
 */
abstract public TransferData[] getSupportedTypes();

/**
 * Returns true if the TransferData data type can be converted 
 * using this transfer agent, or false otherwise (including if transferData is
 * null).
 *
 * @param transferData a platform specific description of a data type; only the data
 *  type fields of the TransferData object need to be filled in
 *
 * @return true if the transferData data type can be converted using this transfer 
 * agent
 */
abstract public boolean isSupportedType(TransferData transferData);

/**
 * Returns the platform specific ids of the  data types that can be converted using 
 * this transfer agent.
 * 
 * @return the platform specific ids of the data types that can be converted using 
 * this transfer agent
 */
abstract protected int[] getTypeIds();

/**
 * Returns the platform specific names of the  data types that can be converted 
 * using this transfer agent.
 * 
 * @return the platform specific names of the data types that can be converted 
 * using this transfer agent.
 */
abstract protected String[] getTypeNames();

/**
 * Converts a java representation of data to a platform specific representation of 
 * the data. 
 *
 * On a successful conversion, the transferData.result field will be set as follows:
 * 

 * Windows: COM.S_OK
 * 
Motif: 1
 * 
GTK: 1
 * 
Photon: 1
 * 

 * 
 * If this transfer agent is unable to perform the conversion, the transferData.result 
 * field will be set to a failure value as follows:
 * 

 * Windows: COM.DV_E_TYMED or COM.E_FAIL
 * 
Motif: 0
 * 
GTK: 0
 * 
Photon: 0
 * 

 *
 * @param object a java representation of the data to be converted; the type of
 * Object that is passed in is dependent on the Transfer subclass.
 *
 * @param transferData an empty TransferData object; this object will be 
 * filled in on return with the platform specific representation of the data
 * 
 * @exception org.eclipse.swt.SWTException 
 *    ERROR_INVALID_DATA - if object does not contain data in a valid format or is null
 * 
 */
abstract protected void javaToNative (Object object, TransferData transferData);

/**
 * Converts a platform specific representation of data to a java representation.
 * 
 * @param transferData the platform specific representation of the data to be 
 * converted
 *
 * @return a java representation of the converted data if the conversion was 
 * successful; otherwise null.  If transferData is null then
 * null is returned.  The type of Object that is returned is 
 * dependent on the Transfer subclass.
 */
abstract protected Object nativeToJava(TransferData transferData);

/**
 * Registers a name for a data type and returns the associated unique identifier.
 *
 * You may register the same type more than once, the same unique identifier 
 * will be returned if the type has been previously registered.
 *
 * Note: On windows, do not call this method with pre-defined 
 * Clipboard Format types such as CF_TEXT or CF_BITMAP because the 
 * pre-defined identifier will not be returned
 *
 * @param formatName the name of a data type
 *
 * @return the unique identifier associated with this data type
 */
public static int registerType(String formatName) {
	// Look name up in the registry
	// If name is not in registry, add it and return assigned value.
	// If name already exists in registry, return its assigned value
	TCHAR chFormatName = new TCHAR(0, formatName, true);
	return OS.RegisterClipboardFormat(chFormatName);
}

/**
 * Test that the object is of the correct format for this Transfer class.
 * 
 * @param object a java representation of the data to be converted
 * 
 * @return true if object is of the correct form for this transfer type
 * 
 * @since 3.1
 */
protected boolean validate(Object object) {
	return true;
}
}