public void dragEnter(DropTargetEvent event) {
			if (event.detail == DND.DROP_DEFAULT) {
				if ((event.operations & DND.DROP_COPY) != 0) {
					event.detail = DND.DROP_COPY;
				} else if ((event.operations & DND.DROP_MOVE) != 0) {
					event.detail = DND.DROP_MOVE;
				} else {
					event.detail = DND.DROP_NONE;
				}
			}
			for (int i = 0; i < event.dataTypes.length; i++) {
				if (textTransfer.isSupportedType(event.dataTypes[i])) {
					event.currentDataType = event.dataTypes[i];
					// 只允许COPY or MOVE
					if (event.detail != DND.DROP_COPY
							&& event.detail != DND.DROP_MOVE) {
						event.detail = DND.DROP_NONE;
					}
					break;
				}
			}
		}