Package com.google.gwt.cell.client

Interface Cell<C>

Type Parameters:
C - the type that this Cell represents
All Known Implementing Classes:
AbstractCell, AbstractEditableCell, AbstractInputCell, AbstractSafeHtmlCell, ActionCell, ButtonCell, ButtonCellBase, CheckboxCell, ClickableTextCell, CompositeCell, DateCell, DatePickerCell, EditTextCell, IconCellDecorator, ImageCell, ImageLoadingCell, ImageResourceCell, NumberCell, SafeHtmlCell, SafeImageCell, SelectionCell, TextButtonCell, TextCell, TextInputCell
public interface Cell<C>
A lightweight representation of a renderable object.

Multiple cell widgets or Columns can share a single Cell instance, but there may be implications for certain stateful Cells. Generally, Cells are stateless flyweights that see the world as row values/keys. If a Column contains duplicate row values/keys, the Cell will not differentiate the value in one row versus another. Similarly, if you use a single Cell instance in multiple Columns, the Cells will not differentiate the values coming from one Column versus another.

However, some interactive Cells (EditTextCell, CheckboxCell, TextInputCell, etc...) have a stateful "pending" state, which is a map of row values/keys to the end user entered pending value. For example, if an end user types a new value in a TextInputCell, the TextInputCell maps the "pending value" and associates it with the original row value/key. The next time the Cell Widget renders that row value/key, the Cell renders the pending value instead. This allows applications to refresh the Cell Widget without clearing out all of the end user's pending changes. In subclass of AbstractEditableCell, the pending state remains until either the original value is updated (a successful commit), or until AbstractEditableCell.clearViewData(Object) is called (a failed commit).

If you share an interactive Cell between two cell widgets (or Columns within the same CellTable), then when the end user updates the pending value in one widget, it will be reflected in the other widget when the other widget is redrawn. You should base your decision on whether or not to share Cell instances on this behavior.

Example

public class CellExample implements EntryPoint {

  /**
   * A custom {@link Cell} used to render a string that contains the name of a
   * color.
   */
  static class ColorCell extends AbstractCell<String> {

    /**
     * The HTML templates used to render the cell.
     */
    interface Templates extends SafeHtmlTemplates {
      /**
       * The template for this Cell, which includes styles and a value.
       * 
       * @param styles the styles to include in the style attribute of the div
       * @param value the safe value. Since the value type is {@link SafeHtml},
       *          it will not be escaped before including it in the template.
       *          Alternatively, you could make the value type String, in which
       *          case the value would be escaped.
       * @return a {@link SafeHtml} instance
       */
      @SafeHtmlTemplates.Template("<div style=\"{0}\">{1}</div>")
      SafeHtml cell(SafeStyles styles, SafeHtml value);
    }

    /**
     * Create a singleton instance of the templates used to render the cell.
     */
    private static Templates templates = GWT.create(Templates.class);

    @Override
    public void render(Context context, String value, SafeHtmlBuilder sb) {
      /*
       * Always do a null check on the value. Cell widgets can pass null to
       * cells if the underlying data contains a null, or if the data arrives
       * out of order.
       */
      if (value == null) {
        return;
      }

      // If the value comes from the user, we escape it to avoid XSS attacks.
      SafeHtml safeValue = SafeHtmlUtils.fromString(value);

      // Use the template to create the Cell's html.
      SafeStyles styles = SafeStylesUtils.forTrustedColor(safeValue.asString());
      SafeHtml rendered = templates.cell(styles, safeValue);
      sb.append(rendered);
    }
  }

  /**
   * The list of data to display.
   */
  private static final List<String> COLORS = Arrays.asList("red", "green", "blue", "violet",
      "black", "gray");

  @Override
  public void onModuleLoad() {
    // Create a cell to render each value.
    ColorCell cell = new ColorCell();

    // Use the cell in a CellList.
    CellList<String> cellList = new CellList<String>(cell);

    // Push the data into the widget.
    cellList.setRowData(0, COLORS);

    // Add it to the root panel.
    RootPanel.get().add(cellList);
  }
}

Warning: The Cell interface may change in subtle but breaking ways as we continuously seek to improve performance. You should always subclass AbstractCell instead of implementing Cell directly.