Time for action – using DialogSettings

A more useful alternative to the Memento pattern is DialogSettings, which provides a properties-like interface for storing strings and other basic primitive values. This stores its information in an XML file, and can be acquired as a standard extension to the UI plug-in or created from a file location. The settings store is used to store values persistently, and is saved automatically when the plug-in shuts down. At startup, it is loaded automatically. Perform the following steps:

  1. To migrate the settings for the last tab selected to use DialogSettings, remove the init() and save() methods from the TimeZoneView and replace them with the following in the createPartControl():
    final IDialogSettings settings =Activator.getDefault().getDialogSettings();
lastTabSelected = settings.get("lastTabSelected");
  2. The call to getDialogSettings() comes from the UIPlugin class. Once the DialogSettings have been acquired, it can be used to store and retrieve values. Update the selection listener to store this in the settings instead:
    tabs.addSelectionListener(new SelectionAdapter() {
  public void widgetSelected(SelectionEvent e) {
    if (e.item instanceof CTabItem) {
      lastTabSelected = ((CTabItem) e.item).getText();
      settings.put("lastTabSelected", lastTabSelected);
    }
  }
});
  3. Now run the Eclipse instance, go to the Time Zone View and note how the settings are saved when either the view is closed or the application shuts down.

What just happened?

The calls to IMemento were replaced with DialogSettings, a much more useful mechanism for storing values. In addition, we created a separate group of settings to create a nested namespace with settings.addNewSection("name") and with settings.getSection("name").

The name DialogSettings comes from the fact that was initially used by dialogs with warnings such as "Do not show this message again". In fact, this is so common that a Dialog with a "Do not show this message again" can be created with a MessageDialogWithToggle as follows:

if (settings.getBoolean("spamalot")) {
  MessageDialogWithToggle dialog = MessageDialogWithToggle.
   openInformation(Display.getCurrent().getActiveShell(),
    "Spam", "Keep being spammed?", "Do not show this spam again",
    false, null, null);
  boolean spamalot = !dialog.getToggleState();
  settings.put("spamalot",spamalot);
}

The MessageDialogWithToggle also has an option to write this to a preference store using the store and the key value in the last two values (null in the example).

The DialogSettings can also be used to store items such as the last-value-used or to restore selection.

The key difference between using an IPreferenceStore and using DialogSettings is that the former can be used for importing/exporting preferences between Eclipse instances, using the File | Import/Export | Preferences menu. The DialogSettings, on the other hand, are supposed to be transient and recreatable if they are lost.

Note

What just happened? E4: As E4 takes off and provides default support for reading and writing preferences, expect to see more uses of the preferences for storing both transient and non-transient preference data.

Pop quiz – understanding preferences

Q1. What is the default style used for the FieldEditorPreferencePage, and how can it be changed to something more aesthetically pleasing?

Q2. What kinds of primitive values can be edited with a FieldEditorPreferencePage?

Q3. How can a preference value be searched for in the preference page?

Q4. Which is the preferred API for storing view-specific information; IMemento or IEclipsePreferences?

Q5. Which class provides the "Do not show this message again" support?

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