Make Settings modular and allow them to be EditEngine'd

Summary: Ref T4103. This starts breaking out settings in a modern way to prepare for global defaults.

Test Plan:
  - Edited diff settings.
  - Saw them take effect in primary settings pane.
  - Set stuff to new automatic defaults.
  - Tried to edit another user's settings.
  - Edited a bot's settings as an administrator.

{F1669077}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103

Differential Revision: https://secure.phabricator.com/D15995

src/__phutil_library_map__.php

@@ -2822,6 +2822,7 @@ phutil_register_library_map(array(
     'PhabricatorObjectSelectorDialog' => 'view/control/PhabricatorObjectSelectorDialog.php',
     'PhabricatorOffsetPagedQuery' => 'infrastructure/query/PhabricatorOffsetPagedQuery.php',
     'PhabricatorOldWorldContentSource' => 'infrastructure/contentsource/PhabricatorOldWorldContentSource.php',
+    'PhabricatorOlderInlinesSetting' => 'applications/settings/setting/PhabricatorOlderInlinesSetting.php',
     'PhabricatorOneTimeTriggerClock' => 'infrastructure/daemon/workers/clock/PhabricatorOneTimeTriggerClock.php',
     'PhabricatorOpcodeCacheSpec' => 'applications/cache/spec/PhabricatorOpcodeCacheSpec.php',
     'PhabricatorOwnerPathQuery' => 'applications/owners/query/PhabricatorOwnerPathQuery.php',
@@ -3348,11 +3349,15 @@ phutil_register_library_map(array(
     'PhabricatorSecurityConfigOptions' => 'applications/config/option/PhabricatorSecurityConfigOptions.php',
     'PhabricatorSecuritySetupCheck' => 'applications/config/check/PhabricatorSecuritySetupCheck.php',
     'PhabricatorSelectEditField' => 'applications/transactions/editfield/PhabricatorSelectEditField.php',
+    'PhabricatorSelectSetting' => 'applications/settings/setting/PhabricatorSelectSetting.php',
     'PhabricatorSendGridConfigOptions' => 'applications/config/option/PhabricatorSendGridConfigOptions.php',
     'PhabricatorSessionsSettingsPanel' => 'applications/settings/panel/PhabricatorSessionsSettingsPanel.php',
+    'PhabricatorSetting' => 'applications/settings/setting/PhabricatorSetting.php',
     'PhabricatorSettingsAddEmailAction' => 'applications/settings/action/PhabricatorSettingsAddEmailAction.php',
     'PhabricatorSettingsAdjustController' => 'applications/settings/controller/PhabricatorSettingsAdjustController.php',
     'PhabricatorSettingsApplication' => 'applications/settings/application/PhabricatorSettingsApplication.php',
+    'PhabricatorSettingsEditController' => 'applications/settings/controller/PhabricatorSettingsEditController.php',
+    'PhabricatorSettingsEditEngine' => 'applications/settings/editor/PhabricatorSettingsEditEngine.php',
     'PhabricatorSettingsMainController' => 'applications/settings/controller/PhabricatorSettingsMainController.php',
     'PhabricatorSettingsMainMenuBarExtension' => 'applications/settings/extension/PhabricatorSettingsMainMenuBarExtension.php',
     'PhabricatorSettingsPanel' => 'applications/settings/panel/PhabricatorSettingsPanel.php',
@@ -3363,6 +3368,7 @@ phutil_register_library_map(array(
     'PhabricatorSetupIssueUIExample' => 'applications/uiexample/examples/PhabricatorSetupIssueUIExample.php',
     'PhabricatorSetupIssueView' => 'applications/config/view/PhabricatorSetupIssueView.php',
     'PhabricatorShortSite' => 'aphront/site/PhabricatorShortSite.php',
+    'PhabricatorShowFiletreeSetting' => 'applications/settings/setting/PhabricatorShowFiletreeSetting.php',
     'PhabricatorSimpleEditType' => 'applications/transactions/edittype/PhabricatorSimpleEditType.php',
     'PhabricatorSite' => 'aphront/site/PhabricatorSite.php',
     'PhabricatorSlowvoteApplication' => 'applications/slowvote/application/PhabricatorSlowvoteApplication.php',
@@ -3568,6 +3574,7 @@ phutil_register_library_map(array(
     'PhabricatorUIExampleRenderController' => 'applications/uiexample/controller/PhabricatorUIExampleRenderController.php',
     'PhabricatorUIExamplesApplication' => 'applications/uiexample/application/PhabricatorUIExamplesApplication.php',
     'PhabricatorUSEnglishTranslation' => 'infrastructure/internationalization/translation/PhabricatorUSEnglishTranslation.php',
+    'PhabricatorUnifiedDiffsSetting' => 'applications/settings/setting/PhabricatorUnifiedDiffsSetting.php',
     'PhabricatorUnitTestContentSource' => 'infrastructure/contentsource/PhabricatorUnitTestContentSource.php',
     'PhabricatorUnitsTestCase' => 'view/__tests__/PhabricatorUnitsTestCase.php',
     'PhabricatorUnknownContentSource' => 'infrastructure/contentsource/PhabricatorUnknownContentSource.php',
@@ -3592,6 +3599,7 @@ phutil_register_library_map(array(
     'PhabricatorUserLogView' => 'applications/people/view/PhabricatorUserLogView.php',
     'PhabricatorUserPHIDResolver' => 'applications/phid/resolver/PhabricatorUserPHIDResolver.php',
     'PhabricatorUserPreferences' => 'applications/settings/storage/PhabricatorUserPreferences.php',
+    'PhabricatorUserPreferencesEditor' => 'applications/settings/editor/PhabricatorUserPreferencesEditor.php',
     'PhabricatorUserPreferencesPHIDType' => 'applications/settings/phid/PhabricatorUserPreferencesPHIDType.php',
     'PhabricatorUserPreferencesQuery' => 'applications/settings/query/PhabricatorUserPreferencesQuery.php',
     'PhabricatorUserPreferencesTransaction' => 'applications/settings/storage/PhabricatorUserPreferencesTransaction.php',
@@ -7413,6 +7421,7 @@ phutil_register_library_map(array(
     'PhabricatorObjectSelectorDialog' => 'Phobject',
     'PhabricatorOffsetPagedQuery' => 'PhabricatorQuery',
     'PhabricatorOldWorldContentSource' => 'PhabricatorContentSource',
+    'PhabricatorOlderInlinesSetting' => 'PhabricatorSelectSetting',
     'PhabricatorOneTimeTriggerClock' => 'PhabricatorTriggerClock',
     'PhabricatorOpcodeCacheSpec' => 'PhabricatorCacheSpec',
     'PhabricatorOwnerPathQuery' => 'Phobject',
@@ -8064,11 +8073,15 @@ phutil_register_library_map(array(
     'PhabricatorSecurityConfigOptions' => 'PhabricatorApplicationConfigOptions',
     'PhabricatorSecuritySetupCheck' => 'PhabricatorSetupCheck',
     'PhabricatorSelectEditField' => 'PhabricatorEditField',
+    'PhabricatorSelectSetting' => 'PhabricatorSetting',
     'PhabricatorSendGridConfigOptions' => 'PhabricatorApplicationConfigOptions',
     'PhabricatorSessionsSettingsPanel' => 'PhabricatorSettingsPanel',
+    'PhabricatorSetting' => 'Phobject',
     'PhabricatorSettingsAddEmailAction' => 'PhabricatorSystemAction',
     'PhabricatorSettingsAdjustController' => 'PhabricatorController',
     'PhabricatorSettingsApplication' => 'PhabricatorApplication',
+    'PhabricatorSettingsEditController' => 'PhabricatorController',
+    'PhabricatorSettingsEditEngine' => 'PhabricatorEditEngine',
     'PhabricatorSettingsMainController' => 'PhabricatorController',
     'PhabricatorSettingsMainMenuBarExtension' => 'PhabricatorMainMenuBarExtension',
     'PhabricatorSettingsPanel' => 'Phobject',
@@ -8079,6 +8092,7 @@ phutil_register_library_map(array(
     'PhabricatorSetupIssueUIExample' => 'PhabricatorUIExample',
     'PhabricatorSetupIssueView' => 'AphrontView',
     'PhabricatorShortSite' => 'PhabricatorSite',
+    'PhabricatorShowFiletreeSetting' => 'PhabricatorSelectSetting',
     'PhabricatorSimpleEditType' => 'PhabricatorEditType',
     'PhabricatorSite' => 'AphrontSite',
     'PhabricatorSlowvoteApplication' => 'PhabricatorApplication',
@@ -8305,6 +8319,7 @@ phutil_register_library_map(array(
     'PhabricatorUIExampleRenderController' => 'PhabricatorController',
     'PhabricatorUIExamplesApplication' => 'PhabricatorApplication',
     'PhabricatorUSEnglishTranslation' => 'PhutilTranslation',
+    'PhabricatorUnifiedDiffsSetting' => 'PhabricatorSelectSetting',
     'PhabricatorUnitTestContentSource' => 'PhabricatorContentSource',
     'PhabricatorUnitsTestCase' => 'PhabricatorTestCase',
     'PhabricatorUnknownContentSource' => 'PhabricatorContentSource',
@@ -8351,6 +8366,7 @@ phutil_register_library_map(array(
       'PhabricatorDestructibleInterface',
       'PhabricatorApplicationTransactionInterface',
     ),
+    'PhabricatorUserPreferencesEditor' => 'AlmanacEditor',
     'PhabricatorUserPreferencesPHIDType' => 'PhabricatorPHIDType',
     'PhabricatorUserPreferencesQuery' => 'PhabricatorCursorPagedPolicyAwareQuery',
     'PhabricatorUserPreferencesTransaction' => 'PhabricatorApplicationTransaction',

src/applications/people/storage/PhabricatorUser.php

@@ -503,6 +503,7 @@ final class PhabricatorUser
     if (!$preferences) {
       $preferences = new PhabricatorUserPreferences();
       $preferences->setUserPHID($this->getPHID());
+      $preferences->attachUser($this);
 
       $default_dict = array(
         PhabricatorUserPreferences::PREFERENCE_TITLES => 'glyph',

src/applications/settings/application/PhabricatorSettingsApplication.php

@@ -34,6 +34,8 @@ final class PhabricatorSettingsApplication extends PhabricatorApplication {
         'adjust/' => 'PhabricatorSettingsAdjustController',
         'timezone/(?P<offset>[^/]+)/'
           => 'PhabricatorSettingsTimezoneController',
+        '(?P<type>user)/(?P<username>[^/]+)/(?:panel/(?P<key>[^/]+)/)?'
+          => 'PhabricatorSettingsEditController',
       ),
     );
   }

src/applications/settings/controller/PhabricatorSettingsEditController.php

@@ -0,0 +1,35 @@
+<?php
+
+final class PhabricatorSettingsEditController
+  extends PhabricatorController {
+
+  public function handleRequest(AphrontRequest $request) {
+    $viewer = $this->getViewer();
+
+    $engine = id(new PhabricatorSettingsEditEngine())
+      ->setController($this);
+
+    switch ($request->getURIData('type')) {
+      case 'user':
+        $user = id(new PhabricatorPeopleQuery())
+          ->setViewer($viewer)
+          ->withUsernames(array($request->getURIData('username')))
+          ->executeOne();
+
+        $preferences = $user->loadPreferences();
+
+        PhabricatorPolicyFilter::requireCapability(
+          $viewer,
+          $preferences,
+          PhabricatorPolicyCapability::CAN_EDIT);
+
+        $engine->setTargetObject($preferences);
+        break;
+      default:
+        return new Aphront404Response();
+    }
+
+    return $engine->buildResponse();
+  }
+
+}

src/applications/settings/editor/PhabricatorSettingsEditEngine.php

@@ -0,0 +1,91 @@
+<?php
+
+final class PhabricatorSettingsEditEngine
+  extends PhabricatorEditEngine {
+
+  const ENGINECONST = 'settings.settings';
+
+  public function isEngineConfigurable() {
+    return false;
+  }
+
+  public function getEngineName() {
+    return pht('Settings');
+  }
+
+  public function getSummaryHeader() {
+    return pht('Edit Settings Configurations');
+  }
+
+  public function getSummaryText() {
+    return pht('This engine is used to edit settings.');
+  }
+
+  public function getEngineApplicationClass() {
+    return 'PhabricatorSettingsApplication';
+  }
+
+  protected function newEditableObject() {
+    return new PhabricatorUserPreferences();
+  }
+
+  protected function newObjectQuery() {
+    return new PhabricatorUserPreferencesQuery();
+  }
+
+  protected function getObjectCreateTitleText($object) {
+    return pht('Create Settings');
+  }
+
+  protected function getObjectCreateButtonText($object) {
+    return pht('Create Settings');
+  }
+
+  protected function getObjectEditTitleText($object) {
+    return pht('Edit Settings');
+  }
+
+  protected function getObjectEditShortText($object) {
+    return pht('Edit Settings');
+  }
+
+  protected function getObjectCreateShortText() {
+    return pht('Create Settings');
+  }
+
+  protected function getObjectName() {
+    return pht('Settings');
+  }
+
+  protected function getEditorURI() {
+    return '/settings/edit/';
+  }
+
+  protected function getObjectCreateCancelURI($object) {
+    return '/settings/';
+  }
+
+  protected function getObjectViewURI($object) {
+    // TODO: This isn't correct...
+    return '/settings/user/'.$this->getViewer()->getUsername().'/';
+  }
+
+  protected function getCreateNewObjectPolicy() {
+    return PhabricatorPolicies::POLICY_ADMIN;
+  }
+
+  protected function buildCustomEditFields($object) {
+    $viewer = $this->getViewer();
+    $settings = PhabricatorSetting::getAllEnabledSettings($viewer);
+
+    $fields = array();
+    foreach ($settings as $setting) {
+      foreach ($setting->newCustomEditFields($object) as $field) {
+        $fields[] = $field;
+      }
+    }
+
+    return $fields;
+  }
+
+}

src/applications/settings/editor/PhabricatorUserPreferencesEditor.php

@@ -0,0 +1,132 @@
+<?php
+
+final class PhabricatorUserPreferencesEditor
+  extends AlmanacEditor {
+
+  public function getEditorObjectsDescription() {
+    return pht('Settings');
+  }
+
+  public function getTransactionTypes() {
+    $types = parent::getTransactionTypes();
+
+    $types[] = PhabricatorUserPreferencesTransaction::TYPE_SETTING;
+
+    return $types;
+  }
+
+  protected function getCustomTransactionOldValue(
+    PhabricatorLiskDAO $object,
+    PhabricatorApplicationTransaction $xaction) {
+
+    $setting_key = $xaction->getMetadataValue(
+      PhabricatorUserPreferencesTransaction::PROPERTY_SETTING);
+
+    switch ($xaction->getTransactionType()) {
+      case PhabricatorUserPreferencesTransaction::TYPE_SETTING:
+        return $object->getPreference($setting_key);
+    }
+
+    return parent::getCustomTransactionOldValue($object, $xaction);
+  }
+
+  protected function getCustomTransactionNewValue(
+    PhabricatorLiskDAO $object,
+    PhabricatorApplicationTransaction $xaction) {
+
+    $actor = $this->getActor();
+
+    $setting_key = $xaction->getMetadataValue(
+      PhabricatorUserPreferencesTransaction::PROPERTY_SETTING);
+
+    $settings = PhabricatorSetting::getAllEnabledSettings($actor);
+    $setting = $settings[$setting_key];
+
+    switch ($xaction->getTransactionType()) {
+      case PhabricatorUserPreferencesTransaction::TYPE_SETTING:
+        $value = $xaction->getNewValue();
+        $value = $setting->getTransactionNewValue($value);
+        return $value;
+    }
+
+    return parent::getCustomTransactionNewValue($object, $xaction);
+  }
+
+  protected function applyCustomInternalTransaction(
+    PhabricatorLiskDAO $object,
+    PhabricatorApplicationTransaction $xaction) {
+
+    $setting_key = $xaction->getMetadataValue(
+      PhabricatorUserPreferencesTransaction::PROPERTY_SETTING);
+
+    switch ($xaction->getTransactionType()) {
+      case PhabricatorUserPreferencesTransaction::TYPE_SETTING:
+        $new_value = $xaction->getNewValue();
+        if ($new_value === null) {
+          $object->unsetPreference($setting_key);
+        } else {
+          $object->setPreference($setting_key, $new_value);
+        }
+        return;
+    }
+
+    return parent::applyCustomInternalTransaction($object, $xaction);
+  }
+
+  protected function applyCustomExternalTransaction(
+    PhabricatorLiskDAO $object,
+    PhabricatorApplicationTransaction $xaction) {
+
+    switch ($xaction->getTransactionType()) {
+      case PhabricatorUserPreferencesTransaction::TYPE_SETTING:
+        return;
+    }
+
+    return parent::applyCustomExternalTransaction($object, $xaction);
+  }
+
+  protected function validateTransaction(
+    PhabricatorLiskDAO $object,
+    $type,
+    array $xactions) {
+
+    $errors = parent::validateTransaction($object, $type, $xactions);
+
+    $actor = $this->getActor();
+    $settings = PhabricatorSetting::getAllEnabledSettings($actor);
+
+    switch ($type) {
+      case PhabricatorUserPreferencesTransaction::TYPE_SETTING:
+        foreach ($xactions as $xaction) {
+          $setting_key = $xaction->getMetadataValue(
+            PhabricatorUserPreferencesTransaction::PROPERTY_SETTING);
+
+          $setting = idx($settings, $setting_key);
+          if (!$setting) {
+            $errors[] = new PhabricatorApplicationTransactionValidationError(
+              $type,
+              pht('Invalid'),
+              pht(
+                'There is no known application setting with key "%s".',
+                $setting_key),
+              $xaction);
+            continue;
+          }
+
+          try {
+            $setting->validateTransactionValue($xaction->getNewValue());
+          } catch (Exception $ex) {
+            $errors[] = new PhabricatorApplicationTransactionValidationError(
+              $type,
+              pht('Invalid'),
+              $ex->getMessage(),
+              $xaction);
+          }
+        }
+        break;
+    }
+
+    return $errors;
+  }
+
+}

src/applications/settings/phid/PhabricatorUserPreferencesPHIDType.php

@@ -32,7 +32,6 @@ final class PhabricatorUserPreferencesPHIDType extends PhabricatorPHIDType {
     $viewer = $query->getViewer();
     foreach ($handles as $phid => $handle) {
       $preferences = $objects[$phid];
-
       $handle->setName(pht('Settings %d', $preferences->getID()));
     }
   }

src/applications/settings/setting/PhabricatorOlderInlinesSetting.php

@@ -0,0 +1,35 @@
+<?php
+
+final class PhabricatorOlderInlinesSetting
+  extends PhabricatorSelectSetting {
+
+  const SETTINGKEY = 'diff-ghosts';
+
+  const VALUE_GHOST_INLINES_ENABLED = 'default';
+  const VALUE_GHOST_INLINES_DISABLED = 'disabled';
+
+  public function getSettingName() {
+    return pht('Show Older Inlines');
+  }
+
+  protected function getControlInstructions() {
+    return pht(
+      'When a revision is updated, Phabricator attempts to bring inline '.
+      'comments on the older version forward to the new changes. You can '.
+      'disable this behavior if you prefer comments stay anchored in one '.
+      'place.');
+  }
+
+  public function getSettingDefaultValue() {
+    return self::VALUE_GHOST_INLINES_ENABLED;
+  }
+
+  protected function getSelectOptions() {
+    return array(
+      self::VALUE_GHOST_INLINES_ENABLED => pht('Enabled'),
+      self::VALUE_GHOST_INLINES_DISABLED => pht('Disabled'),
+    );
+  }
+
+
+}

src/applications/settings/setting/PhabricatorSelectSetting.php

@@ -0,0 +1,55 @@
+<?php
+
+abstract class PhabricatorSelectSetting
+  extends PhabricatorSetting {
+
+  abstract protected function getSelectOptions();
+
+  final protected function newCustomEditField($object) {
+    $setting_key = $this->getSettingKey();
+    $default_value = $object->getDefaultValue($setting_key);
+
+    $options = $this->getSelectOptions();
+
+    if (isset($options[$default_value])) {
+      $default_label = pht('Default (%s)', $options[$default_value]);
+    } else {
+      $default_label = pht('Default (Unknown, "%s")', $default_value);
+    }
+
+    $options = array(
+      '' => $default_label,
+    ) + $options;
+
+    return $this->newEditField($object, new PhabricatorSelectEditField())
+      ->setOptions($options);
+  }
+
+  final public function validateTransactionValue($value) {
+    if (!strlen($value)) {
+      return;
+    }
+
+    $options = $this->getSelectOptions();
+
+    if (!isset($options[$value])) {
+      throw new Exception(
+        pht(
+          'Value "%s" is not valid for setting "%s": valid values are %s.',
+          $value,
+          $this->getSettingName(),
+          implode(', ', array_keys($options))));
+    }
+
+    return;
+  }
+
+  public function getTransactionNewValue($value) {
+    if (!strlen($value)) {
+      return null;
+    }
+
+    return (string)$value;
+  }
+
+}

src/applications/settings/setting/PhabricatorSetting.php

@@ -0,0 +1,96 @@
+<?php
+
+abstract class PhabricatorSetting extends Phobject {
+
+  private $viewer;
+
+  public function setViewer(PhabricatorUser $viewer) {
+    $this->viewer = $viewer;
+    return $this;
+  }
+
+  public function getViewer() {
+    return $this->viewer;
+  }
+
+  abstract public function getSettingName();
+
+  protected function getControlInstructions() {
+    return null;
+  }
+
+  protected function isEnabledForViewer(PhabricatorUser $viewer) {
+    return true;
+  }
+
+  public function getSettingDefaultValue() {
+    return null;
+  }
+
+  final public function getSettingKey() {
+    return $this->getPhobjectClassConstant('SETTINGKEY');
+  }
+
+  public static function getAllSettings() {
+    return id(new PhutilClassMapQuery())
+      ->setAncestorClass(__CLASS__)
+      ->setUniqueMethod('getSettingKey')
+      ->execute();
+  }
+
+  public static function getAllEnabledSettings(PhabricatorUser $viewer) {
+    $settings = self::getAllSettings();
+    foreach ($settings as $key => $setting) {
+      if (!$setting->isEnabledForViewer($viewer)) {
+        unset($settings[$key]);
+      }
+    }
+    return $settings;
+  }
+
+  final public function newCustomEditFields($object) {
+    $fields = array();
+
+    $field = $this->newCustomEditField($object);
+    if ($field) {
+      $fields[] = $field;
+    }
+
+    return $fields;
+  }
+
+  protected function newCustomEditField($object) {
+    return null;
+  }
+
+  protected function newEditField($object, PhabricatorEditField $template) {
+    $setting_property = PhabricatorUserPreferencesTransaction::PROPERTY_SETTING;
+    $setting_key = $this->getSettingKey();
+    $value = $object->getPreference($setting_key);
+    $xaction_type = PhabricatorUserPreferencesTransaction::TYPE_SETTING;
+    $label = $this->getSettingName();
+
+    $template
+      ->setKey($setting_key)
+      ->setLabel($label)
+      ->setValue($value)
+      ->setTransactionType($xaction_type)
+      ->setMetadataValue($setting_property, $setting_key);
+
+    $instructions = $this->getControlInstructions();
+    if (strlen($instructions)) {
+      $template->setControlInstructions($instructions);
+    }
+
+    return $template;
+  }
+
+  public function validateTransactionValue($value) {
+    return;
+  }
+
+  public function getTransactionNewValue($value) {
+    return $value;
+  }
+
+}

src/applications/settings/setting/PhabricatorShowFiletreeSetting.php

@@ -0,0 +1,34 @@
+<?php
+
+final class PhabricatorShowFiletreeSetting
+  extends PhabricatorSelectSetting {
+
+  const SETTINGKEY = 'diff-filetree';
+
+  const VALUE_DISABLE_FILETREE = 0;
+  const VALUE_ENABLE_FILETREE = 1;
+
+  public function getSettingName() {
+    return pht('Show Filetree');
+  }
+
+  protected function getControlInstructions() {
+    return pht(
+      'When viewing a revision or commit, you can enable a sidebar showing '.
+      'affected files. When this option is enabled, press {nav %s} to show '.
+      'or hide the sidebar.',
+      'f');
+  }
+
+  public function getSettingDefaultValue() {
+    return self::VALUE_DISABLE_FILETREE;
+  }
+
+  protected function getSelectOptions() {
+    return array(
+      self::VALUE_DISABLE_FILETREE => pht('Disable Filetree'),
+      self::VALUE_ENABLE_FILETREE => pht('Enable Filetree'),
+    );
+  }
+
+}

src/applications/settings/setting/PhabricatorUnifiedDiffsSetting.php

@@ -0,0 +1,35 @@
+<?php
+
+final class PhabricatorUnifiedDiffsSetting
+  extends PhabricatorSelectSetting {
+
+  const SETTINGKEY = 'diff-unified';
+
+  const VALUE_ON_SMALL_SCREENS = 'default';
+  const VALUE_ALWAYS_UNIFIED = 'unified';
+
+  public function getSettingName() {
+    return pht('Show Unified Diffs');
+  }
+
+  protected function getControlInstructions() {
+    return pht(
+      'Phabricator normally shows diffs in a side-by-side layout on large '.
+      'screens and automatically switches to a unified view on small '.
+      'screens (like mobile phones). If you prefer unified diffs even on '.
+      'large screens, you can select them for use on all displays.');
+  }
+
+  public function getSettingDefaultValue() {
+    return self::VALUE_ON_SMALL_SCREENS;
+  }
+
+  protected function getSelectOptions() {
+    return array(
+      self::VALUE_ON_SMALL_SCREENS => pht('On Small Screens'),
+      self::VALUE_ALWAYS_UNIFIED => pht('Always'),
+    );
+  }
+
+
+}

src/applications/settings/storage/PhabricatorUserPreferences.php

@@ -92,6 +92,21 @@ final class PhabricatorUserPreferences
     return $this;
   }
 
+  public function getDefaultValue($key) {
+    $setting = self::getSettingObject($key);
+
+    if (!$setting) {
+      return null;
+    }
+
+    return $setting->getSettingDefaultValue();
+  }
+
+  private static function getSettingObject($key) {
+    $settings = PhabricatorSetting::getAllSettings();
+    return idx($settings, $key);
+  }
+
   public function getPinnedApplications(array $apps, PhabricatorUser $viewer) {
     $pref_pinned = self::PREFERENCE_APP_PINNED;
     $pinned = $this->getPreference($pref_pinned);
@@ -212,8 +227,7 @@ final class PhabricatorUserPreferences
 
 
   public function getApplicationTransactionEditor() {
-    // TODO: Implement.
-    throw new PhutilMethodNotImplementedException();
+    return new PhabricatorUserPreferencesEditor();
   }
 
   public function getApplicationTransactionObject() {

src/applications/settings/storage/PhabricatorUserPreferencesTransaction.php

@@ -3,6 +3,10 @@
 final class PhabricatorUserPreferencesTransaction
   extends PhabricatorApplicationTransaction {
 
+  const TYPE_SETTING = 'setting';
+
+  const PROPERTY_SETTING = 'setting.key';
+
   public function getApplicationName() {
     return 'user';
   }

src/view/form/control/AphrontFormSelectControl.php

@@ -56,6 +56,7 @@ final class AphrontFormSelectControl extends AphrontFormControl {
     $disabled = array_fuse($disabled);
 
     $tags = array();
+    $already_selected = false;
     foreach ($options as $value => $thing) {
       if (is_array($thing)) {
         $tags[] = phutil_tag(
@@ -65,10 +66,22 @@ final class AphrontFormSelectControl extends AphrontFormControl {
           ),
           self::renderOptions($selected, $thing));
       } else {
+        // When there are a list of options including similar values like
+        // "0" and "" (the empty string), only select the first matching
+        // value. Ideally this should be more precise about matching, but we
+        // have 2,000 of these controls at this point so hold that for a
+        // broader rewrite.
+        if (!$already_selected && ($value == $selected)) {
+          $is_selected = 'selected';
+          $already_selected = true;
+        } else {
+          $is_selected = null;
+        }
+
         $tags[] = phutil_tag(
           'option',
           array(
-            'selected' => ($value == $selected) ? 'selected' : null,
+            'selected' => $is_selected,
             'value'    => $value,
             'disabled' => isset($disabled[$value]) ? 'disabled' : null,
           ),