# HaTS in Chrome

HaTS is the survey feature on ChromeOS.
Let's see how HaTS is implemented as ChromeOS UI.

## HaTS Config
[HatsConfig](https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/ash/hats/hats_config.h;l=14;drc=446950b310f139999a622c7029805504979f798b) struct is created for each type of the survey.
For example, [kHatsGeneralSurvey](https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/ash/hats/hats_config.cc;l=49-54;drc=f4a00cc248dd2dc8ec8759fb51620d47b5114090) constant represents the general survey shown after login.
Each config holds
- `feature`: corresponding flag to decide whether the survey is enabled
- `new_device_threshold`: minimum time after initial login
- `is_selected_pref_name`: whether we are selected for the current survey cycle
- `cycle_end_timestamp_pref_name`: when the current survey cycle end
- `survey_last_interaction_timestamp_pref_name`: last time the user interacted with the survey
- `threshold_time`: minimum time after the previous interaction
- `globcal_cap_opt_out`: opt-out by global cap

For example, the general survey struct is like this:

const HatsConfig kHatsGeneralSurvey = {
    ::features::kHappinessTrackingSystem,  // feature
    base::Days(7),                         // new_device_threshold
    prefs::kHatsDeviceIsSelected,          // is_selected_pref_name
    prefs::kHatsSurveyCycleEndTimestamp,   // cycle_end_timestamp_pref_name
};

The feature flags are in [kHappinessTrackingSystem*](https://source.chromium.org/chromium/chromium/src/+/main:chrome/common/chrome_features.cc;l=697-793;drc=f4a00cc248dd2dc8ec8759fb51620d47b5114090) and they are disabled by default.

## HaTS UI
[HatsNotificationController](https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/ash/hats/hats_notification_controller.h;l=31;drc=fe132eeb21687c455d695d6af346f15454828d01) controls the UI.
Some features (e.g. [OsSettingsHatsManager](https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/ui/webui/ash/settings/services/hats/os_settings_hats_manager.cc;l=44-45;drc=f4a00cc248dd2dc8ec8759fb51620d47b5114090)) create this controller and handle the notification.

[HatsNotificationController::ShowDialog](https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/ash/hats/hats_notification_controller.cc;l=331;drc=f4a00cc248dd2dc8ec8759fb51620d47b5114090) will call the static method [HatsDialog::Show](https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/ash/hats/hats_dialog.cc;l=180;drc=f4a00cc248dd2dc8ec8759fb51620d47b5114090) and construct [HatsDialog](https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/ash/hats/hats_dialog.h;l=19;drc=8dd419d33fa6112ca7d4892e0450fc0c8959b090).
[HatsDialog](https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/ash/hats/hats_dialog.h;l=19;drc=8dd419d33fa6112ca7d4892e0450fc0c8959b090) is a class which represents the happinesss tracking survey dialog.
This is implemented as one of [ui::WebDialogDelegate](https://source.chromium.org/chromium/chromium/src/+/main:ui/web_dialogs/web_dialog_delegate.h;l=36;drc=f4a00cc248dd2dc8ec8759fb51620d47b5114090), the abstract class to receive notifications.

Show is triggered mainly from [MessageCenterImpl](https://source.chromium.org/chromium/chromium/src/+/main:ui/message_center/message_center_impl.cc;l=556;drc=f4a00cc248dd2dc8ec8759fb51620d47b5114090) on click.

## Hats finch
[HatsFinchHelper](https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/ash/hats/hats_finch_helper.h;l=23;drc=8f57934dc9dd175acdc5fc56e2e6b7c3cb8e3b95) provides an API for the controller to retrieve processed information about finch experiment.

Finch is the A/B experiment platform for Chrome.
Whether the feature is enabled on the finch experiment or not for the user is partially stored in this object.
For example, the method [IsEnabledForGooglers](https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/ash/hats/hats_finch_helper.cc;l=76;drc=f4a00cc248dd2dc8ec8759fb51620d47b5114090) returns whether it's enabled for googlers obviously.
Also, we can get the histogram name, client data and so on.