This document gives the best practices on how to use user actions in code and how to document them for the dashboard. User actions come with only a name and a timestamp. They are best used when you care about a sequence--which actions happen in what order. If you don't care about the order, you should be using histograms (likely enumerated histograms).
Often, you want both user actions and histogram logging in your code. They enable different analyses. They're complementary.
Generally you'll want to call
base::RecordAction(), which is defined in https://cs.chromium.org/chromium/src/base/metrics/user_metrics.h.
Prefer to emit at the highest level reasonable, closest to the code that handles the UI interaction. Emitting deep in implementation code can cause problems because that code may get reused (and thus called more times in more places) or may get called fewer times (due to caching for example). In cases like this, the logged user action will not longer correspond with a meaningful action by the user.
Generally a logged user action should correspond with a single, uh, action by the user. :-) As such, they should probably only appear in a single place in the code. If the same user action needs to be logged in multiple places, consider whether you should be using different user action names for these separate call paths.
That said, if you truly need to record the same user action in multiple places, that's okay. Use a compile-time constant of appropriate scope that can be referenced everywhere. Using inline strings in multiple places can lead to errors if you ever need to revise the name and you update one one location and forget another.
Due to the practices about when and how often to emit a user action, actions should not be emitted often enough to cause efficiency issues. (If actions are emitted often enough to cause a problem, they're not being emitted at appropriate times. See advice below.)
A user action should be tied to an actual action taken by a user. Each meaningful unit of action should cause one emit.
Generally a meaningful user action should cause only one emit. For example, if the browser already has a “Back” user action, it‘s poor practice to add a “BackViaKeyboardShortcut” user action. This is mostly redundant. (If you’re trying to determine the breakdown of keyboard-shortcut backs versus all backs, use a histogram.)
Again, choose an appropriately-sized meaningful unit. For example, emit “DragScrolled” for a whole scroll action. Don't emit this action every time the user pauses scrolling if the user remains in the process of scrolling (mouse button still down).
As another example, you may want to emit “FocusOmnibox” (upon focus), “OmniboxEditInProgress” (upon first keystroke), and “OmniboxUse” (upon going somwhere) but forswear “OmniboxKeystroke”. That's probably more detailed than you need.
It‘s okay to emit user actions such as “ShowTranslateInfobar” or “DisplayedImageLinkContextMenu”. However, more detailed impression information, especially those not caused by the user (as in the second example) and not as attention-grabbing (as in the first example), is often not useful for analyzing sequences of user actions. For example, don’t emit “ShowedSecureIconNextToOmnibox”.
Test your user actions using chrome://user-actions. Make sure they're being emitted when you expect and not emitted at other times.
If this is a general UI surface, please try to check every platform. In particular, check Windows (Views-based platforms), Mac (non-Views), Android phone (yet other UI wrapper code), Android tablet (often triggers lookalike but different menus), and iOS (yet more different UI wrapper code).
In addition to testing interactively, you can have unit tests check the number of times a user action was emitted. See user_action_tester.h for details.
The top of go/uma-guide has good advice on how to go about analyzing and interpreting the results of UMA data uploaded by users. If you‘re reading this page, you’ve probably just finished adding a user action to the Chromium source code and you‘re waiting for users to update their version of Chrome to a version that includes your code. In this case, the best advice is to remind you that users who update frequently / quickly are biased. Best take the initial statistics with a grain of salt; they’re probably mostly right but not entirely so.
When changing the semantics of a user action (when it's emitted), make it into a new user action with a new name. Otherwise the dashboard will be mixing two different interpretations of the data and make no sense.
Document user actions in actions.xml. There is also a google-internal version of the file for the rare case when the user action is confidential (added only to Chrome code, not Chromium code; or, an accurate description about how to interpret the user action would reveal information about Google's plans).
If possible, please add the actions.xml description in the same changelist in which you add the user-action-emitting code. This has several benefits. One, it sometimes happens that the actions.xml reviewer has questions or concerns about the user action description that reveal problems with interpretation of the data and call for a different recording strategy. Two, it allows the user action reviewer to easily review the emission code to see if it comports with these best practices, and to look for other errors.
User actions descriptions should be understandable to someone not familiar with your feature. Please add a sentence or two of background if necessary.
It is good practice to note caveats associated with your user actions in this section, such as which platforms are supported (if the set of supported platforms is surprising). E.g., a desktop feature that happens not to be logged on Mac.
User action descriptions should clearly state when the action is emitted.
User actions need to be owned by a person or set of people. These indicate who the current experts on it are. Being the owner means you are responsible for answering questions about it, handling the maintenance if there are functional changes. The owners should be added in the original user action description. If you are using a user action heavily and understand it intimately, feel free to add yourself as an owner. @chromium.org email addresses are preferred.
actions.xml allows you to annotate an action as
not_user_triggered="true". This feature should be used rarely. If you think you want to annotate your action thusly, please re-review the best practices above.
Do not delete actions from actions.xml. Instead, mark unused user actions as obsolete, annotating them with the associated date or milestone in the obsolete tag entry. If your user action is being replaced by a new version, we suggest noting that in the previous user action's description.
A changelist that marks a user action as obsolete should be reviewed by all current owners.
Deleting user action entries would be bad if someone accidentally reused your old user action name and which therefore corrupts new data with whatever old data is still coming in. It‘s also useful to keep obsolete user action descriptions in actions.xml--that way, if someone is searching for a user action to answer a particular question, they can learn if there was a user action at some point that did so even if it isn’t active now.