performance_timeline_2 library Null safety

Performance Timeline Level 2


The object encapsulates a single performance metric that is part of the performance timeline. A performance entry can be directly created by making a performance mark or measure (for example by calling the mark() method) at an explicit point in an application. Performance entries are also created in indirect ways such as loading a resource (such as an image). [...]
The interface is used to observe performance measurement events and be notified of new performance entries as they are recorded in the browser's performance timeline. Note: This feature is available in Web Workers
The interface is a list of peformance events that were explicitly observed via the observe() method. [...]