17. A system of one or more computers comprising one or more processors and memory, the processors further configured to perform operations for managing user interface (UI) building blocks (UIBB), the operations comprising: presenting, by a form repeater UIBB, initial UI representations of two or more structured entities through a graphical user interface (GUI), each initial UI representation of the two or more structured entities comprising one or more fields; receiving a trigger from a user through the form repeater UIBB, the trigger associated with a particular initial UI representation of a particular structured entity of the two or more structured entities; in response to receiving the trigger, identifying an event condition ID associated with the trigger; matching the event condition ID to a stored ID value; based on the matching, identifying i) a substitute UI representation of the particular structured entity, ii) an event condition, and iii) a type of the event condition, wherein the substitute UI representation of the particular structured entity includes one or more fields that differ from the one or more fields of the particular initial UI representation of the particular structured entity; determining that the event condition identifies that an origination of the trigger is associated with the particular initial UI representation, and that the type of the event condition includes a swapping-in event condition; based on the determination that the event condition identifies that the origination of the trigger is associated with the particular initial UI representation, and that the type of the event condition includes the swapping-in event condition, replacing, by a substitute UIBB, the particular initial UI representation of the particular structured entity of the two or more structured entities with the identified substitute UI representation of the particular structured entity; removing the particular initial UI representation of the particular structured entity from the form repeater UIBB; and maintaining the initial UI representation of the remaining structured entities of the two or more structured entities concurrently with replacing the particular initial UI representation of the particular structured entity with the identified substitute UI representation of the particular structured entity.
17. A system of one or more computers comprising one or more processors and memory, the processors further configured to perform operations for managing user interface (UI) building blocks (UIBB), the operations comprising: presenting, by a form repeater UIBB, initial UI representations of two or more structured entities through a graphical user interface (GUI), each initial UI representation of the two or more structured entities comprising one or more fields; receiving a trigger from a user through the form repeater UIBB, the trigger associated with a particular initial UI representation of a particular structured entity of the two or more structured entities; in response to receiving the trigger, identifying an event condition ID associated with the trigger; matching the event condition ID to a stored ID value; based on the matching, identifying i) a substitute UI representation of the particular structured entity, ii) an event condition, and iii) a type of the event condition, wherein the substitute UI representation of the particular structured entity includes one or more fields that differ from the one or more fields of the particular initial UI representation of the particular structured entity; determining that the event condition identifies that an origination of the trigger is associated with the particular initial UI representation, and that the type of the event condition includes a swapping-in event condition; based on the determination that the event condition identifies that the origination of the trigger is associated with the particular initial UI representation, and that the type of the event condition includes the swapping-in event condition, replacing, by a substitute UIBB, the particular initial UI representation of the particular structured entity of the two or more structured entities with the identified substitute UI representation of the particular structured entity; removing the particular initial UI representation of the particular structured entity from the form repeater UIBB; and maintaining the initial UI representation of the remaining structured entities of the two or more structured entities concurrently with replacing the particular initial UI representation of the particular structured entity with the identified substitute UI representation of the particular structured entity. 22. The system of claim 17 , wherein replacing, by the substitute UIBB, the particular initial UI representation of the particular structured entity of the two or more structured entities with the identified substitute UI representation of the particular structured entity comprises swapping the identified substitute UI representation of the particular structured entity with the particular initial UI representation of the particular structured entity based on the type of the event.