OpenEdge Development: Mobile Applications - Product ...

Comparing asynchronous and synchronous execution
JSDO overview
Asynchronous execution is mandatory for the methods that execute the built-in Mobile
CRUD operations. These operations usually involve AppServer access to its data
sources, which are typically OpenEdge databases. The Mobile read operation
executed by the fill( ) method can involve reading and returning hundreds to
thousands (or more) records in multiple temp-tables of a ProDataSet across the
network. The Mobile create, update, and delete operations, which require writes to and
lock management of databases, are executed across the network by the
saveChanges( ) method one record at a time, and for as many records as are marked
as changed in JSDO local storage. This means that completion of these methods can
require detectable wait times. If they were executed synchronously, the Mobile App
user would be prevented from doing any work within the app while these methods were
executing. With asynchronous execution, they can perform other tasks, such as setting
application options, while waiting for a list of customers and orders, for example, to be
displayed in a list view, or while waiting for notification that changes they have posted
have been saved in the database.
Asynchronous execution is the default for non-built-in invocation methods, again,
because these operations execute across the network and can involve complex
database interactions on the AppServer. However, it is also possible that an invocation
method might perform a simple function and return a primitive value used in an
expression—for example, an age or credit limit. In this case, you might prefer to
execute the method synchronously in the expression to complete the calculation with
its return value. This you can do by passing false as an additional Boolean parameter
that indicates that the invocation method is to be executed synchronously. For more
information on the execution options for invocation methods, see the “Accessing
non-built-in invoke operations” section on page 115.
Named events for asynchronous execution
For all methods that execute asynchronously, the JSDO supports a set of named
events to which you can subscribe event handler functions, functions that execute with
a signature defined to receive the results of a particular event. The JSDO defines a set
of unique before and after events for each Mobile operation and also for the
saveChanges( ) method, itself, because in one call, this method can execute all three
of the create, update, and delete operations for any number of records in JSDO local
storage.
In other words, there is a unique event that fires before a given Mobile operation
executes across the network and one that fires after a given Mobile operation
executes. In addition, there is one event that fires before the saveChanges( ) method
executes any Mobile operations and another that fires after all Mobile operations
executed by the saveChanges( ) method have completed. Note that the after events
all fire (including for saveChanges( )) whether Mobile operations complete
successfully or with an error.
OpenEdge ® Development: Mobile Applications 99