exportcert refactor

[sharan8]

The certificate creation and export can be abstracted out into a new utility class so as to allow for better separation of concerns and scope for future extensions. The organisation is now as follows:

ExportCertCommand:

• Affirms and assigns the export path based on the ability to create a folder to export to
• Validates the index input and raises a CommandException if there is a violation
• Checks if the volunteer has non-zero event records
• Retrieves the records and corresponding events for the selected volunteer from the model
• Creates a new CertGenerator object by supplying the save path, volunteer and (record, event) pairs.
• Calls the generatePdf method of the CertGenerator class and handles the potential resulting IOExceptions.

CertGenerator:

• Gets instantiated with a save path, volunteer object and volunteer data in the from of (record, event) pairs.
• Populates a PDF document with the supplied volunteer information
• Exports the PDF document to the supplied savepath

Some design considerations:

• Save path is supplied by ExportCertCommand so as to support future functionality to get the filepath from the user itself. As such, CertGenerator is excluded from the save path decision making process.
• The retrieval of volunteer information is done in ExportCertCommand itself so as to prevent the need for both CertGenerator and ExportCertCommand to interact with the model. This is inspired by the Law of Demeter.

[coveralls]

Coverage increased (+0.09%) to 67.353% when pulling b15d821 on sharan8:exportcert-refactor into 76ad597 on CS2103-AY1819S1-W16-2:master.