Part 2 of 4—Database Details—Using Delphi LiveBindings With TObject and TObjectList

Source Code

The source code in the form of a Delphi Project Group is available at:

https://github.com/Pasquina/delphilivebindingobjects.git

The project was developed using Delphi RAD Studio Tokyo (10.2.2)

The README file contains additional useful information.

Database Details

The application database is defined by three classes contained in the file DBLBO.pas. When instantiated, they form a tree structure not unlike a relational database having two master/detail relationships. Diagrammatically, they appear as follows (a full-sized PDF of this diagram is also to be found in the Docs folder of the Github project or can be downloaded here):

Delphi Livebinding Objects Data Structure

The Database Classes

TCorp

This class is instantiated a single time and contains information about the fictitious corporation used in the sample application. In addition, it contains a TObjectList<TBranch> property that is used to store all of the instances of Branches owned by the corporation.

TBranch

This class is instantiated once for each Branch owned by the corporation and that appears in the TObjectList<TBranch> property of the instantiated TCorp.

TBranch also contains a TObjectList<TEmployee> property that contains a list of the instances of TEmployee for all of the employees who are assigned to the corresponding branch.

TEmployee

This class is instantiated once for each Employee assigned to the Branch and who appears in the TObjectList<TEmployee> property of the corresponding TBranch.

Instantiation Hierarchy

The Create constructor of TCorp not only instantiates the TCorp object; it also creates three arbitrary Branches by invoking the TBranch.Create constructor for each of the branches.

Each invocation of the TBranch.Create constructor instantiates the branch and provides arbitrary properties, but also invokes the TEmployee.Create constructor for each of the employees assigned to the corresponding Branch.

In this was, as single call to TCorp.Create will result in the instantiation of the entire database.

Arbitrary Data

Each of the database object Create constructors provides for the specification of arbitrary data when invoking the constructor. In this way, the invoker provides the data needed for the instantiation.

Overloaded Creates

The constructors for TBranch and TEmployee have overloaded versions. In one version, the data used to populate the fields of the instance is provided by the invoker. This is the form used when the database objects are first instantiated.

The second form of Create does not provide for any values for the properties of the new object. This is the form used when the TBindNavigator components process the Add (+) button. 

Type Aliases

The DBLBO.pas file also specifies three Type Aliases:

• CorpObjectBSWrapper that is a TObjectBindSourceAdapter<TCorp>.
• BranchListBSWrapper that is a TListBindSourceAdapter<TBranch>.
• EmployeeListBSWrapper that is a TListBindSourceAdapter<TEmployee>.

Using these Type Aliases solves a couple of problems.

1. The naming conventions used by LiveBindings are incredibly bad. Class names like TObjectBindSourceAdapter, TListBindSourceAdapter and TAdapterBindSource make repeated use of the words "Source," "Bind," and "Adapter" with no clear indication of what a given class is supposed to do. Using a TypeAlias to change the name to something meaningful (they really are classes that wrap around Objects and Lists) makes it much easier to develop and understand the code.
2. The objects referenced by the Type Aliases are, in fact, generic in nature as the Type Alias indicates. This is what prevents their specification as a component in the first place. By using a Type Alias the cumbersome generic notation disappears from view, again facilitating cleaner, more easily understood code.

This completes the examination of the database objects. Next, we'll consider the Model Details.

Using Delphi LiveBindings With TObject and TObjectList<T>—Part 1 of 4—Overview<==Previous
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