Used for accessing data via the BDE, third-party BDE replacement products,
or TDataSet descendants. The DBPipeline is connected via the DataSource
Used to access data in ASCII text files.
Just-In-Time pipeline for accessing data in Delphi objects.
Regardless of the type of pipeline or the type of data being accessed,
the data pipeline component has two basic purposes:
1. Supply data
2. Control data traversal
Data pipelines provide data via field objects. For instance, the following
code would retrieve the current field value of a field called
lValue := DataPipeline1.Fields[‘Company’].Value;
Each time a data-aware report component prints, it uses this approach to
retrieve the data from the data pipeline. Data-aware report components have
two properties that determine the data they will retrieve:
DataField. The name of the appropriate data pipeline component is assigned
to the DataPipeline. The DataField property is assigned the name of the
field that contains the data to be displayed. Once these two properties are
assigned, the data-aware component has the ability to retrieve data directly
from the data pipeline, independent of the report where the data-aware
Controlling Data Traversal
The second purpose of the data pipeline is to control data
traversal. Data traversal is the act of moving from the first record of the
data to the last record. When a report is printed, the report engine
traverses the data by doing the following steps:
1. Opens the data pipeline.
2. Goes to the first record.
3. Begins printing the page and then gives the detail band the opportunity
4. Goes to the next record.
5. Gives the detail band the opportunity to print.
6. Continues steps 4 and 5 until there is no more page space.
7. Completes the page.
8. Continues steps 4 through 7 until all records have been exhausted.
It is important to note that when the detail band is given the
opportunity to print, the data-aware components within the detail band are
rendered – at which point they retrieve the field value of the current
record. The engine then moves to the next record and prints the detail band
again. It is this combination of the report traversing the data and the
data-aware components retrieving the data that creates the pages of the
report. If the data pipeline is not assigned to either of these entities
(data-aware component or report), then the report will not work. Therefore,
the report must be assigned to a data pipeline and each data-aware component
must be assigned to a datapipeline and a datafield.
We’ve said that the report engine traverses the data. That isn’t
completely true. The report engine makes requests of the data pipeline (such
as open, first, next, last), and then relies on the data pipeline to do the
work. This means that the data pipeline controls the data traversal. This
control can be used to great advantage.
For instance, let’s say you are
displaying a database grid on a form. The user has selected an individual
record of the grid and wants to print that record. If you have a
datapipeline pointed at the same dataset as the grid, then it can access all
of the records. However, we can instruct the data pipeline to traverse only
the current record by simply setting the RangeBegin and RangeEnd properties
to CurrentRecord. When the report prints, it will send traversal requests to
the data pipeline, and the data pipeline will traverse only one record. It
will then inform the report engine that all records have been traversed and
the report will print only one record.
Let’s take this example further and
say that you let the user select multiple records from the database grid.
You then want the report to contain only the selected records. In this case
we can assign the Bookmarks from the grid to the BookmarkList property of
the data pipeline. When the report prints, the data pipeline will traverse
only those records in the list of bookmarks and only those records will
appear in the report. Essentially, the engine makes the same traversal
requests of the data pipeline for every report, but the data pipeline
controls how the data is actually traversed.