When we talk about iText 7's basic building blocks, we refer to all classes that implement the
IElement interface. iText 7 is originally written in Java, then ported to C#. Because of our experience with both programming languages, we've adopted the convenient habit - typical for C# developers - to start every name of an interface with the letter
Figure 0.1 shows an overview of the relationship between
IElement and some other interfaces.
Figure 0.1: Overview of the interfaces
At the top of the hierarchy, we find the
IPropertyContainer interface. This interface defines methods to set, get, and delete properties. This interfaces has two direct subinterfaces:
IElement interface will be implemented by objects such as
Table. These are the objects that we'll add to a document, either directly or indirectly. The
IRenderer interface will be implemented by objects such as
TableRenderer. These renderers are used internally by iText, but we can subclass them if we want to tweak the way an object is rendered.
IElement interface has two subinterfaces of its own. The
ILeafElement interface will be implemented by building blocks that can't contain any other elements. For instance: you can add a
Text or an
Image element to a
Paragraph object, but you can't add any object to a
Text or an
Image implement the
ILeafElement interface to reflect this. Finally, there's the
LargeElement interface that allows you to render an object before you've finished adding all the content. It's implemented by the
Table class, which means that you add a table to a document before you've completed adding all the
Cell objects. By doing so, you can reduce the memory use: all the table content that can be rendered before the content of the table is completed, can be flushed from memory.
IPropertyContainer interface is implemented by the abstract
ElementPropertyContainer class. This class has three subclasses; see figure 0.2.
Figure 0.2: Implementations of the IPropertyContainer interface
Style class is a container for all kinds of style attributes such as margins, paddings and rotation. It inherits style values such as widths, heights, colors, borders and alignments from the abstract
RootElement class defines methods to add content, using either an
add() method or a
showTextAligned() method. The
Document object will add this content to a page. The
Canvas object doesn't know the concept of a page. It acts as a bridge between the high-level layout API and the low-level kernel API.
Figure 0.3 gives us an overview of the
Figure 0.3: Implementations of the IElement interface
All classes derived from the
AbstractElement class implement the
Link also implement the
ILeafElement interface. The
ILargeElement interface is only implemented by the
Table class. The basic building blocks make it very easy for you to create tagged PDF.
Tagged PDF is a requirement for PDF/A level A, a standard for long-term preservation of document, and for PDF/UA, an accessibility standard. A properly tagged PDF includes semantic information about all the relevant content.
The semantic meaning of all elements that implement the
IAccessibleElement interface will be added to the resulting PDF if we define a
PdfDocument as a tagged PDF using the
setTagged() method. In that case, iText will create a structure tree so that a
Table is properly tagged as a table, a
List properly tagged as a list, and so on. This is true for the
LineSeparator objects. The
Tab or the
AreaBreak object don't implement the
IAccessibleElement interface because they objects don't have any real content. The whitespace they create doesn't have any semantic meaning.
In this tutorial, we won't create tagged PDF; iText will just render the content to the document using the appropriate
IRenderer implementation. Figure 0.4 shows an overview of the
Figure 0.4: Implementations of the IRenderer interface
When you compare figure 0.4 with 0.3, you'll discover that each
AbstractElement and each
RootElement has its corresponding renderer. We won't discuss figure 0.4 in much detail. The concept of renderers will become clear the moment we start making some examples.