C＃iText7从表中删除某些标头

Question

I want to remove certain header from created table.

Table table = new Table(2).SetWidth(iText.Layout.Properties.UnitValue.CreatePercentValue(100));
Table headerTable1 = new Table(2).SetWidth(iText.Layout.Properties.UnitValue.CreatePercentValue(100));
headerTable1.AddCell(new Cell().Add(new Paragraph("Test1")).SetWidth(iText.Layout.properties.UnitValue.CreatePercentValue(50)));
headerTable1.AddCell(new Cell().Add(new Paragraph("Test2")).SetWidth(iText.Layout.properties.UnitValue.CreatePercentValue(50)));

table.AddHeaderCell(new Cell(1,2).Add(headerTable1));

Table headerTable2 = new Table(2).SetWidth(iText.Layout.Properties.UnitValue.CreatePercentValue(100));
headerTable2.AddCell(new Cell().Add(new Paragraph("Test1")).SetWidth(iText.Layout.properties.UnitValue.CreatePercentValue(50)));
headerTable2.AddCell(new Cell().Add(new Paragraph("Test2")).SetWidth(iText.Layout.properties.UnitValue.CreatePercentValue(50)));

table.AddHeaderCell(new Cell(1,2).Add(headerTable2));

Table headerTable3 = new Table(2).SetWidth(iText.Layout.Properties.UnitValue.CreatePercentValue(100));
headerTable3.AddCell(new Cell().Add(new Paragraph("Test1")).SetWidth(iText.Layout.properties.UnitValue.CreatePercentValue(50)));
headerTable3.AddCell(new Cell().Add(new Paragraph("Test2")).SetWidth(iText.Layout.properties.UnitValue.CreatePercentValue(50)));

table.AddHeaderCell(new Cell(1,2).Add(headerTable3));

{Add Multiple Cells to Table}

Once the height of table is bigger than PageSize.A4.GetHeight()(suppose we are using A4 Content), it automatically generates several pages and add above header. From second page, i want to remove headerTable2. How can i do it? Please help me. Thanks.

Answer 1

I've been mulling over your questionn quite for some time and, unfortunately, haven't come to any plausible solution.

In iText7 in order to override the behavior of certain layout element one should extend its default renderer (for a table it would be the TableRenderer ) and then set the instance of this custom renderer on the element one wants to process.

One can get the header of the table with Table#getHeader() element and set a rendere on it, however there are two issus that are not easy to overcome:

1) for each page iText recreates the renderer, so you cannot suppose that one renderer instance will be propagated to overflow renderer of the table. To know whether the renderer have been in use or not, one can introduce some static fields, of course, but

2) the header's borders are processed implicitly inside the parent table's layout method. And because there are a lot of edge cases with tables, the default layout method is indeed very complex, it'd be a nigthmare to override it. More than that, a lot of TableRenderer 's fields have either private or default access, so it'd caus eyou additional difficulties.

That's why I think you have to face up to the fact that it's impossible to process the table headers in a way you want.

But what about just headers? iText provides customers with the IEvent functionality. One can implement a BEGIN_PAGE (or END_PAGE) event (in your case it would be the header drawing) and make the document listen to it. Once the event has been caught, the drawing will be performed.

Let's look how it could be achieved.

Handling the event (TableHeaderEventHandler in the example below) is quite simple:

    TableHeaderEventHandler handler = new TableHeaderEventHandler(doc);
    pdfDoc.addEventHandler(PdfDocumentEvent.END_PAGE, handler);

And that's how one can implement this event:

    public class TableHeaderEventHandler implements IEventHandler {
    protected Table table;
    protected float tableHeight;
    protected Document doc;

    public TableHeaderEventHandler(Document doc) {
        this.doc = doc;
        initTable(false);
    }

    @Override
    public void handleEvent(Event event) {
        PdfDocumentEvent docEvent = (PdfDocumentEvent) event;
        PdfDocument pdfDoc = docEvent.getDocument();
        PdfPage page = docEvent.getPage();
        PdfCanvas canvas = new PdfCanvas(page.newContentStreamBefore(), page.getResources(), pdfDoc);
        Rectangle rect = new Rectangle(pdfDoc.getDefaultPageSize().getX() + doc.getLeftMargin(),
                pdfDoc.getDefaultPageSize().getTop() - doc.getTopMargin(), 523, getTableHeight());
        new Canvas(canvas, pdfDoc, rect)
                .add(table);

        if (1 == doc.getPdfDocument().getPageNumber(page)) {
            initTable(true);
        }
    }

    public float getTableHeight() {
        return tableHeight;
    }

    private void initTable(boolean removeTheSecondRow) {
        table = new Table(3);
        table.addCell("row 1, 1");
        table.addCell(new Cell(1, 2).add(new Paragraph("row 1, 2 3")));
        if (!removeTheSecondRow) {
            table.addCell(new Cell(1, 3).add(new Paragraph("row 2, 1 2 3")));
        }
        table.addCell("row 3, 1");
        table.addCell("row 3, 2");
        table.addCell("row 3, 3");
        TableRenderer renderer = (TableRenderer) table.createRendererSubTree();
        renderer.setParent(new Document(new PdfDocument(new PdfWriter(new ByteArrayOutputStream()))).getRenderer());
        tableHeight = renderer.layout(new LayoutContext(new LayoutArea(0, PageSize.A4))).getOccupiedArea().getBBox().getHeight();
        doc.setMargins(120 + tableHeight, 36, 36, 36);
    }
}

In the snippet above I reinitialize the header once it's been placed on the first page. Please mind the processing of the document's margins: you may not want the page's content to interfere with the header, and that requires special attention:

        TableRenderer renderer = (TableRenderer) table.createRendererSubTree();
        renderer.setParent(new Document(new PdfDocument(new PdfWriter(new ByteArrayOutputStream()))).getRenderer());
        tableHeight = renderer.layout(new LayoutContext(new LayoutArea(0, PageSize.A4))).getOccupiedArea().getBBox().getHeight();
        doc.setMargins(120 + tableHeight, 36, 36, 36);

The sample which I've been playing with you can find on iText's website or github: https://github.com/itext/i7js-examples/blob/develop/src/test/java/com/itextpdf/samples/sandbox/events/TableHeader.java

There are, however, some very obvious flaws in the solution above:

1) One should handle border-collapsing by themselves.

That's a tough one, and a general solution would require a lot of contemplation. However to solve a particular case, one can just calculate the space in a such way that the borders are placed correctly.

2) One should update the document's area wisely.

Let's look at the resultant pdf produce by the code above. You may wonder why there is a gap between the header and the document's other elements on the second page. And what's more amazing, why there is no gap on the third+ page:

The reason is as follows: when the event is handled, the next page has already been added to the document. How to overcome it? As I said in the beginning, usually one overrides a renderer (here it's the DocumentRenderer ) to handle it. For more information, please look at this answer: IText 7 How To Add Div or Paragraph in Header Without Overlapping Page Content?

Hope that would be useful!

PS Although the answer has been given in Java, there should be no problem for you to reproduce it in C#, because the API is quite the same.