Switch document renderer - Cannot draw elements on already flushed pages

According to this case's answer by Alexey, we can utilize different renderers with customized RootLayoutArea to achieve such layout behavior as below.

Customized Layout Document

Here is the code:

public void ManipulatePdf(String dest)
{
    PdfDocument pdfDoc = new PdfDocument(new PdfWriter(dest));
    Document doc = new Document(pdfDoc, PageSize.A4);
    doc.SetMargins(36, 36, 36, 36);

    Paragraph p = new Paragraph();
    p.SetBorder(new SolidBorder(0.5f));
    for (int i = 1; i <= 500; i++)
    {
        p.Add(new Text(i + " "));
    }
    doc.Add(p);

    RootLayoutArea endOfFullWidthContentArea = (RootLayoutArea)doc.GetRenderer().GetCurrentArea();

    ExtendedDocumentRenderer renderer1 = new ExtendedDocumentRenderer(doc,
        new RootLayoutArea(endOfFullWidthContentArea.GetPageNumber(), endOfFullWidthContentArea.GetBBox().Clone().SetWidth(200)));
    doc.SetRightMargin(doc.GetRightMargin() + doc.GetPageEffectiveArea(PageSize.A4).GetWidth() - 200);
    doc.SetRenderer(renderer1);

    //The paragraph drawn in the left
    Paragraph p1 = new Paragraph();
    p1.SetBorder(new SolidBorder(0.5f));
    for (int i = 1; i <= 500; i++)
    {
        p1.Add(new Text(i + " "));
    }
    doc.Add(p1);

    ExtendedDocumentRenderer renderer2 = new ExtendedDocumentRenderer(doc,
        new RootLayoutArea(endOfFullWidthContentArea.GetPageNumber(),
            endOfFullWidthContentArea.GetBBox().Clone().MoveRight(200 + 20)
                .SetWidth(endOfFullWidthContentArea.GetBBox().GetWidth() - 200 - 20)));
    doc.SetRightMargin(36);
    doc.SetLeftMargin(200 + 36 + 20);
    doc.SetRenderer(renderer2);
    Paragraph p2 = new Paragraph();
    for (int i = 1; i <= 1000; i++)
    {
        p2.Add(new Text(i + " "));
    }
    doc.Add(p2);

    //Compute which free area is lower in the document
    RootLayoutArea areaColumn1 = (RootLayoutArea)renderer1.GetCurrentArea();
    RootLayoutArea areaColumn2 = (RootLayoutArea)renderer2.GetCurrentArea();
    RootLayoutArea downArea =
        areaColumn1.GetPageNumber() > areaColumn2.GetPageNumber() ? areaColumn1 :
        (areaColumn1.GetPageNumber() < areaColumn2.GetPageNumber() ? areaColumn2 :
            (areaColumn1.GetBBox().GetTop() < areaColumn2.GetBBox().GetTop() ? areaColumn1 : areaColumn2));

    doc.SetMargins(36, 36, 36, 36);
    DocumentRenderer renderer3 = new ExtendedDocumentRenderer(doc,
        new RootLayoutArea(downArea.GetPageNumber(), downArea.GetBBox().Clone().SetX(36).SetWidth(doc.GetPageEffectiveArea(PageSize.A4).GetWidth())));
    doc.SetRenderer(renderer3);

    Paragraph p3 = new Paragraph();
    for (int i = 1; i <= 1000; i++)
    {
        p3.Add(new Text(i + " "));
    }
    doc.Add(p3);

    doc.Close();
}

private class ExtendedDocumentRenderer : DocumentRenderer
{
    public ExtendedDocumentRenderer(Document document, RootLayoutArea currentArea) : base(document)
    {
        this.currentArea = new RootLayoutArea(currentArea.GetPageNumber(), currentArea.GetBBox().Clone());
        this.currentPageNumber = this.currentArea.GetPageNumber();
    }
}

We can see that the Paragraph 1 in the left column spans two pages, and the Paragraph 2 spans three pages, under this scenario the code runs well.

But if we add more contents to Paragraph 1 and make it span 3 pages as following:

//The paragraph drawn in the left has more contents
Paragraph p1 = new Paragraph();
p1.SetBorder(new SolidBorder(0.5f));
for (int i = 1; i <= 800; i++)
{
    p1.Add(new Text(i + " "));
}
doc.Add(p1);

When the Paragraph 2 is rendering, it throws the exception of "Cannot draw elements on already flushed pages". How to solve this problem? Thank you!

Solution

If you elements can span multiple pages then you need to apply immediateFlush=false setting of the DocumentRenderer which goes to its constructor.

You will need to modify your ExtendedDocumentRenderer custom implementation in the following way:

private class ExtendedDocumentRenderer : DocumentRenderer
{
    public ExtendedDocumentRenderer(Document document, RootLayoutArea currentArea) : base(document, false)
    {
        this.currentArea = new RootLayoutArea(currentArea.GetPageNumber(), currentArea.GetBBox().Clone());
        this.currentPageNumber = this.currentArea.GetPageNumber();
    }
}

And then you will need to call Document#Flush whenever you are done with another document renderer. Essentially in your code sample that call needs to be done after adding each new element. Here is the full code:

Document doc = new Document(pdfDocument, PageSize.A4);
doc.SetMargins(36, 36, 36, 36);

Paragraph p = new Paragraph();
p.SetBorder(new SolidBorder(0.5f));
for (int i = 1; i <= 500; i++)
{
    p.Add(new Text(i + " "));
}
doc.Add(p);

RootLayoutArea endOfFullWidthContentArea = (RootLayoutArea)doc.GetRenderer().GetCurrentArea();

ExtendedDocumentRenderer renderer1 = new ExtendedDocumentRenderer(doc,
    new RootLayoutArea(endOfFullWidthContentArea.GetPageNumber(), endOfFullWidthContentArea.GetBBox().Clone().SetWidth(200)));
doc.SetRightMargin(doc.GetRightMargin() + doc.GetPageEffectiveArea(PageSize.A4).GetWidth() - 200);
doc.SetRenderer(renderer1);

//The paragraph drawn in the left
Paragraph p1 = new Paragraph();
p1.SetBorder(new SolidBorder(0.5f));
for (int i = 1; i <= 800; i++)
{
    p1.Add(new Text(i + " "));
}
doc.Add(p1);
doc.Flush();

ExtendedDocumentRenderer renderer2 = new ExtendedDocumentRenderer(doc,
    new RootLayoutArea(endOfFullWidthContentArea.GetPageNumber(),
        endOfFullWidthContentArea.GetBBox().Clone().MoveRight(200 + 20)
            .SetWidth(endOfFullWidthContentArea.GetBBox().GetWidth() - 200 - 20)));
doc.SetRightMargin(36);
doc.SetLeftMargin(200 + 36 + 20);
doc.SetRenderer(renderer2);
Paragraph p2 = new Paragraph();
for (int i = 1; i <= 1000; i++)
{
    p2.Add(new Text(i + " "));
}
doc.Add(p2);
doc.Flush();

//Compute which free area is lower in the document
RootLayoutArea areaColumn1 = (RootLayoutArea)renderer1.GetCurrentArea();
RootLayoutArea areaColumn2 = (RootLayoutArea)renderer2.GetCurrentArea();
RootLayoutArea downArea =
    areaColumn1.GetPageNumber() > areaColumn2.GetPageNumber() ? areaColumn1 :
    (areaColumn1.GetPageNumber() < areaColumn2.GetPageNumber() ? areaColumn2 :
        (areaColumn1.GetBBox().GetTop() < areaColumn2.GetBBox().GetTop() ? areaColumn1 : areaColumn2));

doc.SetMargins(36, 36, 36, 36);
DocumentRenderer renderer3 = new ExtendedDocumentRenderer(doc,
    new RootLayoutArea(downArea.GetPageNumber(), downArea.GetBBox().Clone().SetX(36).SetWidth(doc.GetPageEffectiveArea(PageSize.A4).GetWidth())));
doc.SetRenderer(renderer3);

Paragraph p3 = new Paragraph();
for (int i = 1; i <= 1000; i++)
{
    p3.Add(new Text(i + " "));
}
doc.Add(p3);
doc.Flush();

doc.Close();