iText7 large table autoLayout() - autolayout

Question

According to the docs
public Table(int numColumns,
boolean largeTable)
Constructs a Table with specified number of columns. The final column widths depend on selected table layout. Since 7.0.2 table layout algorithms were introduced. Auto layout is default, except large tables. For large table fixed layout set implicitly. Since 7.1 table will have undefined column widths, that will be determined during layout. In oder to set equal percent width as column width, use UnitValue.createPercentArray(int)
I render a large table using https://developers.itextpdf.com/examples/tables/clone-large-tables
Is there a way to define autoLayout? Maybe after adding the first row, get the cell widths and set them on the table, but that doesn't seem possible, because the column widths are null because I am using the constructor with number of columns.
Or adding some sort of autoLayout when end page is reached.
I don't want to define the widths for the columns because we have lots of tables.

Answer 1

First of all I would like to mention that auto layout requires the content of the whole table. The content is used when calculating the column widths. But you are using large table, which probably means you have a lot of data and you don't want to keep everything in memory (if that's not the case, just don't use large tables).
Thus, all you can do is calculate an approximation of the automatic column widths given some initial cells. Basically, it is possible to implement your first idea, however, it takes some code to be written. But if you have very different content in cells across different rows (e.g. images vs inner tables vs some text), then this method might not work very well because as I said, to estimate column widths well you need all the content.
Please also bear in mind that this approach is quite dirty and might not work for some corner cases. But it does solve the goal and frees you of the necessity to define column widths.
To describe the solution in a few words, we take cells of several initial rows, add them to a temporary table and layout it (estimate positions etc), without actually drawing it anywhere. Then we extract the cell widths from the layout step information and can use them for the large table constructor.
The method estimating column widths looks like this:
private UnitValue[] estimateWidths(Document document, Cell[][] cells) {
int numOfColumns = cells[0].length;
Table table = new Table(numOfColumns);
for (int i = 0; i < cells.length; i++) {
for (int j = 0; j < cells[i].length; j++) {
table.addCell(cells[i][j]);
}
}
LayoutContext context = new LayoutContext(document.getRenderer().getCurrentArea().clone());
TableRenderer tableRenderer = (TableRenderer)table.createRendererSubTree();
LayoutResult result = tableRenderer.setParent(document.getRenderer()).layout(context);
if (result.getStatus() == LayoutResult.PARTIAL) {
tableRenderer = (TableRenderer) result.getSplitRenderer();
}
UnitValue[] widths = new UnitValue[numOfColumns];
List<IRenderer> subList = tableRenderer.getChildRenderers().subList(0, numOfColumns);
for (int i = 0; i < subList.size(); i++) {
IRenderer cell = subList.get(i);
widths[i] = UnitValue.createPointValue(cell.getOccupiedArea().getBBox().getWidth());
}
return widths;
}
So assuming you have a Cell[][] cells array of cells for initial couple of rows (can be one row as well, but the more the better), where cells[i][j] refers to the cell at row i and column j, you can create your large table like this:
Table table = new Table(estimateWidths(doc, cells), true);
But don't forget to explicitly add cells from cells array to the large table before adding new content.