Size of the layout depending of the number of features

Question

I have a point layer named 'observation', with different attributes and one called "Année". My project also have a variable called @Année.

In the layout, I have a A0 page (1189x841), but I want an extra space to put my attribute table (1cm for each row). So I'm trying to calculate the size with this formula, and it's worked fine:

841 + (10 * layer_property('Observation', 'feature_count'))

My problem is that my attribute table shows only the observations for the given year (@Année), with this formula:

to_string(year("Année")) LIKE @Année

How can I adapt layer_property('Observation', 'feature_count') to add a filter, so it will count only the observation for a given year?

(EDIT) I tried the solution with aggregate, but it doesn't work, cause it says "No feature was found on this layer to evaluate the expression", so I can't click on OK.

(EDIT2) The expression works to calculate a field for exemple: But as an expression to set the size of the page, I still have the problem... and it's exactly the same expression:

Answer 1

You can try the following (tested)

with_variable('Année', '2019',
  aggregate(
    layer:='Observation',
    aggregate:='count',
    expression:="any_existing_field_name_from_observation_layer",
    filter:=to_string(year("Année")) LIKE to_string(@Année)
  )
)

Edit:

Unable to make things work in the expression for page size, you can try to use the custom Python function

from datetime import datetime

from qgis.core import *
from qgis.gui import *
from qgis.utils import *

@qgsfunction(args='auto', group='Custom')
def get_count_filtered(layer_name, field_name, variable_val_name, feature, parent, context):
    """ Returns the value of the variable 'var_name' 
        Example usage: env('user_full_name') """
    project = QgsProject.instance()
    layer = project.mapLayersByName(layer_name)[0]
    variable_val = QgsExpressionContextUtils.projectScope(project).variable(variable_val_name)
    vals = [str(feat[field_name]) for feat in layer.getFeatures() if feat[field_name] is not None]
    vals = [d for d in vals if str(datetime.strptime(d, '%Y-%m-%d').year) in variable_val]
    return len(vals)

Then use in the expression

get_count_filtered('Observation', 'Année du relevé', 'Année')