Improve CollectionView loading speed

Question

I have a calendar view with a year overview:

For that I have the following View hierarchy:

• ViewController
  • View
    • Collection View with 12 cells (One for each month)
      • Colleciton View with 49 cells (One for each day + in and out dates + days of week)

Now when it comes to the following code part where I load the cells for the days inside the overview:

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath) as! CalendarPageYearCollectionCellMonthCell;

    print("cell for item at: \(indexPath.item)");

    cell.layer.shouldRasterize = true;
    cell.layer.rasterizationScale = UIScreen.main.scale;

    cell.label.text = self.days[indexPath.item];

    return cell;
}

This code part takes over 9 seconds. Meanwhile the UI is frozen. How can I improve the loading and rendering speed of those cells significantly so that it wont take more than 1 second to load the view?

EDIT I found out which part of the code is consuming this amount of time. I created a UICollectionViewCell programmatically:

import Foundation
import UIKit
import SnapKit

class CalendarPageYearCollectionCellMonthCell : UICollectionViewCell {

    var label: UILabel! = nil;

    override init(frame: CGRect) {
        super.init(frame: frame);

        self.setupView();
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    func setupView() {
        self.backgroundColor = UIColor.green;

        self.label = UILabel();
        self.label.font = UIFont(name: "HelveticaNeue", size: 12);
        self.label.textColor = PaintBox.colorWithName(VKBColor.black, andAlpha: 1.0);
        self.label.backgroundColor = UIColor.red;
        self.label.textAlignment = .center;
        self.sizeToFit();
        self.label.center = self.center;

        self.addSubview(self.label);
    }
}

So when I use this code the Calendar loads really fast. But with that code, ofcourse, you will never see the label which you added as a subview. So I need to add some constraints:

func setupView() {
    self.backgroundColor = UIColor.green;

    self.label = UILabel();
    self.label.font = UIFont(name: "HelveticaNeue", size: 12);
    self.label.textColor = PaintBox.colorWithName(VKBColor.black, andAlpha: 1.0);
    self.label.backgroundColor = UIColor.red;
    self.label.textAlignment = .center;
    self.sizeToFit();
    self.label.center = self.center;

    self.addSubview(self.label);

    self.label.snp.makeConstraints { make in
        make.edges.equalToSuperview();
    }
}

And now it takes ages for the view to load. So the real problem seems to be the constraints.

Isnt there a ways to just create a cell only one time and copy that to all other cells with just setting the text?

Answer 1

So after lot of researching I came to the conclusion that I cannot speed this up while using storyboards and constraints. It is always slow as hell.

So I tried to get rid of all xib files, storyboard files and constraints. The result was incredible. I made it from a view loading speed of 9 seconds to a loading speed of 230 ms.

So what I recommend to everybody: If you have performance issues just dont use xib's, storyboard's or constraints. Just code everything yourself and set all frames of your views directly.