快速[弱自我]引起的内存泄漏

Question

func addAdditionalElement(_ additionalSelectedElementsIDs: [String], startX: CGFloat, containerView: UIView, viewHeight: CGFloat) -> CGFloat {
    var totalWidth = startX

    if additionalSelectedElementsIDs.count > 0 {

        let paddingBeforeLabel = (totalWidth==0 ? 0:paddingBetweenLabels)
        totalWidth += paddingBeforeLabel
        var countLabel = createLabel("+\(additionalSelectedElementsIDs.count)", shouldStrikethrough: false)
        if let filter = filter as? DsFilter {
            if filter.isRAOptimization() {
                countLabel = createLabel("...", shouldStrikethrough: false)
            }
        }
        countLabel.frame = CGRect(x: totalWidth, y: 0, width: countLabel.frame.size.width, height: viewHeight)
        countLabel.layer.cornerRadius = countLabel.frame.size.height / 2
        addLabel(countLabel, containerView: containerView)
        DispatchQueue.main.async { [weak self] in
            guard let isSubset = self?.filter.isSelectedElementsSubset(elementsIDs: additionalSelectedElementsIDs) else { return }
            self?.updateLabelSelectedState(countLabel, isTemporarySelected: !isSubset)
        }
        totalWidth += countLabel.frame.size.width
    }
    return totalWidth
}

leak find in DispatchQueue.main.async { [weak self] in line through instruments. It can be fixed by use [unowned self] , but I can't guarantee the nil situation.

I'm just curious the reason of memory leak, why would [weak self] cause a memory leak? It's not a nested closure, the capture looks fine... Also, why use [unowned self] can fix this?

Answer 1

'weak' reference is supposed to take more resources, than 'unowned'. At runtime when the object that is referenced weekly gets deallocated, there is a process of resetting 'weak' pointers to this object to 'nil'. On the other hand, 'unowned' pointer is not being kept track of like this.