NSCollectionViewFlowLayout-左对齐 [英] NSCollectionViewFlowLayout - left alignment

问题描述

NSCollectionViewFlowLayout生成一个布局，其中的项目在右边距对齐；或者，如果容器仅够容纳一个项目，则将项目居中.我期待一个对齐选项，例如委托，但在文档中找不到任何内容.是否需要通过子类化NSCollectionViewFlowLayout来实现这一目标?

NSCollectionViewFlowLayout produces a layout with items justified on the right margin or, if the container is only wide enough for one item, centres items. I was expecting an alignment option, e.g. on the delegate, but am not finding anything in the docs. Does it require subclassing NSCollectionViewFlowLayout to achieve this?

推荐答案

以下是产生左对齐流布局的子类:

Here is a subclass that produces a left justified flow layout:

class LeftFlowLayout: NSCollectionViewFlowLayout {

    override func layoutAttributesForElementsInRect(rect: CGRect) -> [NSCollectionViewLayoutAttributes] {

        let defaultAttributes = super.layoutAttributesForElementsInRect(rect)

        if defaultAttributes.isEmpty {
            // we rely on 0th element being present,
            // bail if missing (when there's no work to do anyway)
            return defaultAttributes
        }

        var leftAlignedAttributes = [NSCollectionViewLayoutAttributes]()

        var xCursor = self.sectionInset.left // left margin

        // if/when there is a new row, we want to start at left margin
        // the default FlowLayout will sometimes centre items,
        // i.e. new rows do not always start at the left edge

        var lastYPosition = defaultAttributes[0].frame.origin.y

        for attributes in defaultAttributes {
            if attributes.frame.origin.y > lastYPosition {
                // we have changed line
                xCursor = self.sectionInset.left
                lastYPosition = attributes.frame.origin.y
            }

            attributes.frame.origin.x = xCursor
            // by using the minimumInterimitemSpacing we no we'll never go
            // beyond the right margin, so no further checks are required
            xCursor += attributes.frame.size.width + minimumInteritemSpacing

            leftAlignedAttributes.append(attributes)
        }
        return leftAlignedAttributes
    }
}

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