默認center 模式 居中
btn.contentHorizontalAlignment = UIControl.ContentHorizontalAlignment.right
right 和 trailing效果一樣
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fill
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leading 和 left 效果一樣
if #available(iOS 11.0, *) {
btn.contentHorizontalAlignment = UIControl.ContentHorizontalAlignment.leading
} else {
// Fallback on earlier versions
}
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