如何组合多个可为空的 NSPredicate？

例如，类似的东西：

    var finalPredicate = NSPredicate(format: "")

    if (screen != nil) {
        screenPredicate = NSPredicate(format: "screen = %@", screen!)
        finalPredicate = NSCompoundPredicate(andPredicateWithSubpredicates: [screenPredicate!])
    }

    if (feature != nil) {
        featurePredicate = NSPredicate(format: "feature = %@", feature!)
        finalPredicate = NSCompoundPredicate(andPredicateWithSubpredicates: [finalPredicate, featurePredicate!])
    }

    if (shouldDisplayPredicate != nil) {
        shouldDisplayPredicate = NSPredicate(format: "shouldDisplay = %@", shouldDisplay!)
        finalPredicate = NSCompoundPredicate(andPredicateWithSubpredicates: [finalPredicate, shouldDisplayPredicate!])
    }
    if (hasDisplayed != nil) {
        displayPredicate = NSPredicate(format: "hasDisplayed = %@", hasDisplayed!)
        finalPredicate = NSCompoundPredicate(andPredicateWithSubpredicates: [finalPredicate, displayPredicate!])
    }

在谓词可能为空或不为空的情况下，是否有更好的方法来做到这一点？

Thanks

首先，应避免强行拆包并更换

if screen != nil {
    // ... add predicate for `screen!` ...
}

通过可选的绑定：

if let screenValue = screen {
    // ... add predicate for `screenValue` ...
}

Compare 我什么时候应该将可选值与 nil 进行比较？ https://stackoverflow.com/questions/29717210/when-should-i-compare-an-optional-value-to-nil以获得对该主题的良好概述。 使用以下方法可以更紧凑地实现相同的效果map()方法 的Optional:

screen.map { /* ... add predicate for `$0` ... }

仅当以下情况时才会调用闭包screen != nil， 进而$0里面 闭包是展开的值。

其次，用所有必需的谓词填充数组更简单 首先，只创建一次复合谓词。 这还允许您检查是否设置了任何搜索属性。

然后你的代码就变成了

var predicates: [NSPredicate] = []
if let screenValue = screen {
    predicates.append(NSPredicate(format: "screen = %@", screenValue))
}
if let featureValue = feature {
    predicates.append(NSPredicate(format: "feature = %@", featureValue))
}
// ... other search attributes ...
if !predicates.isEmpty {
    let finalPredicate = NSCompoundPredicate(andPredicateWithSubpredicates:predicates)
}

or

var predicates: [NSPredicate] = []
screen.map { predicates.append(NSPredicate(format: "screen = %@", $0)) }
feature.map { predicates.append(NSPredicate(format: "feature = %@", $0)) }
// ... other search attributes ...
if !predicates.isEmpty {
    let finalPredicate = NSCompoundPredicate(andPredicateWithSubpredicates:predicates)
}