Question

例如，像：

    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null的情况下，有更好的方法吗？

由于

Answer 1

首先，你应该避免强制解包并替换

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

通过可选的绑定：

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

比较When should I compare an optional value to nil?以获得该主题的详细概述。使用map()方法可以更紧凑地实现相同的目的 Optional：

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

仅在screen != nil，然后在$0内部调用闭包封闭是未包装的价值。

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。这也允许您检查是否设置了任何搜索属性。

您的代码将变为

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)
}

或

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)
}

如何组合多个可以为空的NSPredicates？

1 个答案: