Question

如何在Swift中实现以下模式？

使用包含字典的JSON数组初始化Container类。这些词典用于初始化Entry类。但是，当访问Entry或entries属性时，searchEntries对象的初始化会延迟发生。

@interface Container

@property (readonly, nonatomic) NSArray *entryDicts;

@property (readonly, nonatomic) NSArray* entries;
@property (readonly, nonatomic) NSDictionary *searchEntries;

@end



@implementation Container

- (instancetype)initWithArray:(NSArray *)array
{
    self = [super init];
    if (self) {
        _entryDicts = array;
    }
    return self;
}

@synthesize entries = _entries;
- (NSArray *)entries
{
    [self loadEntriesIfNeeded];
    return _entries;
}

@synthesize entriesByNumber = _entriesByNumber;
- (NSDictionary *)entriesByNumber
{
    [self loadEntriesIfNeeded];
    return _entriesByNumber;
}

- (void)loadEntriesIfNeeded
{
    if (_entries == nil) {
        // Load entries
        NSMutableArray *entries = [NSMutableArray arrayWithCapacity:[self.entriesDict count]];
        NSMutableDictionary *entriesByNumber = [NSMutableDictionary dictionaryWithCapacity:[self.entriesDict count]];

        [self.entriesDict enumerateKeysAndObjectsUsingBlock:^(NSString *number, NSDictionary *entryDict, BOOL *stop) {
            Entry *entry = [[Entry alloc] initWithDictionary:entryDict container:self];
            [entries addObject:entry];
            entriesByNumber[number] = entry;
        }];

        _entries = [entries copy];
        _entriesByNumber = [entriesByNumber copy];

        // Delete dictionaries
        _entriesDict = nil;
    }
}

@end

Answer 1

这个怎么样：

class Container {

    lazy var entries: [String] = self.newEntries()

    func newEntries() -> [String] {

        // calculate and return entries

    }

}

Answer 2

似乎这个问题在很大程度上得到了回答，但回到原帖，这里（恕我直言）是一个相对简洁的Swift翻译。关键是你可以链接惰性属性。请注意，我使用了类函数和闭包 - 两者都没问题。

import Swift

println("begin")

class ClassWithLazyProperties {

    lazy var entries:[String] = ClassWithLazyProperties.loadStuff()
    lazy var entriesByNumber:Dictionary<Int, String> = {

        var d = Dictionary<Int, String>()
        for i in 0..<self.entries.count {
            d[i] = self.entries[i]
        }
        return d
    }()

    private class func loadStuff() -> [String] {
        return ["Acai", "Apples", "Apricots", "Avocado", "Ackee", "Bananas", "Bilberries"]
    }

}

let c = ClassWithLazyProperties()
c.entriesByNumber
    // 0: "Acai", 1: "Apples", 2: "Apricots", 3: "Avocado", 4: "Ackee", 5: "Bananas", 6: "Bilberries"]


println("end")

Answer 3

您可以使用optional作为实例变量。然后创建一个函数，如果它存在则返回可选项，如果不模拟延迟加载则创建一个新对象。

class Lazy {
    var lazyVariable:String?

    func lazilyGetEntries() -> String {
        if let possibleVariable = self.lazyVariable { // optional already exists
            return possibleVariable
        }
        else {                                        // optional does not exist, create it
            self.lazyVariable = String()
            return self.lazyVariable!
        }
    }
}

Answer 4

延迟工作的方式是初始化程序（或init方法）仅在首次访问变量或属性时运行。我看到它在代码中不能工作（至少是直接）的一个主要原因，那是因为你将两个惰性实例化代码打包到一个方法中（loadEntriesIfNeeded）。

要使用延迟实例化，您可能需要扩展NSMutableArray和NSDictionary，并为您的延迟实例化覆盖或创建自定义初始化程序。然后，将loadEntriesIfNeeded内的代码分发到各自的初始值设定项中。

在条目初始化程序中：

NSMutableArray *entries = [NSMutableArray arrayWithCapacity:[self.entriesDict count]];
[self.entriesDict enumerateKeysAndObjectsUsingBlock:^(NSString *number, NSDictionary *entryDict, BOOL *stop) {
            Entry *entry = [[Entry alloc] initWithDictionary:entryDict container:self];
            [entries addObject:entry];}];
_entries = [entries copy];

然后在entriesByNumber初始化程序中：

NSMutableDictionary *entriesByNumber = [NSMutableDictionary dictionaryWithCapacity:[self.entriesDict count]];
// Then do fast enumeration accessing self.entries to assign values to entriesByNumber.
// If self.entries is null, the lazy instantiation should kick in, calling the above code
// and populating the entries variable.
_entriesByNumber = [entriesByNumber copy];

然后，您可以通过调用自定义初始值设定项来创建惰性变量。

@lazy var entries: CustomArray = custominitforarray()
@lazy var entriesByNumber: CustomDictionary = custominitfordictionary()

PS：你怎么没有entryByNumber的财产？我猜它是私人的吗？请测试一下并回复结果，因为我自己太懒了。

Answer 5

您可以在Swift中使用Lazy Stored Properties来实现Lazy Instantiation模式。这是通过在声明存储属性之前添加@lazy属性来完成的。

要记住两件事：

声明
懒惰属性只能用于结构或类的成员（因此我们需要使用DataManager）

这里有一些代码可以放入Playground以查看@lazy属性的工作原理

// initialize your lazily instantiated data
func initLazyData() -> String[] {
    return ["lazy data"]
}

// a class to manage the lazy data (along with any other data you want)
class DataManager {
    @lazy var lazyData = initLazyData()

    var otherData = "Other data"
}

// when we create this object, the "lazy data" array is not initialized
let manager = DataManager()

// even if we access another property, the "lazy data" array stays nil
manager.otherData += ", more data"
manager

// as soon as we access the "lazy data" array, it gets created
manager.lazyData
manager

有关详细信息，请查看“Swift编程语言指南”的“属性”页面上的Lazy Stored Properties部分。请注意，该链接是预发布文档。

Answer 6

通过在声明之前编写@lazy属性来指示延迟存储的属性。“

@lazy var lazyVariable:String? = ""

请注意，懒惰的财产必须有初始化。

Answer 7

Swift中有@lazy个属性。我发现了一个小帖here，我建议观看来自Apple here的三个Swift视频（Swift简介，中级Swift，高级Swift）。这些视频显示了很多内容，而高级视频确实很先进......

Answer 8

我在PageBaseApplication

中找到了这个

var modelController: ModelController {
    // Return the model controller object, creating it if necessary.
    // In more complex implementations, the model controller may be passed to the view controller.
    if !_modelController {
        _modelController = ModelController()
    }
    return _modelController!
}

var _modelController: ModelController? = nil

类似于@Brian Tracy所提到的但是使用变量而不是func

Answer 9

由于entries属性只是entriesByNumber中值的数组，因此您可以在entriesByNumber中执行所有加载，只需entries取决于{{1} }}

entriesByNumber

Swift中的惰性属性初始化

9 个答案: