Swift中的惰性属性初始化

Question

你将如何在Swift中实现以下模式？

所述Container类被初始化,其中包含的词典JSON数组.这些词典用于初始化Entry类.但是,Entry当访问entries或searchEntries访问属性时,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

那这个呢:

class Container {

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

    func newEntries() -> [String] {

        // calculate and return entries

    }

}

Answer 2

似乎这个问题已经在很大程度上得到了回答,但是回到最初的帖子,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,我同时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!
        }
    }
}