Swift中的惰性属性初始化

你如何在Swift中实现以下模式？

Container类使用包含字典的JSON数组进行初始化.这些词典用于初始化Entry类.但是,当访问条目或searchEntries属性时,Entry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

似乎这个问题已基本得到回答,但要回到原帖,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,我同时使用了类函数和闭包 – 两者都很好.

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")

Swift中的惰性属性初始化

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