Objective c iOS-在Obj-C/Swift代码之间挣扎

我一直在将我的应用程序的一部分从Obj-C翻译成Swift，我面临着一个我无法理解或绕过的问题

看看这个Objective-C类：

模型类预测

@class City;

@interface Forecast : ForecastInterface

@property (nonatomic, retain) City *city;

@end

模型级预测48h

@class City;

@interface Forecast48h : ForecastInterface

@property (nonatomic, retain) City *city;

@end

模型类摘要

@class City;

@interface Summary : ForecastInterface

@property (nonatomic, retain) City *city;

@end

最后

模型预测界面

@interface ForecastInterface : NSManagedObject

@property (nonatomic, retain) NSDate * date;
@property (nonatomic, retain) NSString * descriptionLabel;
@property (nonatomic, retain) NSString * windDirection;
@property (nonatomic, retain) NSString * seaState;
@property (nonatomic, retain) NSNumber * day;
@property (nonatomic, retain) NSString * name;
@property (nonatomic, retain) NSString * probFreeze;
@property (nonatomic, retain) NSString * probSnow;
@property (nonatomic, retain) NSString * probRain;
@property (nonatomic, retain) NSNumber * tempMap;
@property (nonatomic, retain) NSString * tmpMax;
@property (nonatomic, retain) NSNumber * tmpSea;
@property (nonatomic, retain) NSString * tmpMin;
@property (nonatomic, retain) NSString * speedWind;
@property (nonatomic, retain) NSString * endDate;
@property (nonatomic, retain) NSString * startDate;

@end

好的，现在您有了我的CoreData模型的演示

我想了解为什么我不能将Forecast或Forecast48h类型转换为Summary类型

看看这个例子：

@interface Summary (Additions)

+(NSArray*)allSummaryOfCity:(NSManagedObjectContext *)context city:(City*)aCity;

+(NSArray *)arrayInformationsWithSummary:(Summary*)aSummary;

+(NSFetchedResultsController *)fetchControllerForAllSummaryOfCity:(NSManagedObjectContext *)context cityIndicatif:(NSString*)anIndicatifCity;
@end

现在是我挣扎的部分

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as? MyCollectionViewCell
        cell?.layoutIfNeeded()

        var tmpForecast = self.listForecasts[indexPath.row]

        cell?.delegate = self
        cell?.load(tmpForecast as! Summary)

        return cell!
    }

当我转储listForecasts时，它只包含Forecast和Forecast48h对象

当我想加载我的手机时，我有运行时错误：

无法将“Forecast48h\u Forecast48h\u”类型的值强制转换为“Summary”

首先，为什么会在错误消息中键入此双预测48h

加载方法的原型：

-(void)loadCell:(Summary*)weatherSummary;

我确实需要向loadCell方法传递一个Summary对象。 为什么Objective-C接受这一点，而Swift不接受

看看这段代码，它是用Objective-C编写的：

Summary* aSummary = [_listForecasts objectAtIndex:indexPath.row];
[cell setDelegate:self];
[cell loadCell:aSummary];
return cell;

这在Objective-C中起作用

有什么帮助吗？请随时询问更多信息。这个项目很复杂，很难解释，我试着让它尽可能简单

编辑：

@interface Summary (Additions)

+(NSArray*)allSummaryOfCity:(NSManagedObjectContext *)context city:(City*)aCity;

+(NSArray *)arrayInformationsWithSummary:(Summary*)aSummary;

+(NSFetchedResultsController *)fetchControllerForAllSummaryOfCity:(NSManagedObjectContext *)context cityIndicatif:(NSString*)anIndicatifCity;
@end

---

多态性意味着您始终可以将对象强制转换为其超类类型，但如果两个对象具有派生自同一超类的类型，则无法从一种类型强制转换为另一种类型。在协议的情况下，同样有效

在您的情况下，使用Forecasterface类型就足够了，因为您确信任何获取的对象都实现了Forecasterface协议：

var listForecasts = Array<ForecastInterface>()
self.listForecasts = city.bestForecastArray()

...

-(void)loadCell:(ForecastInterface*)weatherSummary;

...


func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as? MyCollectionViewCell
    cell?.layoutIfNeeded()

    // tmpForecast type: ForeCastInterface
    var tmpForecast = self.listForecasts[indexPath.row]

    cell?.delegate = self
    cell?.load(tmpForecast)

    return cell!
}

---

是否可能在某个时候从Forecasterface类型返回到Summary类型？@仅当对象确实是Summary类型时才平衡。如果对象的类型为Forecast48h，则强制转换将始终失败。如果Forecast48是Summary的一个子类，那么它就会成功，这不是您的情况。我的意思是，为了现在处理ForecastInterface类型，我更改了loadCell方法。但是在这个方法中，我确实需要Summary+Additions文件中实现的Summary类型中的一些特定方法。您需要哪些方法？你能发布Summary+Additions.swift的内容吗？如果你强制施展，如果你有其他对象类型（例如Forecast48h），应用程序将崩溃。

var listForecasts = Array<ForecastInterface>()
self.listForecasts = city.bestForecastArray()

...

-(void)loadCell:(ForecastInterface*)weatherSummary;

...


func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as? MyCollectionViewCell
    cell?.layoutIfNeeded()

    // tmpForecast type: ForeCastInterface
    var tmpForecast = self.listForecasts[indexPath.row]

    cell?.delegate = self
    cell?.load(tmpForecast)

    return cell!
}

var tmpForecast = self.listForecasts[indexPath.row]
if let summary = tmpForecast as? Summary {
    ...
}