Question

如果从字典中的空键/值中获取数据，如何将默认值放入字符串中。

所以[myObject setMyString:[dictionary valueForKey:@"myKey"]];

那么如果我做NSString *newString = myObject.myString，我会得到unrecognized selector error。

因此，如果键值为空，我只需要一种方法来插入默认字符串;

Answer 1

如果dictionary是NSDictionary，您应该使用objectForKey，因为valueForKey用于KVC。它适用于NSDictionary但如果密钥与某些NSDictionary KVC密钥发生冲突，可能会咬你，例如“@allKeys”或“@count”。

我认为最短的可能是：

[dictionary objectForkey:@"myKey"] ?: @"defaultValue"

有一种可怕的方法是滥用现有的字典方法来生成get by key或者如果你因某些原因不想使用某个条件则返回一个默认值...

[[dictionary objectsForKeys:[NSArray arrayWithObject:@"myKey"]
             notFoundMarker:@"defaultValue"]
 objectAtIndex:0]

你没有听到我的消息：）

Answer 2

这个怎么样？

NSString *value = [dictionary valueForKey:@"myKey"];
if (!value) value = @"defaultValue";
[myObject setMyString:value];

Answer 3

对于一般情况使用[NSNull null]，并在返回时处理它。

在你的问题中，你不会得到“无法识别的选择器”; newString将设置为nil（而不是[NSNull null]，因此我怀疑您可能还有其他意义 - 也许如何设置NSUserDefaults的默认值？

Answer 4

我使用简单的NSDictionary扩展程序完成了这项工作。

的NSDictionary + NSDictionaryExtensions.h

#import <Foundation/Foundation.h>

@interface NSDictionary (NSDictionaryExtensions)

- (id)objectForKey:(id)aKey defaultObject: (id) defObj;

@end

的NSDictionary + NSDictionaryExtensions.m

#import "NSDictionary+NSDictionaryExtensions.h"

@implementation NSDictionary (NSDictionaryExtensions)

- (id)objectForKey:(id)aKey defaultObject: (id) defObj
{
    id ret = [self objectForKey: aKey];
    if ( ret == nil )
        return defObj;
    else
        return ret;
}

@end

然后我可以按如下方式访问：

NSString* str = [dict objectForKey: @"a_key" defaultObject: @"default"];

Answer 5

尝试

if ( myObject == nil ) {
   [myObject setMyString:@""];
}

或

if ( [dictionary valueForKey:@"myKey"] == nil ) {
    [dictionary setObject:@"" forKey:@"myKey"];
}

Answer 6

无论您为字符串分配了什么默认值，都不会使myObject.myString返回“无法识别的选择器错误” - 通常属性myString存在或不存在（通常是'，因为您可以深入研究Objective-C运行时的内核和玩游戏，但这很少（如果有的话）你想做什么！）

你想做什么？如果您的目标是在myObject.myString未设置的情况下抛出异常，那么您可以在属性实现中执行此操作。

例如，如果没有为给定密钥设置值，valueForKey;将返回nil，因此您可以在您的媒体资源中检查该内容：

- (void) setMyString:(NSString *)value
{  myString = value;
}

- (NSString *) myString
{
   if (myString == nil)
      @throw [NSException exceptionWithName:@"UnrecognizedSelector" reason:@"No value set for myString" userInfo:nil];
   return myString;
}

注意：

在终端输入的代码，可能包含拼写错误
代码假设垃圾收集/ ARC开启，如果不是，则需要添加适当的保留/复制/发布

但你真的想抛出异常吗？如果字符串应该，那么通常只会这样做，而不是例外条件 - 你不会抛出默认值。

Answer 7

根据@Paul Lynch你必须检查[NSNull null]除了nil之外。建议检查您要查找的数据类型：

- (id) objectForKey: (id) key withDefault: (id) defaultValue
{
    id value = self[key];

    if (value == nil || [value isEqual: [NSNull null]] || ![defaultValue isKindOfClass: [value class]])
    {
        value = defaultValue;
    }

    return value;
}

Answer 8

您可以手动检查以下不同类型的零值：

extension NSDictionary {

func convertToString() -> String {
    let jsonData = try! JSONSerialization.data(withJSONObject: self, options: JSONSerialization.WritingOptions.prettyPrinted) as NSData!

    if jsonData  == nil{
        return "{}"
    }else {
        return String(data: jsonData as! Data, encoding: String.Encoding.utf8)!
    }
}

func object_forKeyWithValidationForClass_Int(aKey: String) -> Int {

    // CHECK FOR EMPTY
    if(self.allKeys.count == 0) {
        return Int()
    }

    // CHECK IF KEY EXIST
    if let val = self.object(forKey: aKey) {
        if((val as AnyObject).isEqual(NSNull())) {
            return Int()
        }
    } else {
        // KEY NOT FOUND
        return Int()
    }

    // CHECK FOR NIL VALUE
    let aValue : AnyObject = self.object(forKey: aKey)! as AnyObject
    if aValue.isEqual(NSNull()) {
        return Int()
    }
    else if(aValue.isKind(of: NSString.self)){
        return Int((aValue as! NSString).intValue)
    }
    else {

        if aValue is Int {
            return self.object(forKey: aKey) as! Int
        }
        else{
            return Int()
        }
    }
}

func object_forKeyWithValidationForClass_CGFloat(aKey: String) -> CGFloat {

    // CHECK FOR EMPTY
    if(self.allKeys.count == 0) {
        return CGFloat()
    }

    // CHECK IF KEY EXIST
    if let val = self.object(forKey: aKey) {
        if((val as AnyObject).isEqual(NSNull())) {
            return CGFloat()
        }
    } else {
        // KEY NOT FOUND
        return CGFloat()
    }

    // CHECK FOR NIL VALUE
    let aValue : AnyObject = self.object(forKey: aKey)! as AnyObject
    if aValue.isEqual(NSNull()) {
        return CGFloat()
    }
    else {

        if aValue is CGFloat {
            return self.object(forKey: aKey) as! CGFloat
        }
        else{
            return CGFloat()
        }
    }
}

func object_forKeyWithValidationForClass_String(aKey: String) -> String {

    // CHECK FOR EMPTY
    if(self.allKeys.count == 0) {
        return String()
    }

    // CHECK IF KEY EXIST
    if let val = self.object(forKey: aKey) {
        if((val as AnyObject).isEqual(NSNull())) {
            return String()
        }
    } else {
        // KEY NOT FOUND
        return String()
    }

    // CHECK FOR NIL VALUE
    let aValue : AnyObject = self.object(forKey: aKey)! as AnyObject
    if aValue.isEqual(NSNull()) {
        return String()
    }
    else if(aValue.isKind(of: NSNumber.self)){
        return String(format:"%f", (aValue as! NSNumber).doubleValue)
    }
    else {

        if aValue is String {
            return self.object(forKey: aKey) as! String
        }
        else{
            return String()
        }
    }
}

func object_forKeyWithValidationForClass_StringInt(aKey: String) -> String {

    // CHECK FOR EMPTY
    if(self.allKeys.count == 0) {
        return String()
    }

    // CHECK IF KEY EXIST
    if let val = self.object(forKey: aKey) {
        if((val as AnyObject).isEqual(NSNull())) {
            return String()
        }
    } else {
        // KEY NOT FOUND
        return String()
    }

    // CHECK FOR NIL VALUE
    let aValue : AnyObject = self.object(forKey: aKey)! as AnyObject
    if aValue.isEqual(NSNull()) {
        return String()
    }
    else if(aValue.isKind(of: NSNumber.self)){
        return String(format:"%d", (aValue as! NSNumber).int64Value)


    }
    else {

        if aValue is String {
            return self.object(forKey: aKey) as! String
        }
        else{
            return String()
        }
    }
}

func object_forKeyWithValidationForClass_Bool(aKey: String) -> Bool {
    // CHECK FOR EMPTY
    if(self.allKeys.count == 0) {
        return Bool()
    }

    // CHECK IF KEY EXIST
    if let val = self.object(forKey: aKey) {
        if((val as AnyObject).isEqual(NSNull())) {
            return Bool()
        }
    } else {
        // KEY NOT FOUND
        return Bool()
    }

    // CHECK FOR NIL VALUE
    let aValue : AnyObject = self.object(forKey: aKey)! as AnyObject
    if aValue.isEqual(NSNull()) {
        return Bool()
    }
    else {

        if aValue is Bool {
            return self.object(forKey: aKey) as! Bool
        }
        else{
            return Bool()
        }
    }
}

func object_forKeyWithValidationForClass_NSArray(aKey: String) -> NSArray {
    // CHECK FOR EMPTY
    if(self.allKeys.count == 0) {
        return NSArray()
    }

    // CHECK IF KEY EXIST
    if let val = self.object(forKey: aKey) {
        if((val as AnyObject).isEqual(NSNull())) {
            return NSArray()
        }
    } else {
        // KEY NOT FOUND
        return NSArray()
    }

    // CHECK FOR NIL VALUE
    let aValue : AnyObject = self.object(forKey: aKey)! as AnyObject
    if aValue.isEqual(NSNull()) {
        return NSArray()
    }
    else {
        if aValue is NSArray {
            return self.object(forKey: aKey) as! NSArray
        }
        else{
            return NSArray()
        }
    }
}

func object_forKeyWithValidationForClass_NSMutableArray(aKey: String) -> NSMutableArray {
    // CHECK FOR EMPTY
    if(self.allKeys.count == 0) {
        return NSMutableArray()
    }

    // CHECK IF KEY EXIST
    if let val = self.object(forKey: aKey) {
        if((val as AnyObject).isEqual(NSNull())) {
            return NSMutableArray()
        }
    } else {
        // KEY NOT FOUND
        return NSMutableArray()
    }

    // CHECK FOR NIL VALUE
    let aValue : AnyObject = self.object(forKey: aKey)! as AnyObject
    if aValue.isEqual(NSNull()) {
        return NSMutableArray()
    }
    else {

        if aValue is NSMutableArray {
            return self.object(forKey: aKey) as! NSMutableArray
        }
        else{
            return NSMutableArray()
        }
    }
}

func object_forKeyWithValidationForClass_NSDictionary(aKey: String) -> NSDictionary {
    // CHECK FOR EMPTY
    if(self.allKeys.count == 0) {
        return NSDictionary()
    }

    // CHECK IF KEY EXIST
    if let val = self.object(forKey: aKey) {
        if((val as AnyObject).isEqual(NSNull())) {
            return NSDictionary()
        }
    } else {
        // KEY NOT FOUND
        return NSDictionary()
    }

    // CHECK FOR NIL VALUE
    let aValue : AnyObject = self.object(forKey: aKey)! as AnyObject
    if aValue.isEqual(NSNull()) {
        return NSDictionary()
    }
    else {

        if aValue is NSDictionary {
            return self.object(forKey: aKey) as! NSDictionary
        }
        else{
            return NSDictionary()
        }
    }
}

func object_forKeyWithValidationForClass_NSMutableDictionary(aKey: String) -> NSMutableDictionary {
    // CHECK FOR EMPTY
    if(self.allKeys.count == 0) {
        return NSMutableDictionary()
    }

    // CHECK IF KEY EXIST
    if let val = self.object(forKey: aKey) {
        if((val as AnyObject).isEqual(NSNull())) {
            return NSMutableDictionary()
        }
    } else {
        // KEY NOT FOUND
        return NSMutableDictionary()
    }

    // CHECK FOR NIL VALUE
    let aValue : AnyObject = self.object(forKey: aKey)! as AnyObject
    if aValue.isEqual(NSNull()) {
        return NSMutableDictionary()
    }
    else {
        if aValue is NSMutableDictionary {
            return self.object(forKey: aKey) as! NSMutableDictionary
        }
        else{
            return NSMutableDictionary()
        }
    }
}

}

NSString和NSDictionary valueForKey - nil？

8 个答案: