來源
當(dāng)你要發(fā)送valueForKeyPath:消息給KVC對象時,你可以把集合運(yùn)算包含在key path里.集合運(yùn)算符是前面帶有(@)的關(guān)鍵字.它可以在數(shù)據(jù)返回前執(zhí)行特定的操作.NSObject提供的valueForKeyPath:默認(rèn)實(shí)現(xiàn)實(shí)現(xiàn)了這種行為.
當(dāng)key path包含集合運(yùn)算符時,在運(yùn)算符前面的是left key path,它表示接收運(yùn)算符的集合.如果你直接把消息發(fā)送給集合對象(比如:NSArray對象),left key path可能會被忽略.
運(yùn)算符后面的,就是right key path,它表示運(yùn)算符是在哪個集合屬性上發(fā)生作用.所有的集合運(yùn)算符(除了@count外),都需要一個right key path.
集合運(yùn)算的三種基本運(yùn)算類型:
- 匯總運(yùn)算:以某種方式合并集合對象,并返回一個單一的對象,這個對象的類型是和right key path里的屬性一樣的.
@count運(yùn)算符是特例,你沒有right key path并總是返回NSNumber實(shí)例. - 數(shù)組運(yùn)算:總是返回集合里的子集
- 嵌套運(yùn)算:作用在包含了其它集合的集合,并返回一個
NSArray或NSSet實(shí)例.以某種方式合并了嵌套集合.
例子
@interface BankAccount : NSObject
@property (nonatomic) NSNumber* currentBalance; // An attribute
@property (nonatomic) Person* owner; // A to-one relation
@property (nonatomic) NSArray< Transaction* >* transactions; // A to-many relation
@end
@interface Transaction : NSObject
@property (nonatomic) NSString* payee; // To whom
@property (nonatomic) NSNumber* amount; // How much
@property (nonatomic) NSDate* date; // When
@end
為了討論,假設(shè)BankAccount實(shí)例,有以下數(shù)據(jù)在transactions數(shù)組里:
| payee (values) | amount (values formatted as currency) | date (values formatted as month day, year) |
|---|---|---|
| Green Power | $120.00 | Dec 1, 2015 |
| Green Power | $150.00 | Jan 1, 2016 |
| Green Power | $170.00 | Feb 1, 2016 |
| Car Loan | $250.00 | Jan 15, 2016 |
| Car Loan | $250.00 | Feb 15, 2016 |
| Car Loan | $250.00 | Mar 15, 2016 |
| General Cable | $120.00 | Dec 1, 2015 |
| General Cable | $155.00 | Jan 1, 2016 |
| General Cable | $120.00 | Feb 1, 2016 |
| Mortgage | $1,250.00 | Jan 15, 2016 |
| Mortgage | $1,250.00 | Feb 15, 2016 |
| Mortgage | $1,250.00 | Mar 15, 2016 |
| Animal Hospital | $600.00 | Jul 15, 2016 |
你可以簡單的通過把 self 作為操作符后面的 key path 來獲取一個由 NSNumber 組成的數(shù)組或者集合的總值，例如對于數(shù)組 @[@(1), @(2), @(3)] 可使用 valueForKeyPath:@"@max.self" 來獲取最大值。
@avg:
valueForKeyPath:讀取集合里每個元素的right key path屬性,把它轉(zhuǎn)成double(nil值用0表示),并計算它們的算術(shù)平均值.最后把結(jié)果存在NSNumber實(shí)例里并返回.
NSNumber *transactionAverage = [self.transactions valueForKeyPath:@"@avg.amount"];
///transactionAverage 的結(jié)果是$456.54
@count
valueForKeyPath:返回集合里有多少個對象,并存在NSNumber實(shí)例里.如果出現(xiàn)right key path將被忽略.
NSNumber *numberOfTransactions = [self.transactions valueForKeyPath:@"@count"];
//numberOfTransactions的值為13
@max
valueForKeyPath:將搜索集合里全部的right key path的實(shí)體,并返回最大的那個.這個搜索是用compare:方法來對比的.所以right key path所指向的實(shí)體必須是可以使用這個方法的.搜索會忽略nil.
NSDate *latestDate = [self.transactions valueForKeyPath:@"@max.date"];
//latestDate的值是 Jul 15, 2016
@min
與@max相反
NSDate *earliestDate = [self.transactions valueForKeyPath:@"@min.date"];
// earliestDate的值是 Dec 1, 2015
@sum
valueForKeyPath:讀取集合里每個元素的right key path屬性,把它轉(zhuǎn)成double(nil值用0表示),并計算它們的合.最后把結(jié)果存在NSNumber實(shí)例里并返回.
NSNumber *amountSum = [self.transactions valueForKeyPath:@"@sum.amount"];
// amountSum的值是$5935.00
Array Operators
valueForKeyPath:返回一個right key path所指向?qū)傩缘臄?shù)組,這個屬性是集合元素相關(guān)聯(lián)的.
如果所有返回的屬性都是nil,
valueForKeyPath:方法會拋出異常
@distinctUnionOfObjects
會返回right key path所指向?qū)ο蟮臄?shù)組,是重復(fù)的將被忽略
NSArray *distinctPayees = [self.transactions valueForKeyPath:@"@distinctUnionOfObjects.payee"];
//distinctPayees的結(jié)果:Car Loan, General Cable, Animal Hospital, Green Power, Mortgage.
@unionOfObjects
和@distinctUnionOfObjects相似的行為,但這個并不移除重復(fù)的對象.
NSArray *payees = [self.transactions valueForKeyPath:@"@unionOfObjects.payee"];
// payees的結(jié)果:Green Power, Green Power, Green Power, Car Loan, Car Loan, Car Loan, General Cable, General Cable, General Cable, Mortgage, Mortgage, Mortgage, Animal Hospital.注意有重復(fù)的.
Nesting Operators
NSArray* moreTransactions = @[<# transaction data #>];
NSArray* arrayOfArrays = @[self.transactions, moreTransactions];
moreTransactions里的數(shù)據(jù)
| payee (values) | amount (values formatted as currency) | date (values formatted as month day, year) |
|---|---|---|
| General Cable - Cottage | $120.00 | Dec 18, 2015 |
| General Cable - Cottage | $155.00 | Jan 9, 2016 |
| General Cable - Cottage | $120.00 | Dec 1, 2016 |
| Second Mortgage | $1,250.00 | Nov 15, 2016 |
| Second Mortgage | $1,250.00 | Sep 20, 2016 |
| Second Mortgage | $1,250.00 | Feb 12, 2016 |
| Hobby Shop | $600.00 | Jun 14, 2016 |
@distinctUnionOfArrays
NSArray *collectedDistinctPayees = [arrayOfArrays valueForKeyPath:@"@distinctUnionOfArrays.payee"];
// collectedDistinctPayees的結(jié)果:Hobby Shop, Mortgage, Animal Hospital, Second Mortgage, Car Loan, General Cable - Cottage, General Cable, Green Power
@unionOfArrays
和@distinctUnionOfArrays一樣,但包含重復(fù)的數(shù)據(jù)
NSArray *collectedPayees = [arrayOfArrays valueForKeyPath:@"@unionOfArrays.payee"];
collectedPayees的結(jié)果:Green Power, Green Power, Green Power, Car Loan, Car Loan, Car Loan, General Cable, General Cable, General Cable, Mortgage, Mortgage, Mortgage, Animal Hospital, General Cable - Cottage, General Cable - Cottage, General Cable - Cottage, Second Mortgage, Second Mortgage, Second Mortgage, Hobby Shop.
@distinctUnionOfSets
這個和@distinctUnionOfArrays是一樣的,只不過它用在NSSet實(shí)例包含NSSet實(shí)例,而不是NSArray包含NSArray.并且,它也是返回一個NSSet實(shí)例.
