KVC---使用集合运算
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当你要发送valueForKeyPath:消息给KVC对象时,你可以把集合运算包含在key path里.集合运算符是前面带有(@)的关键字.它可以在数据返回前执行特定的操作.NSObject提供的valueForKeyPath:默认实现实现了这种行为.
当key path包含集合运算符时,在运算符前面的是left key path,它表示接收运算符的集合.如果你直接把消息发送给集合对象(比如:NSArray对象),left key path可能会被忽略.
运算符后面的,就是right key path,它表示运算符是在哪个集合属性上发生作用.所有的集合运算符(除了@count外),都需要一个right key path.
key path 运算格式
集合运算的三种基本运算类型:
- 汇总运算:以某种方式合并集合对象,并返回一个单一的对象,这个对象的类型是和right key path里的属性一样的.
@count运算符是特例,你没有right key path并总是返回NSNumber实例. - 数组运算:总是返回集合里的子集
- 嵌套运算:作用在包含了其它集合的集合,并返回一个
NSArray或NSSet实例.以某种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
为了讨论,假设BankAccount实例,有以下数据在transactions数组里:
| 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 |
你可以简单的通过把 self 作为操作符后面的 key path 来获取一个由 NSNumber 组成的数组或者集合的总值，例如对于数组 @[@(1), @(2), @(3)] 可使用 valueForKeyPath:@"@max.self" 来获取最大值。
@avg:
valueForKeyPath:读取集合里每个元素的right key path属性,把它转成double(nil值用0表示),并计算它们的算术平均值.最后把结果存在NSNumber实例里并返回.
NSNumber *transactionAverage = [self.transactions valueForKeyPath:@"@avg.amount"];
///transactionAverage 的结果是$456.54
@count
valueForKeyPath:返回集合里有多少个对象,并存在NSNumber实例里.如果出现right key path将被忽略.
NSNumber *numberOfTransactions = [self.transactions valueForKeyPath:@"@count"];
//numberOfTransactions的值为13
@max
valueForKeyPath:将搜索集合里全部的right key path的实体,并返回最大的那个.这个搜索是用compare:方法来对比的.所以right key path所指向的实体必须是可以使用这个方法的.搜索会忽略nil.
NSDate *latestDate = [self.transactions valueForKeyPath:@"@max.date"];
//latestDate的值是 Jul 15, 2016
@min
与@max相反
NSDate *earliestDate = [self.transactions valueForKeyPath:@"@min.date"];
// earliestDate的值是 Dec 1, 2015
@sum
valueForKeyPath:读取集合里每个元素的right key path属性,把它转成double(nil值用0表示),并计算它们的合.最后把结果存在NSNumber实例里并返回.
NSNumber *amountSum = [self.transactions valueForKeyPath:@"@sum.amount"];
// amountSum的值是$5935.00
Array Operators
valueForKeyPath:返回一个right key path所指向属性的数组,这个属性是集合元素相关联的.
如果所有返回的属性都是nil,
valueForKeyPath:方法会抛出异常
@distinctUnionOfObjects
会返回right key path所指向对象的数组,是重复的将被忽略
NSArray *distinctPayees = [self.transactions valueForKeyPath:@"@distinctUnionOfObjects.payee"];
//distinctPayees的结果:Car Loan, General Cable, Animal Hospital, Green Power, Mortgage.
@unionOfObjects
和@distinctUnionOfObjects相似的行为,但这个并不移除重复的对象.
NSArray *payees = [self.transactions valueForKeyPath:@"@unionOfObjects.payee"];
// payees的结果:Green Power, Green Power, Green Power, Car Loan, Car Loan, Car Loan, General Cable, General Cable, General Cable, Mortgage, Mortgage, Mortgage, Animal Hospital.注意有重复的.
Nesting Operators
NSArray* moreTransactions = @[<# transaction data #>];
NSArray* arrayOfArrays = @[self.transactions, moreTransactions];
moreTransactions里的数据
| 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的结果:Hobby Shop, Mortgage, Animal Hospital, Second Mortgage, Car Loan, General Cable - Cottage, General Cable, Green Power
@unionOfArrays
和@distinctUnionOfArrays一样,但包含重复的数据
NSArray *collectedPayees = [arrayOfArrays valueForKeyPath:@"@unionOfArrays.payee"];
collectedPayees的结果: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
这个和@distinctUnionOfArrays是一样的,只不过它用在NSSet实例包含NSSet实例,而不是NSArray包含NSArray.并且,它也是返回一个NSSet实例.
