Tag: KVC
NSArray 和 KVC
by Elton on 十一.04, 2009, under Mac
Theocoacao有篇不错的介绍NSArray和KVC的文章，我给粗略翻译了一下。
NSArray的-valueForKey: 有一个不是很显著的特性. 你可以使用它返回一个由tree对象包装的值对象的数组。这句话可能不是很容易理解，看下面的例子就清楚了。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 | NSMutableArray * tree = [NSMutableArray array]; NSDictionary * p = nil; // parent NSDictionary * c = nil; // child NSNumber * n = nil; // value int i; for ( i = 0; i < 10; i++ ) { n = [NSNumber numberWithInt: i]; c = [NSDictionary dictionaryWithObject:n forKey: @"someKey"]; p = [NSDictionary dictionaryWithObject: c forKey: @"storage"]; [tree addObject: p]; } NSLog (@"%@", tree); // here's the important part! NSArray * justValues; justValues = [tree valueForKeyPath: @"storage.someKey"]; NSLog (@"%@", justValues); |
第一个NSLog输出的内容t:
1 2 3 4 5 6 7 8 9 10 11 12 13 | NSLog (@"%@", tree); ( {storage = {someKey = 0; }; }, {storage = {someKey = 1; }; }, {storage = {someKey = 2; }; }, {storage = {someKey = 3; }; }, {storage = {someKey = 4; }; }, {storage = {someKey = 5; }; }, {storage = {someKey = 6; }; }, {storage = {someKey = 7; }; }, {storage = {someKey = 8; }; }, {storage = {someKey = 9; }; } ) |
第二个NSLog 返回一个数组的值，这个数组是由封装它的tree对象的 @”storage.someKey”:这个keypath提供的：
1 2 3 4 | NSArray * justValues; justValues = [tree valueForKeyPath: @"storage.someKey"]; NSLog (@"%@", justValues); (0, 1, 2, 3, 4, 5, 6, 7, 8, 9) |
我发现它真的非常方便，当你使用Core Data要排序一个managed objects对象里面的一个数组的时候。 你可以用一个attribute就把它里面数组的值列出来。.
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