iOS中 valueForKeyPath常用用法

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+ (NSString *)caculateArray:(NSArray *)array
{
  CGFloat sum = [[array valueForKeyPath:@"@sum.floatValue"] floatValue];
  CGFloat avg = [[array valueForKeyPath:@"@avg.floatValue"] floatValue];
  CGFloat max =[[array valueForKeyPath:@"@max.floatValue"] floatValue];
  CGFloat min =[[array valueForKeyPath:@"@min.floatValue"] floatValue];
  NSLog(@"%fn%fn%fn%f",sum,avg,max,min);
  return [NSString stringWithFormat:@"%f",sum];
}

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NSArray *arr = @[@{@"city":@"beijing",@"person":@{@"name":@"zhangsan"}},@{@"city":@"chengdu"}];
[arr valueForKeyPath:@"city"] 可以获取到city数组 @[@"beijing",@"chengdu"]

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NSDictionary *dict1 = @{@"dict1":@{@"dict2":@{@"name":@"lishi",@"info":@{@"age":@"12"}}}};
id res = [dict1 valueForKeyPath:@"dict1.dict2.name"];//(http://dict1.dict2.name/)
NSLog(@"res = %@",res);// 输出lishi

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NSArray *array = @[@"qq", @"wechat", @"qq", @"msn", @"wechat"];
NSArray *deleltNewA = [array valueForKeyPath:@"@distinctUnionOfObjects.self"];
NSLog(@"deleltNewA = %@",deleltNewA);// 输出( qq, wechat,  msn)

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NSArray *array = @[@{@"name" : @"xiaoming",
         @"code" : @1},
         @{@"name": @"judy",
         @"code" : @2},
         @{@"name": @"judy",
         @"code" : @3},
         @{@"name": @"xiaohua",
         @"code" : @4}];
NSArray *deleltNewA = [array valueForKeyPath:@"@distinctUnionOfObjects.name"];
NSLog(@"deleltNewA = %@",deleltNewA);// 输出(judy, xiaohua, xiaoming);

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[addressTextField setValue:[UIColor redColor] forKeyPath:@”_placeholderLabel.textColor”];
比起重写 - (void)drawPlaceholderInRect:(CGRect)rect; 要方便太多！