valueForKeyPath

Perhaps many people do not use - (id)valueForKeyPath:(NSString *)keyPath very often in day-to-day development.

However, this method is very powerful. For example:

NSArray *array = @[@"name", @"w", @"aa", @"jimsa"];
NSLog(@"%@", [array valueForKeyPath:@"uppercaseString"]);

Output

(
    NAME,
    W,
    AA,
    JIMSA
)

This is equivalent to calling uppercaseString on each element in the array, then returning the results as a new array. Since uppercaseString works, other NSString methods work too, for example:

[array valueForKeyPath:@"length"]

This returns an array containing the length of each string. Any instance method you can think of can be used this way.

If you think this method only does that much, you are wrong. Let’s look at some concrete examples.

Quickly Calculating Sum, Average, Maximum, and Minimum for an NSArray of NSNumbers

NSArray *array = @[@1, @2, @3, @4, @10];
NSNumber *sum = [array valueForKeyPath:@"@sum.self"];
NSNumber *avg = [array valueForKeyPath:@"@avg.self"];
NSNumber *max = [array valueForKeyPath:@"@max.self"];
NSNumber *min = [array valueForKeyPath:@"@min.self"];

Or specify the output type:

NSNumber *sum = [array valueForKeyPath:@"@sum.floatValue"];
NSNumber *avg = [array valueForKeyPath:@"@avg.floatValue"];
NSNumber *max = [array valueForKeyPath:@"@max.floatValue"];
NSNumber *min = [array valueForKeyPath:@"@min.floatValue"];

One thing to note: the collection operators @sum and @avg first convert objects to double values, and then output an NSNumber containing a double, so do not make this mistake:

NSArray *arr = @[@(0),@(10),@(40)];
NSInteger avg = [[arr valueForKeyPath:@"@avg.self"] integerValue];
NSLog(@"---%ld----",avg);

After calling integerValue, the result becomes 0 directly.

Removing Duplicate Data

NSArray *array = @[@"name", @"w", @"aa", @"jimsa", @"aa"];
NSLog(@"%@", [array valueForKeyPath:@"@distinctUnionOfObjects.self"]);

(
    name,
    w,
    jimsa,
    aa
)

Quick Value Extraction

    NSArray *array = @[@{@"name" : @"cookeee",
                         @"code" : @1},
                       @{@"name" : @"sswwre",
                         @"code" : @2}];
    NSLog(@"%@", [array valueForKeyPath:@"name"]);

This directly returns an array containing the values for the name key in each dictionary. Clearly, this is more convenient and faster than looping through the values and adding them to a new array.

(
    cookeee,
    jim,
    jim,
    jbos
)

Nested Usage

Remove duplicate values from the name field and then extract the values.

NSArray *array = @[@{@"name" : @"cookeee",@"code" : @1},
                    @{@"name": @"jim",@"code" : @2},
                    @{@"name": @"jim",@"code" : @1},
                    @{@"name": @"jbos",@"code" : @1}];

NSLog(@"%@", [array valueForKeyPath:@"@distinctUnionOfObjects.name"]);

(
    cookeee,
    jim,
    jbos
)

Get the maximum value of the number field.

NSArray *array = @[@{@"number" : @1, @"title" : @""},
                   @{@"number" : @2, @"title" : @""},
                   @{@"number" : @20, @"title" : @""},
                   @{@"number" : @4, @"title" : @""},
                   @{@"number" : @5, @"title" : @""}
];
NSNumber *max = [array valueForKeyPath:@"@max.number"];

Changing the Placeholder Color of a UITextField

    [searchField setValue:[UIColor whiteColor] forKeyPath:@"_placeholderLabel.textColor"];

This is much more convenient than overriding - (void)drawPlaceholderInRect:(CGRect)rect;.

Practical Example

NSArray *array = @[
                   @[@1, @2],
                   @[@1, @2, @3, @4],
                   @[@1, @2, @3]
                   ];
NSNumber *count = [[array valueForKeyPath:@"@unionOfObjects.@count"] valueForKeyPath:@"@sum.self"];

Calculate the total number of elements in a nested array. For example, this returns 9.