Passing a method as a parameter type

Question

Passing a method as a parameter type

As I delve into learning JavaScript from Objective-C, I find myself pondering whether it is possible to have a method with parameter types in Objective-C. Let's take the example of the findIndex() JavaScript function that identifies and returns the index of the first array element passing a specified test function. How could this be implemented in Objective-C using blocks, if at all?

If I were to create a class category for NSArray, how would I go about passing a block to a method and applying that condition to the NSArray itself? If this is not feasible, I am eager to understand the reasons behind it.

    var ages = [3, 10, 18, 20];

    function checkAdult(age) {
        return age >= 18;
    }

    function myFunction() {
        document.getElementById("demo").innerHTML = ages.findIndex(checkAdult);
    }

I came across a valuable piece of information on StackOverflow regarding block syntax, which might aid in understanding the implementation of the findIndex() function.

Block Syntax

Block as a method parameter Template

- (void)aMethodWithBlock:(returnType (^)(parameters))blockName {
        // your code
}

Example

-(void) saveWithCompletionBlock: (void (^)(NSArray *elements, NSError *error))completionBlock{
        // your code
}

Block as a method argument Template

[anObject aMethodWithBlock: ^returnType (parameters) {
    // your code
}];

Example

[self saveWithCompletionBlock:^(NSArray *array, NSError *error) {
    // your code
}];

Block as a local variable** Template

returnType (^blockName)(parameters) = ^returnType(parameters) {
    // your code
};

Example

void (^completionBlock) (NSArray *array, NSError *error) = ^void(NSArray *array, NSError *error){
    // your code
};

Block as a typedef Template

typedef returnType (^typeName)(parameters);

typeName blockName = ^(parameters) {
    // your code
}

FindIndex() Definition

Array.prototype.findIndex ( predicate [ , thisArg ] )

NOTE predicate should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. findIndex calls predicate once for each element of the array, in ascending order, until it finds one where predicate returns true. If such an element is found, findIndex immediately returns the index of that element value. Otherwise, findIndex returns -1.

javascript objective-c arrays nsarray objective-c-blocks

Answer 1

Answer №1

Equivalent code snippet in Objective-C:

@implementation NSArray (Extension)

- (NSUInteger)findIndexWithCondition:(BOOL (^)(id item, NSUInteger index, NSArray* array))condition {
    NSUInteger foundIndex = NSNotFound;
    if (condition != NULL) {
        for (NSUInteger i = 0; i < self.count; i++) {
            if (condition(self[i], i, self)) {
                foundIndex = i;
                break;
            }
        }
    }
    return foundIndex;
}

@end

It's worth noting the change in the default return value from -1 to NSNotFound because the index is unsigned.

Usage of the method:

NSUInteger indexFound = [@[@3, @10, @18, @20] findIndexWithCondition:^BOOL(id element, NSUInteger index, NSArray *array) {
    return [element integerValue] >= 18;
}];

You have the option to simplify by not including extra parameters, as demonstrated in this JavaScript-inspired implementation:

@implementation NSArray (Extension)

- (NSUInteger)findIndexWithPredicate:(BOOL (^)(id item))predicate {
    NSUInteger foundIndex = NSNotFound;
    if (predicate != NULL) {
        for (NSUInteger i = 0; i < self.count; i++) {
            if (predicate(self[i])) {
                foundIndex = i;
                break;
            }
        }
    }
    return foundIndex;
}

@end

NSUInteger indexFound = [@[@3, @10, @18, @20] findIndexWithPredicate:^BOOL(id element) {
    return [element integerValue] >= 18;
}];

An alternative approach without creating a custom category involves utilizing the enumerateObjectsUsingBlock: method:

__block NSUInteger indexToFind;
[@[@3, @10, @18, @20] enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
    if ([obj integerValue] >= 18) {
        *stop = YES;
        indexToFind = idx;
    }
}];

Furthermore, there are options like C function pointers instead of blocks, but blocks offer more versatility.

Answer 2

Equivalent code snippet in Objective-C:

@implementation NSArray (Extension)

- (NSUInteger)findIndexWithCondition:(BOOL (^)(id item, NSUInteger index, NSArray* array))condition {
    NSUInteger foundIndex = NSNotFound;
    if (condition != NULL) {
        for (NSUInteger i = 0; i < self.count; i++) {
            if (condition(self[i], i, self)) {
                foundIndex = i;
                break;
            }
        }
    }
    return foundIndex;
}

@end

It's worth noting the change in the default return value from -1 to NSNotFound because the index is unsigned.

Usage of the method:

NSUInteger indexFound = [@[@3, @10, @18, @20] findIndexWithCondition:^BOOL(id element, NSUInteger index, NSArray *array) {
    return [element integerValue] >= 18;
}];

You have the option to simplify by not including extra parameters, as demonstrated in this JavaScript-inspired implementation:

@implementation NSArray (Extension)

- (NSUInteger)findIndexWithPredicate:(BOOL (^)(id item))predicate {
    NSUInteger foundIndex = NSNotFound;
    if (predicate != NULL) {
        for (NSUInteger i = 0; i < self.count; i++) {
            if (predicate(self[i])) {
                foundIndex = i;
                break;
            }
        }
    }
    return foundIndex;
}

@end

NSUInteger indexFound = [@[@3, @10, @18, @20] findIndexWithPredicate:^BOOL(id element) {
    return [element integerValue] >= 18;
}];

An alternative approach without creating a custom category involves utilizing the enumerateObjectsUsingBlock: method:

__block NSUInteger indexToFind;
[@[@3, @10, @18, @20] enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
    if ([obj integerValue] >= 18) {
        *stop = YES;
        indexToFind = idx;
    }
}];

Furthermore, there are options like C function pointers instead of blocks, but blocks offer more versatility.

Answer 3

Answer №2

Here is a solution I came up with that is similar to Boris's:

NSArray *numbers = @[@"3", @"10", @"18", @"20"];

BOOL(^check)(NSInteger integer) = ^BOOL(NSInteger integer) {
    if (integer >= 18) {
        return YES;
    }
    return NO;
};

NSInteger(^findIndex)(BOOL(^check)(), NSArray *array) = ^NSInteger(BOOL(^check)(), NSArray *array) {
    for (NSUInteger i = 0; i < array.count; i++) {
        NSInteger currentNumber = [array[i] intValue];
        if (check(currentNumber) == YES) {
            return i;
        }
    }

    return -1;
};

NSInteger resultIndex = findIndex(check, numbers);

Answer 4

Here is a solution I came up with that is similar to Boris's:

NSArray *numbers = @[@"3", @"10", @"18", @"20"];

BOOL(^check)(NSInteger integer) = ^BOOL(NSInteger integer) {
    if (integer >= 18) {
        return YES;
    }
    return NO;
};

NSInteger(^findIndex)(BOOL(^check)(), NSArray *array) = ^NSInteger(BOOL(^check)(), NSArray *array) {
    for (NSUInteger i = 0; i < array.count; i++) {
        NSInteger currentNumber = [array[i] intValue];
        if (check(currentNumber) == YES) {
            return i;
        }
    }

    return -1;
};

NSInteger resultIndex = findIndex(check, numbers);

Passing a method as a parameter type

Answer №1

Answer №2

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