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//Vamos a observar la propiedad cartUnits del objeto _product
[_product addObserver:self forKeyPath:@"cartUnits" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if([keyPath isEqualToString:@"cartUnits"])
    {
        //Acción
    }
}

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CGSize expectedLabelSize = [yourString sizeWithFont:yourLabel.font 
                        constrainedToSize:maximumLabelSize 
                        lineBreakMode:yourLabel.lineBreakMode]; 

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ProductView *v = [[[NSBundle mainBundle] loadNibNamed:@"ProductView" owner:self options:nil] objectAtIndex:0];

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    [UIView animateWithDuration:DURATION
                          delay:0.0
                        options: UIViewAnimationCurveEaseOut
                     animations:^{


                     } 
                     completion:^(BOOL finished){
                         
                     }];

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//Assuming the view controller's size is WIDTHxHEIGHT

//Calling the view Controller:
    CustomViewController *vc = [[CustomViewController alloc] init];
    vc.modalPresentationStyle = UIModalPresentationFormSheet;
    [self presentModalViewController:vc animated:YES];

//In the view controller:
-(void)viewWillAppear:(BOOL)animated
{

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//
//  UIImage-Extensions.h
//
//  Created by Hardy Macia on 7/1/09.
//  Copyright 2009 Catamount Software. All rights reserved.
//
#import <Foundation/Foundation.h>

@interface UIImage (CS_Extensions)
- (UIImage *)imageAtRect:(CGRect)rect;

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//
//  UIImage-Extensions.m
//
//  Created by Hardy Macia on 7/1/09.
//  Copyright 2009 Catamount Software. All rights reserved.
//

#import "UIImage-Extensions.h"

CGFloat DegreesToRadians(CGFloat degrees) {return degrees * M_PI / 180;};

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static inline double radians (double degrees) {return degrees * M_PI/180;}

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Formula to find the width of UILabel with the text length

#define MY_LABEL_FONT_SIZE 18
#define MY_LABEL_TEXT_WIDTH_MULTIPLIER(MY_LABEL_FONT_SIZE/2 + 1)

For example if we have a String of length 8 characters then we can find the width of the UILabel which can fit the text as :
 
 8* MY_LABEL_TEXT_WIDTH_MULTIPLIER
and setting the Font Size of the UILabel text as  MY_LABEL_FONT_SIZE 

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- (UITableViewCell*) tableView:(UITableView*)tableView cellForRowAtIndexPath:(NSIndexPath*)indexPath
{
    static NSString* customCellIdentifier = @"CustomCellIdentifier";
    CustomCell* cell = (CustomCell *) [tableView dequeueReusableCellWithIdentifier:customCellIdentifier];

    if (cell == nil)// tableview not associated (registered) with nib file.
    {
        UINib* customCellNib = [UINib nibWithNibName:@"CustomCell" bundle:nil]; // Register this nib file with cell identifier.
        [tableView registerNib: customCellNib forCellReuseIdentifier:customCellIdentifier];
    }