ScroolView Zoom

gistfile1.m

//
//  ZoomView.m
//  ZoomView
//
//  Created by Carl Jahn on 15.08.13.
//  Copyright (c) 2013 Carl Jahn. All rights reserved.
//

#import "ZoomView.h"

@interface ZoomView ()<UIScrollViewDelegate>

@property (nonatomic, strong) UIImageView *imageView;
@property (nonatomic, strong) UIScrollView *scrollView;
@end

@implementation ZoomView

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
      
      [self generateGUI];
      
    }
    return self;
}

- (void)generateGUI {

  self.scrollView = [[UIScrollView alloc] initWithFrame: self.bounds];
  self.scrollView.delegate = self;
  self.scrollView.showsHorizontalScrollIndicator = NO;
  self.scrollView.showsVerticalScrollIndicator = NO;
  self.scrollView.maximumZoomScale = 4.0;

  [self addSubview: self.scrollView];
  
  self.imageView = [[UIImageView alloc] initWithFrame: self.bounds];
  [self.scrollView addSubview: self.imageView];
}

- (void)goForIt {
  
  dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
  dispatch_async(queue, ^{
    
    NSURL *url = [NSURL URLWithString:@"http://lorempixel.com/1000/500/"];
    NSData *data = [NSData dataWithContentsOfURL: url];

    UIImage *image = [UIImage imageWithData: data scale:2.0f];
    
    dispatch_sync(dispatch_get_main_queue(), ^{

      
      self.imageView.image = image;
      self.imageView.bounds = CGRectMake(0, 0, image.size.width, image.size.height);
      
      CGFloat minimumZoomScale = self.scrollView.frame.size.width / image.size.width;
      CGFloat tmpZoomScale = self.scrollView.frame.size.height / image.size.height;
      
      if (tmpZoomScale < minimumZoomScale) {
        minimumZoomScale = tmpZoomScale;
      }

      self.scrollView.contentSize = self.imageView.image.size;
      self.scrollView.minimumZoomScale = minimumZoomScale;
      self.scrollView.zoomScale = minimumZoomScale;
      
    });
    
  });

}

- (CGRect)centeredFrameForScrollView:(UIScrollView *)scroll andUIView:(UIView *)rView {
  
  CGSize boundsSize = scroll.bounds.size;
  CGRect frameToCenter = rView.frame;
  frameToCenter.origin = CGPointZero;

  // center horizontally
  if (frameToCenter.size.width < boundsSize.width) {
    frameToCenter.origin.x = (boundsSize.width - frameToCenter.size.width) / 2;
  }

  // center vertically
  if (frameToCenter.size.height < boundsSize.height) {
    frameToCenter.origin.y = (boundsSize.height - frameToCenter.size.height) / 2;
  }
  
  return frameToCenter;
}

- (void)scrollViewDidZoom:(UIScrollView *)scrollView {
  
  self.imageView.frame = [self centeredFrameForScrollView:scrollView andUIView:self.imageView];
}

- (UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView {
  return self.imageView;
}

@end