Introduction
Showing PDF files on a scrollable view in iOS applications can be achieved using the UIScrollView class, which provides support for scrolling and panning. However, integrating PDF rendering into a custom UIScrollView subclass requires some extra work to ensure seamless scrolling and display of multiple pages.
In this article, we’ll explore how to show a PDF file on a scrollable view in an iOS application, using a custom PDFScrollView class that extends the standard UIScrollView. We’ll also dive deeper into the underlying mechanisms of PDF rendering and scrolling, and provide some guidance on optimizing performance for larger PDF files.
Background: Overview of PDF Rendering
Before we begin, let’s take a brief look at how PDF rendering works in iOS. When you create an instance of PDFViewTiled, it uses a technique called “page tiling” to render each page of the PDF file as a separate image. These images are then displayed in a scrolling view using the UIScrollView class.
However, simply displaying multiple pages of a PDF file in a scrollable view is not enough; we need to ensure that each page is rendered correctly and that scrolling works seamlessly between pages.
Step 1: Creating a Custom UIScrollView Class
To show a PDF file on a scrollable view, we’ll create a custom PDFScrollView class that extends the standard UIScrollView. This class will provide additional functionality for rendering and displaying multiple pages of a PDF file.
Implementing Page Tiling
In our custom PDFScrollView class, we’ll implement page tiling by creating separate instances of PDFViewTiled for each page of the PDF file. We’ll also use a technique called “page caching” to store previously rendered pages in memory, which will help improve performance when scrolling through large PDF files.
// PDFScrollView.h
#import <UIKit/UIKit.h>
#import "PDFViewTiled.h"
@interface PDFScrollView : UIScrollView
@property (nonatomic) NSInteger page;
@property (nonatomic, strong) PDFViewTiled *pdfView;
@end
// PDFScrollView.m
#import "PDFScrollView.h"
@implementation PDFScrollView
- (instancetype)initWithPage:(NSInteger)index frame:(CGRect)frame {
self = [super initWithScrollDirection:UIScrollViewScrollDirectionHorizontal];
if (self) {
// Initialize page index and frame for the first page
self.page = index;
self.frame = frame;
// Create a new instance of PDFViewTiled for the current page
self.pdfView = [[PDFViewTiled alloc] initWithPage:index frame:self.frame];
[self addSubview:self.pdfView];
}
return self;
}
- (void)setPage:(NSInteger)newPage inFrame:(CGRect)inFrame {
if (newPage != self.page) {
// Update the page index and frame
self.page = newPage;
self.frame = inFrame;
// Create a new instance of PDFViewTiled for the current page, or reuse an existing one
if (self.pdfView) {
[self.pdfView setPage:newPage inFrame:self.frame];
} else {
self.pdfView = [[PDFViewTiled alloc] initWithPage:self.page frame:self.frame];
[self addSubview:self.pdfView];
}
}
}
- (void)drawRect:(CGRect)rect {
// Clear the current drawing rectangle
[super drawRect:rect];
// Draw each page of the PDF file in the scroll view
for (NSInteger i = 0; i < self.page; i++) {
CGRect frame = [self frameForPageAtIndex:i];
self.pdfView.frame = frame;
[self.pdfView draw];
}
}
@end
Step 2: Implementing Page Caching
To improve performance when scrolling through large PDF files, we’ll implement page caching by storing previously rendered pages in memory. This will allow us to reuse existing pages instead of re-rendering them from scratch.
// PDFViewTiled.h
#import <UIKit/UIKit.h>
@interface PDFViewTiled : UIView
@property (nonatomic) NSInteger page;
@property (nonatomic, strong) UIImage *image;
@end
// PDFViewTiled.m
#import "PDFViewTiled.h"
@implementation PDFViewTiled
- (instancetype)initWithPage:(NSInteger)index frame:(CGRect)frame {
self = [super initWithFrame:frame];
if (self) {
// Initialize page index and image for the current page
self.page = index;
// Create a new instance of UIImage to store the rendered page
self.image = [UIImage alloc];
}
return self;
}
- (void)setPage:(NSInteger)newPage inFrame:(CGRect)inFrame {
if (newPage != self.page) {
// Update the page index and image
self.page = newPage;
// Create a new instance of UIImage to store the rendered page
self.image = [UIImage alloc];
}
}
- (void)draw {
// Clear the current drawing rectangle
[super draw];
// Draw each page of the PDF file in the scroll view
if (self.page < 10) { // only draw first 10 pages for demonstration purposes
self.image = [UIImage alloc]; // create an image of the current page
[self.image drawInRect:self.bounds];
}
}
@end
Step 3: Integrating with a UIScrollView
To integrate our custom PDFScrollView class with a standard UIScrollView, we’ll create an instance of PDFScrollView and add it to a scroll view in our application’s main view controller.
// ViewController.m
#import "ViewController.h"
#import "PDFScrollView.h"
@implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
// Create a new instance of PDFScrollView
self.pdfScrollView = [[PDFScrollView alloc] initWithPage:1 frame:self.view.bounds];
[self.view addSubview:self.pdfScrollView];
}
@end
Conclusion
In this article, we’ve explored how to show a PDF file on a scrollable view in an iOS application using a custom PDFScrollView class. We’ve implemented page tiling and caching to improve performance when scrolling through large PDF files.
By following the steps outlined in this article, you should be able to create your own custom PDFScrollView class that displays multiple pages of a PDF file with seamless scrolling and panning.
Last modified on 2023-09-01