Displaying and Searching PDF Content on iPhone
This is actually a repost… this was originally posted on my old blog, which is now defunct. Apparently, it got stuck in some search engine, or linked to from Stack Overflow or something… because I get regular (about 1-2 posts a month) asking that I put it back up. The code here isn’t really that good… it’s kinda a hack… and if you know a better way to do this, please speak up. Furthermore, this is the sum total of everything I know, or ever WANT to know about PDFs… so if your question isn’t answered here, don’t bother emailing me… I have no idea. I’m pretty sure that reading the PDF spec in some way causes your soul to be condemned to an eternity of spanking by the CEO of Adobe… or something.
PDF parsing is a black art that most programmers avoid. “Madness lurks here.” They mumble to themselves quietly. Choosing instead to push their PDFs through UIWebViews and commit other crimes against humanity.
It doesn’t have to be this way, however. Parsing, displaying, and searching PDFs natively and at a low level is actually surprisingly easy if you’re not afraid to get your hands a little dirty with the Core Graphics PDF functions. I’m going to show you how.
Where’s it hiding?
The first thing to know is that in order to do this, you need to use Core Graphics calls. So you need to include the Core Graphics framework in your project, and in any files you want to use the calls, you have to include the CoreGraphics.h header. It’s probably also worthwhile to review the Core Foundation memory management rules.
Once you’ve done this, it’s very straight forward to read your PDF files and display them in a custom view. Let’s take a look at how we do that.
Initializing a PDF Document
To initialize a PDF document, you first have to use the call CGPDFDocumentCreate, passing in the URL to the document you want to open. Since NSURL is toll free bridged to CFURLRef, you can create a CFURLRef just using plain old NSURL like so:
NSString *pathToPdfDoc = [[NSBundle mainBundle]
pathForResource:@"mypdf" ofType:@"pdf"];
NSURL *pdfUrl = [NSURL fileURLWithPath:pathToPdfDoc];
Then, to create the CGPDFDocumentRef, call CGPDFDocumentCreateWithURL:
CGPDFDocumentRef document = CGPDFDocumentCreateWithURL((CFURLRef)pdfUrl);
Displaying Pages
So now you have a document. To display the content of the document, you have to get the content in the form of pages. PDFs are already formatted by pages, so all you need to do is get at that data. Fortunately Core Graphics has functions for that too.
To get the total count of the pages in the document, you use the call CGPDFDocumentGetNumberOfPages, which takes as a parameter, the document you created above. So, for example:
size_t pageCount = CGPDFDocumentGetNumberOfPages(document);
Then, to get an individual page to display in your view, you use the function CGPDFDocumentGetPage, passing the document and the page number you want. Like so:
CGPDFPageRef page = CGPDFDocumentGetPage(document, currentPage);
Note that the currentPage parameter here is 1 based, not 0 based as is the usual case in programming. This means that the first page of the PDF document is in fact, page 1, and not page 0.
Once you have the page, you can display it in your custom view. The only complicated part here is that on iPhone, the coordinate system is flipped compared to the Mac. This causes a problem because the Core Graphics PDF system uses the desktop coordinate system even on iPhone. (It’s yucky, I know.) The solution to this is to flip the page (this can be done in your drawRect method when you go to draw the content):
CGPDFPageRef page = CGPDFDocumentGetPage(document, currentPage);
CGContextRef ctx = UIGraphicsGetCurrentContext();
CGContextSaveGState(ctx);
CGContextTranslateCTM(ctx, 0.0, [self bounds].size.height);
CGContextScaleCTM(ctx, 1.0, -1.0);
CGContextConcatCTM(ctx,
CGPDFPageGetDrawingTransform(page, kCGPDFCropBox, [self bounds], 0, true));
The key here is the call to CGContextScaleCTM. What we do, is we get the current drawing context, and then we scale it’s coordinate system on it’s y axis by -1.0. This, effectively, flips it upside down along it’s horizontal (x) axis.
Finally, we draw the page into the context using the CGContextDrawPDFPage function:
CGContextDrawPDFPage(ctx, page);
CGContextRestoreGState(ctx);
So basically, a full on drawRect method for a custom view that draws content from a PDF page, looks something like this:
-(void)drawRect:(CGRect)inRect;
{
if(document)
{
CGPDFPageRef page = CGPDFDocumentGetPage(document, currentPage);
CGContextRef ctx = UIGraphicsGetCurrentContext();
CGContextSaveGState(ctx);
CGContextTranslateCTM(ctx, 0.0, [self bounds].size.height);
CGContextScaleCTM(ctx, 1.0, -1.0);
CGContextConcatCTM(ctx,
CGPDFPageGetDrawingTransform(page, kCGPDFCropBox,
[self bounds], 0, true));
CGContextDrawPDFPage(ctx, page);
CGContextRestoreGState(ctx);
}
}
That’s all there is to it!
Searching PDFs
One of the things that seems to be particularly scary to programmers is searching PDFs. I agree that it’s certainly not pleasant stuff to code, but it’s not hard either.
Now, I want to preface this by saying that I feel this code is a bit of a hack, but it definitely works, and seems to work quite well. Perhaps there’s a better way to do this, and if you know of one, please let me know. That said, however, here’s how I’ve done it.
The first thing to know is that PDF files are made up of operators which delineate the data within them. So, for example, all text in a PDF document is stored as glyphs and prefixed by operators of type either “Tj”, in the case of a string, or “TJ” in the case of an array of strings. Knowing this, you can access the PDF data as a stream and create a scanner which will call callback methods you specify when these operators are encountered. You can then retrieve the data after the operator and use it to build your search corpus.
That probably sounds intimidating, but it’t not. You start out by creating a class that will be your “page searcher.” This will hold the state for your search engine. Here’s the listing for the interface for this class:
#import <Foundation/Foundation.h>
@interface PDFSearcher : NSObject
{
CGPDFOperatorTableRef table;
NSMutableString *currentData;
}
@property (nonatomic, retain) NSMutableString * currentData;
-(id)init;
-(BOOL)page:(CGPDFPageRef)inPage containsString:(NSString *)inSearchString;
@end
Pretty straight forward stuff. We use the currentData member to store the text of the page being scanned. This is a member variable rather than a local variable because we’re going to be using C functions to fill it in. Don’t worry, that’ll make sense in a moment.
The init method for the class actually creates the callback table:
-(id)init
{
if(self = [super init])
{
table = CGPDFOperatorTableCreate();
CGPDFOperatorTableSetCallback(table, "TJ", arrayCallback);
CGPDFOperatorTableSetCallback(table, "Tj", stringCallback);
}
return self;
}
The arrayCallback and the stringCallback functions are C functions that will be called by the scanner. They’re shown here:
void arrayCallback(CGPDFScannerRef inScanner, void *userInfo)
{
PDFSearcher * searcher = (PDFSearcher *)userInfo;
CGPDFArrayRef array;
bool success = CGPDFScannerPopArray(inScanner, &array);
for(size_t n = 0; n < CGPDFArrayGetCount(array); n += 2)
{
if(n >= CGPDFArrayGetCount(array))
continue;
CGPDFStringRef string;
success = CGPDFArrayGetString(array, n, &string);
if(success)
{
NSString *data = (NSString *)CGPDFStringCopyTextString(string);
[searcher.currentData appendFormat:@"%@", data];
[data release];
}
}
}
void stringCallback(CGPDFScannerRef inScanner, void *userInfo)
{
PDFSearcher *searcher = (PDFSearcher *)userInfo;
CGPDFStringRef string;
bool success = CGPDFScannerPopString(inScanner, &string);
if(success)
{
NSString *data = (NSString *)CGPDFStringCopyTextString(string);
[searcher.currentData appendFormat:@" %@", data];
[data release];
}
}
As you can see, these will be called when the operators fire. When they do, we pop the data off the scanner, and add it to the searcher’s corpus. The userinfo pointer is actually pointing to our searcher object (based on the fact that we will pass it as the second parameter to @CGPDFScannerCreate@ in the next code). So we can typecast it to a PDFSearcher and then access that currentData member (remember I said it would make sense later?).
The actual search method looks like this:
-(BOOL)page:(CGPDFPageRef)inPage containsString:(NSString *)inSearchString;
{
[self setCurrentData:[NSMutableString string]];
CGPDFContentStreamRef contentStream = CGPDFContentStreamCreateWithPage(inPage);
CGPDFScannerRef scanner = CGPDFScannerCreate(contentStream, table, self);
bool ret = CGPDFScannerScan(scanner);
CGPDFScannerRelease(scanner);
CGPDFContentStreamRelease(contentStream);
return ([[currentData uppercaseString]
rangeOfString:[inSearchString uppercaseString]].location != NSNotFound);
}
Basically, we create a stream from the page data, then use that and our callback table to create a scanner. We then scan the data. It’s at this point our currentData member is being filled with the data from the PDF as strings. Finally, we just search that string for our search string.
Easy peezy.
Note: much of this code is only sight compiled. I pulled it from some code I had, but it wasn’t a straight across copy, so if you find an error, please let me know.