Memory Pressure Crash When Generating PDF - ios

Question

My app takes in the contents of a UIWebView and generates a PDF of the web page. This works fine on smaller pages but when it reaches about 10 pages it crashes "Due to Memory Pressure". Also, this is an ARC app.
The predominant answer I have seen is to use UIGraphicsBeginPDFContextToFile instead of UIGraphicsBeginPDFContextToData and after changing to use File I still get a Memory Pressure crash. I don't understand why it's not clearing the pages from memory. I also added the #autoreleasepool { ... } in the loop as recommended in another question. Any ideas on what I am doing wrong here?
Here's the PDF creation code:
UIGraphicsBeginPDFContextToFile(dataFile, CGRectZero, nil);
for (int i = 0; i < pages; i++) {
#autoreleasepool {
NSLog(#"Creating Page %i", i);
// Check to see if page draws more than the height of the UIWebView
if ((i+1) * 720 > height) {
CGRect f = [_appWebView frame];
f.size.height -= (((i+1) * 720.0) - height);
[_appWebView setFrame: f];
}
UIGraphicsBeginPDFPage();
CGContextRef currentContext = UIGraphicsGetCurrentContext();
CGContextTranslateCTM(currentContext, 36, 36); // Translate for 0.5" margins
[[[_appWebView subviews] lastObject] setContentOffset:CGPointMake(0, 720 * i) animated:NO];
[_appWebView.layer renderInContext:currentContext];
}
}
UIGraphicsEndPDFContext();
Here's the full method if it helps any:
-(void) generatePDF {
startingFrame = _appWebView.frame;
// Memory warning seems to happen on almost every PDF, clear cache here to be proactive.
[[NSURLCache sharedURLCache] removeAllCachedResponses];
UIWebView *webView = [[UIWebView alloc] initWithFrame: CGRectMake(0, 0, 6.5 * 72, 9 * 72)];
[webView setDelegate: self];
// Adjust to letter size paper size in portrait mode
CGRect frame = _appWebView.frame;
frame.size.height = 10*72; // 11" - 1" Margins = 720px (72px / inch)
frame.size.width = 7.5*72; // 8.5 - 1" Margins = 612px (72px / inch)
_appWebView.frame = frame;
[_appWebView stringByEvaluatingJavaScriptFromString:#"window.scroll(0, 0);"];
// Get the height of our webView
NSString *heightStr = [_appWebView stringByEvaluatingJavaScriptFromString:#"document.body.scrollHeight;"];
int height = [heightStr intValue];
// Get the number of pages needed to print. 10 * 72 = 720
int pages = ceil(height / 720.0);
// File
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *dataFile = [documentsDirectory stringByAppendingPathComponent:#"Configuration.pdf"];
NSLog(#"File: %#", dataFile);
UIGraphicsBeginPDFContextToFile(dataFile, CGRectZero, nil);
for (int i = 0; i < pages; i++) {
#autoreleasepool {
NSLog(#"Creating Page %i", i);
// Check to see if page draws more than the height of the UIWebView
if ((i+1) * 720 > height) {
CGRect f = [_appWebView frame];
f.size.height -= (((i+1) * 720.0) - height);
[_appWebView setFrame: f];
}
UIGraphicsBeginPDFPage();
CGContextRef currentContext = UIGraphicsGetCurrentContext();
CGContextTranslateCTM(currentContext, 36, 36); // Translate for 0.5" margins
[[[_appWebView subviews] lastObject] setContentOffset:CGPointMake(0, 720 * i) animated:NO];
[_appWebView.layer renderInContext:currentContext];
}
}
UIGraphicsEndPDFContext();
// Adjust to original size
_appWebView.frame = startingFrame;
}

Answer 1

I don't know of a reason you are getting the memory error, but I had a similar issue with searching of PDF's. Basically the OS was loading the entire PDF into memory, then searching, then removing the pdf even thought I was going page by page. My solution was to only do one page at a time, and that resolved the memory issue for me.
My code looks like this:
NSOperationQueue *operationQueue = [[NSOperationQueue alloc] init];
operationQueue.MaxConcurrentOperationCount = 1;
for(int i = 1; i <= totalPages; i++)
{
// a block of operation
[operationQueue addOperationWithBlock: ^ {
}];
}