UIWebView content sometimes not rendered - ios

Question

Briefly, my problem is: some PDFs loaded into some web views don't show up, while others do, as shown in the following pictures (please note that I edited the images placing fake pdf content, the original cannot be published).
Three PDFs loaded correctly into three UIWebViews
Second UIWebView height is right and comes from the PDF height, but its content is not rendered
My app has to show very short PDFs into a series of web views (assume that this is a mandatory requirement and that I cannot switch to anything else. The PDFs are LaTex generated and contain text which cannot be displayed into UILabel and so on. Also, they are about 4-5 rows length).
The layout is as following: there is a vertical scroll view which has a view (content view) as direct child. A single UIWebView (yellow background) is embedded inside a custom class that inherits from UIView (PDFView, blue background), so I can assign that class as the webview's delegate and perform the webview initialization+constraints settings in just one place. The webview userInteraction is disabled. Into the content view, I dynamically add one PDFView per PDF, setting up the constraints and calling loadData:MIMEType:textEncodingName:baseURL: to load the PDF (which comes as NSData from a file on disk, the white frame). Since PDFView is the delegate of its UIWebView, when it finishes to load its content (the pdf), I change the webview height constraint to match the webview.scrollview.contentsize.height. This allows me to resize the webview so that it displays the entire PDF. Autolayout takes care of setting the scrollview contentsize.height so the user can scroll the entire list of pdf.
Everything seems to work fine but sometimes, on some webviews, the content is not rendered. When the call for webViewDidFinishLoad: triggers, the webView.scrollView.contentSize.height value is correct (in fact, the webview has the right height) and after autolayout finishes his work, the frame of the webview whose content has not been rendered has the correct dimensions. This leads me to think that it must be some sort of rendering problem, since it seems to happen randomly on different webviews and on different pdfs. Sometimes I have to manually push and pop the controller like 10 times before the problem arises, sometimes it appears just on the first push. Memory consumption seems to be fine and there aren't any memory warnings.
I checked every data and view: no one is nil. Also the delegate webView:didFailLoadWithError: is never invoked so the webview always succeeds in loading its content (webViewDidFinishLoad: is called every time, even for the webviews that don't display any content). The PDF data is fine since I loaded it into memory, saved again in another pdf file and opened with a PDF viewer. I'm not using storyboards but xibs. I'm targeting iOS 7+.
Code is as following:
PDFView.h
#interface PDFView : UIView
-(instancetype) initWithData:(NSData *)pdfData at:(NSInteger)position;
#end
PDFView.m
#import "PDFView.h"
#interface PDFView() <UIWebViewDelegate>
#property (strong, nonatomic) UIWebView * contentUIWV;
#property (strong, nonatomic) NSLayoutConstraint * contentHeightNSLC;
#end
#implementation PDFView
-(instancetype) initWithData:(NSData *)pdfData at:(NSInteger)position
{
self = [super init];
if (self)
{
[self initViews];
[self setConstraints];
[self fillViews:pdfData];
self.tag = position;
}
return self;
}
-(void) initViews
{
_contentUIWV = [[UIWebView alloc] init];
[_contentUIWV setContentHuggingPriority:250 forAxis:UILayoutConstraintAxisVertical];
_contentUIWV.userInteractionEnabled = NO;
_contentUIWV.delegate = self;
_contentUIWV.scalesPageToFit = YES;
_contentUIWV.backgroundColor = [UIColor yellowColor];
[self addSubview:_contentUIWV];
}
-(void) setConstraints
{
_contentUIWV.translatesAutoresizingMaskIntoConstraints = NO;
NSDictionary * viewsNSD = #{ #"contentUIWV":_contentUIWV };
[self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:#"H:|-8-[contentUIWV]-8-|" options:0 metrics:nil views:viewsNSD]];
[self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:#"V:|-8-[contentUIWV]-8-|" options:0 metrics:nil views:viewsNSD]];
NSArray * constraintsNSA = [NSLayoutConstraint constraintsWithVisualFormat:#"V:[contentUIWV(==1)]" options:0 metrics:nil views:viewsNSD];
self.contentHeightNSLC = constraintsNSA.firstObject;
[self.contentUIWV addConstraints:constraintsNSA];
}
-(void) fillViews:(NSData *)pdfData
{
#ifdef DEBUG
NSLog(#"PDFView::fillViews: %ld %#", (long)self.tag, pdfData?#"NOT null":#"NULL");
#endif
[_contentUIWV loadData:pdfData MIMEType:#"application/pdf" textEncodingName:#"utf-8" baseURL:nil];
}
#pragma mark - UIWebViewDelegate
- (BOOL)webView:(UIWebView *)webView shouldStartLoadWithRequest:(NSURLRequest *)request navigationType:(UIWebViewNavigationType)navigationType {
return YES;
}
-(void) webView:(UIWebView *)webView didFailLoadWithError:(NSError *)error
{
#ifdef DEBUG
NSLog(#"PDFView::webView:didFailLoadWithError: %#", error.localizedDescription);
#endif
}
-(void) webViewDidFinishLoad:(UIWebView *)webView
{
#ifdef DEBUG
NSLog(#"PDFView::webViewDidFinishLoad: %ld new height (%ld)", (long)self.tag, (long)webView.scrollView.contentSize.height);
#endif
self.contentHeightNSLC.constant = webView.scrollView.contentSize.height;
}
#end
Controller.m
#import "Controller.h"
#import "PDFView.h"
#interface Controller ()
// This Mutable Array stores the NSData of each PDF (PDF are loaded form disk and stored as NSData)
#property (strong, nonatomic) NSMutableArray * exerciseDataNSMA;
#property (weak, nonatomic) IBOutlet UIView *mainUIV;
#property (weak, nonatomic) IBOutlet UIScrollView *scrollUISV;
#property (weak, nonatomic) IBOutlet UIView *contentUIV;
#end
#implementation Controller
- (void)viewDidLoad
{
[super viewDidLoad];
// ... omitted code inits self.exerciseDataNSMA and stores pdf content in it
[self loadContentView];
}
#pragma mark - Private Methods
-(void) loadContentView
{
// Subviews by name (used for constraint bindings)
NSMutableDictionary * subviews = [[NSMutableDictionary alloc] init];
// String in Visual Language Format to create vertical scrollview's contentview constraint
NSMutableString *strVerticalConstraint = [[NSMutableString alloc] initWithString:#"V:|"];
for (NSInteger i=0; i<self.exerciseDataNSMA.count; i++)
{
PDFView * pdfView = [[PDFView alloc] initWithData:self.exerciseDataNSMA[i] at:i];
pdfView.delegate = self;
pdfView.backgroundColor = [UIColor blueColor];
pdfView.translatesAutoresizingMaskIntoConstraints = NO;
[self.contentUIV addSubview:pdfView];
[self.contentUIV addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:#"H:|[pdfView]|" options:0 metrics:nil views:#{#"pdfView":pdfView}]];
NSString * exerciseUIVName = [NSString stringWithFormat:#"pdfView%ld", (long)i];
[strVerticalConstraint appendString:[NSString stringWithFormat:#"[%#]-8-", exerciseUIVName]];
subviews[exerciseUIVName] = pdfView;
}
[strVerticalConstraint appendString:#"|"];
// Add content view constraints
[self.contentUIV addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:strVerticalConstraint options:0 metrics:nil views:subviews]];
}
#end
AppDelegate.m
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
// Without the cache disabled, the webviews do not resize their height
[[NSUserDefaults standardUserDefaults] setBool:NO forKey:#"WebKitDiskImageCacheEnabled"];
[[NSUserDefaults standardUserDefaults] synchronize];
// ... unrelated stuff
return YES;
}
Constraints on Controller are as following:
Note that the height constraint is removed at runtime.
Any idea why this happens? Any idea on how to solve it?
Here what I already tried:
Reproduced the problem on both iOS 7 and iOS8, on iPhone and on iPad. The number of webviews whose content is not rendered is greater on low specs devices (like iPhone 4s) than those with better resources (as the iPad 4);
Executing loadContentView without loading pdfData into the webview. After the controller has appeared, load the pdfData into the webviews;
Load the pdfdata sequentially, meaning that i do the same as B) and then I call loadData:MIMEType:textEncodingName:baseURL: for the first webview, wait for webViewDidFinishLoad: and then repeat this sequence for the second webview and so on;
Force every webview to reload its content (since I don't have a way to detect a webview that doesn't render its content. This results in some webviews rendering their content again, some others not);
Use javascript to resize the webview (it didn't work at all);