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This is a guard that tracks down UIKit access on threads other than main. This snippet is taken from the commercial iOS PDF framework, but relicensed under MIT. Works because a lot of calls internally call setNeedsDisplay or setNeedsLayout. Won't catch everything, but it's very lightweight and usually does the job.You might n…
// Taken from the commercial iOS PDF framework
// Copyright (c) 2014 Peter Steinberger, PSPDFKit GmbH. All rights reserved.
// Licensed under MIT (
// You should only use this in debug builds. It doesn't use private API, but I wouldn't ship it.
#import <objc/runtime.h>
#import <objc/message.h>
#import <UIKit/UIKit.h>
// Compile-time selector checks.
#define PROPERTY(propName) NSStringFromSelector(@selector(propName))
#define PROPERTY(propName) @#propName
#define PSPDFAssert(expression, ...) \
do { if(!(expression)) { \
NSLog(@"%@", [NSString stringWithFormat: @"Assertion failure: %s in %s on line %s:%d. %@", #expression, __PRETTY_FUNCTION__, __FILE__, __LINE__, [NSString stringWithFormat:@"" __VA_ARGS__]]); \
abort(); }} while(0)
BOOL PSPDFReplaceMethodWithBlock(Class c, SEL origSEL, SEL newSEL, id block) {
PSPDFAssert(c && origSEL && newSEL && block);
if ([c instancesRespondToSelector:newSEL]) return YES; // Selector already implemented, skip silently.
Method origMethod = class_getInstanceMethod(c, origSEL);
// Add the new method.
IMP impl = imp_implementationWithBlock(block);
if (!class_addMethod(c, newSEL, impl, method_getTypeEncoding(origMethod))) {
// PSPDFLogError(@"Failed to add method: %@ on %@", NSStringFromSelector(newSEL), c);
return NO;
}else {
Method newMethod = class_getInstanceMethod(c, newSEL);
// If original doesn't implement the method we want to swizzle, create it.
if (class_addMethod(c, origSEL, method_getImplementation(newMethod), method_getTypeEncoding(origMethod))) {
class_replaceMethod(c, newSEL, method_getImplementation(origMethod), method_getTypeEncoding(newMethod));
}else {
method_exchangeImplementations(origMethod, newMethod);
return YES;
SEL _PSPDFPrefixedSelector(SEL selector) {
return NSSelectorFromString([NSString stringWithFormat:@"pspdf_%@", NSStringFromSelector(selector)]);
//#define PSPDFAssert(expression, ...) \
//do { if(!(expression)) { \
//NSLog(@"%@", [NSString stringWithFormat: @"Assertion failure: %s in %s on line %s:%d. %@", #expression, __PRETTY_FUNCTION__, __FILE__, __LINE__, [NSString stringWithFormat:@"" __VA_ARGS__]]); \
//abort(); }} while(0)
void PSPDFAssertIfNotMainThread(void) {
PSPDFAssert(NSThread.isMainThread, @"\nERROR: All calls to UIKit need to happen on the main thread. You have a bug in your code. Use dispatch_async(dispatch_get_main_queue(), ^{ ... }); if you're unsure what thread you're in.\n\nBreak on PSPDFAssertIfNotMainThread to find out where.\n\nStacktrace: %@", NSThread.callStackSymbols);
__attribute__((constructor)) static void PSPDFUIKitMainThreadGuard(void) {
@autoreleasepool {
for (NSString *selStr in @[PROPERTY(setNeedsLayout), PROPERTY(setNeedsDisplay), PROPERTY(setNeedsDisplayInRect:)]) {
SEL selector = NSSelectorFromString(selStr);
SEL newSelector = NSSelectorFromString([NSString stringWithFormat:@"pspdf_%@", selStr]);
if ([selStr hasSuffix:@":"]) {
PSPDFReplaceMethodWithBlock(UIView.class, selector, newSelector, ^(__unsafe_unretained UIView *_self, CGRect r) {
// Check for window, since *some* UIKit methods are indeed thread safe.
Drawing to a graphics context in UIKit is now thread-safe. Specifically:
The routines used to access and manipulate the graphics context can now correctly handle contexts residing on different threads.
String and image drawing is now thread-safe.
Using color and font objects in multiple threads is now safe to do.
if (_self.window) PSPDFAssertIfNotMainThread();
((void ( *)(id, SEL, CGRect))objc_msgSend)(_self, newSelector, r);
}else {
PSPDFReplaceMethodWithBlock(UIView.class, selector, newSelector, ^(__unsafe_unretained UIView *_self) {
if (_self.window) {
if (!NSThread.isMainThread) {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wdeprecated-declarations"
dispatch_queue_t queue = dispatch_get_current_queue();
#pragma clang diagnostic pop
// iOS 8 layouts the MFMailComposeController in a background thread on an UIKit queue.
if (!queue || !strstr(dispatch_queue_get_label(queue), "UIKit")) {
((void ( *)(id, SEL))objc_msgSend)(_self, newSelector);

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commented Sep 15, 2016

I am successfully able to run this code in one of my sample demo app. But when I use the same way in my actual project then it does not work. I have added a UILabel and trying to set the text on background thread but its not complaining. Am i missing something in Build Settings ? (NOTE: PSPDFUIKitMainThreadGuard.m already added to Compile Source)

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