maicoder · March 20, 2017 08:16 · Jul 6, 2016 · Jun 10, 2016 · Sep 16, 2014 · Sep 16, 2014
diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -4,6 +4,8 @@
 //
 // You should only use this in debug builds. It doesn't use private API, but I wouldn't ship it.
 
+// PLEASE DUPE rdar://27192338 (https://openradar.appspot.com/27192338) if you would like to see this in UIKit.
+
 #import <objc/runtime.h>
 #import <objc/message.h>
 

diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -16,7 +16,11 @@
 
 // http://www.mikeash.com/pyblog/friday-qa-2010-01-29-method-replacement-for-fun-and-profit.html
 BOOL PSPDFReplaceMethodWithBlock(Class c, SEL origSEL, SEL newSEL, id block) {
-    PSPDFAssert(c && origSEL && newSEL && block);
+    NSCParameterAssert(c);
+    NSCParameterAssert(origSEL);
+    NSCParameterAssert(newSEL);
+    NSCParameterAssert(block);
+
     if ([c instancesRespondToSelector:newSEL]) return YES; // Selector already implemented, skip silently.
 
     Method origMethod = class_getInstanceMethod(c, origSEL);

diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -1,5 +1,5 @@
 // Taken from the commercial iOS PDF framework http://pspdfkit.com.
-// Copyright (c) 2013 Peter Steinberger. All rights reserved.
+// Copyright (c) 2014 Peter Steinberger, PSPDFKit GmbH. All rights reserved.
 // Licensed under MIT (http://opensource.org/licenses/MIT)
 //
 // You should only use this in debug builds. It doesn't use private API, but I wouldn't ship it.

diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -14,6 +14,44 @@
 #define PROPERTY(propName) @#propName
 #endif
 
+// http://www.mikeash.com/pyblog/friday-qa-2010-01-29-method-replacement-for-fun-and-profit.html
+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:)]) {

diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -14,70 +14,45 @@
 #define PROPERTY(propName) @#propName
 #endif
 
-// A better assert. NSAssert is too runtime dependant, and assert() doesn't log.
-// http://www.mikeash.com/pyblog/friday-qa-2013-05-03-proper-use-of-asserts.html
-// Accepts both:
-// - PSPDFAssert(x > 0);
-// - PSPDFAssert(y > 3, @"Bad value for y");
-#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)
-
-///////////////////////////////////////////////////////////////////////////////////////////
-#pragma mark - Helper for Swizzling
-
-BOOL PSPDFReplaceMethodWithBlock(Class c, SEL origSEL, SEL newSEL, id block) {
-    PSPDFAssert(c && origSEL && newSEL && block);
-    Method origMethod = class_getInstanceMethod(c, origSEL);
-    const char *encoding = method_getTypeEncoding(origMethod);
-
-    // Add the new method.
-    IMP impl = imp_implementationWithBlock(block);
-    if (!class_addMethod(c, newSEL, impl, encoding)) {
-        NSLog(@"Failed to add method: %@ on %@", NSStringFromSelector(newSEL), c);
-        return NO;
-    }else {
-        // Ensure the new selector has the same parameters as the existing selector.
-        Method newMethod = class_getInstanceMethod(c, newSEL);
-        PSPDFAssert(strcmp(method_getTypeEncoding(origMethod), method_getTypeEncoding(newMethod)) == 0, @"Encoding must be the same.");
-
-        // If original doesn't implement the method we want to swizzle, create it.
-        if (class_addMethod(c, origSEL, method_getImplementation(newMethod), encoding)) {
-            class_replaceMethod(c, newSEL, method_getImplementation(origMethod), encoding);
-        }else {
-            method_exchangeImplementations(origMethod, newMethod);
-        }
-    }
-    return YES;
-}
-
-///////////////////////////////////////////////////////////////////////////////////////////
-#pragma mark - Tracks down calls to UIKit from a Thread other than Main
-
-static 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]);
-}
-
-// This installs a small guard that checks for the most common threading-errors in UIKit.
-// This won't really slow down performance but still only is compiled in DEBUG versions of PSPDFKit.
-// @note No private API is used here.
 __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) {
-                    PSPDFAssertIfNotMainThread();
+                    // Check for window, since *some* UIKit methods are indeed thread safe.
+                    // https://developer.apple.com/library/ios/#releasenotes/General/WhatsNewIniPhoneOS/Articles/iPhoneOS4.html
+                    /*
+                     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) {
-                    PSPDFAssertIfNotMainThread();
+                    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.
+                            // https://github.com/PSPDFKit/PSPDFKit/issues/1423
+                            if (!queue || !strstr(dispatch_queue_get_label(queue), "UIKit")) {
+                                PSPDFAssertIfNotMainThread();
+                            }
+                        }
+                    }
                     ((void ( *)(id, SEL))objc_msgSend)(_self, newSelector);
                 });
             }
         }
     }
-}
+}
diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -6,7 +6,6 @@
 
 #import <objc/runtime.h>
 #import <objc/message.h>
-#import <QuartzCore/QuartzCore.h>
 
 // Compile-time selector checks.
 #if DEBUG

diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -8,6 +8,7 @@
 #import <objc/message.h>
 #import <QuartzCore/QuartzCore.h>
 
+// Compile-time selector checks.
 #if DEBUG
 #define PROPERTY(propName) NSStringFromSelector(@selector(propName))
 #else

diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -24,6 +24,9 @@
 NSLog(@"%@", [NSString stringWithFormat: @"Assertion failure: %s in %s on line %s:%d. %@", #expression, __PRETTY_FUNCTION__, __FILE__, __LINE__, [NSString stringWithFormat:@"" __VA_ARGS__]]); \
 abort(); }} while(0)
 
+///////////////////////////////////////////////////////////////////////////////////////////
+#pragma mark - Helper for Swizzling
+
 BOOL PSPDFReplaceMethodWithBlock(Class c, SEL origSEL, SEL newSEL, id block) {
     PSPDFAssert(c && origSEL && newSEL && block);
     Method origMethod = class_getInstanceMethod(c, origSEL);

diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -1,6 +1,6 @@
 // Taken from the commercial iOS PDF framework http://pspdfkit.com.
 // Copyright (c) 2013 Peter Steinberger. All rights reserved.
-// Licensed under MIT.
+// Licensed under MIT (http://opensource.org/licenses/MIT)
 //
 // You should only use this in debug builds. It doesn't use private API, but I wouldn't ship it.
 

diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -25,7 +25,7 @@
 abort(); }} while(0)
 
 BOOL PSPDFReplaceMethodWithBlock(Class c, SEL origSEL, SEL newSEL, id block) {
-    NSCParameterAssert(c && origSEL && newSEL && block);
+    PSPDFAssert(c && origSEL && newSEL && block);
     Method origMethod = class_getInstanceMethod(c, origSEL);
     const char *encoding = method_getTypeEncoding(origMethod);
 
@@ -37,7 +37,7 @@ BOOL PSPDFReplaceMethodWithBlock(Class c, SEL origSEL, SEL newSEL, id block) {
     }else {
         // Ensure the new selector has the same parameters as the existing selector.
         Method newMethod = class_getInstanceMethod(c, newSEL);
-        NSCAssert(strcmp(method_getTypeEncoding(origMethod), method_getTypeEncoding(newMethod)) == 0, @"Encoding must be the same.");
+        PSPDFAssert(strcmp(method_getTypeEncoding(origMethod), method_getTypeEncoding(newMethod)) == 0, @"Encoding must be the same.");
 
         // If original doesn't implement the method we want to swizzle, create it.
         if (class_addMethod(c, origSEL, method_getImplementation(newMethod), encoding)) {

diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -1,12 +1,18 @@
 // Taken from the commercial iOS PDF framework http://pspdfkit.com.
 // Copyright (c) 2013 Peter Steinberger. 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 <QuartzCore/QuartzCore.h>
 
+#if DEBUG
 #define PROPERTY(propName) NSStringFromSelector(@selector(propName))
+#else
+#define PROPERTY(propName) @#propName
+#endif
 
 // A better assert. NSAssert is too runtime dependant, and assert() doesn't log.
 // http://www.mikeash.com/pyblog/friday-qa-2013-05-03-proper-use-of-asserts.html
@@ -18,9 +24,6 @@
 NSLog(@"%@", [NSString stringWithFormat: @"Assertion failure: %s in %s on line %s:%d. %@", #expression, __PRETTY_FUNCTION__, __FILE__, __LINE__, [NSString stringWithFormat:@"" __VA_ARGS__]]); \
 abort(); }} while(0)
 
-// You should only use this in debug builds. It doesn't use private API, but I wouldn't ship it.
-#ifdef DEBUG
-
 BOOL PSPDFReplaceMethodWithBlock(Class c, SEL origSEL, SEL newSEL, id block) {
     NSCParameterAssert(c && origSEL && newSEL && block);
     Method origMethod = class_getInstanceMethod(c, origSEL);
@@ -74,6 +77,4 @@ static void PSPDFAssertIfNotMainThread(void) {
             }
         }
     }
-}
-
-#endif
+}
diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -1,3 +1,7 @@
+// Taken from the commercial iOS PDF framework http://pspdfkit.com.
+// Copyright (c) 2013 Peter Steinberger. All rights reserved.
+// Licensed under MIT.
+
 #import <objc/runtime.h>
 #import <objc/message.h>
 #import <QuartzCore/QuartzCore.h>

diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -58,12 +58,12 @@ static void PSPDFAssertIfNotMainThread(void) {
             SEL selector = NSSelectorFromString(selStr);
             SEL newSelector = NSSelectorFromString([NSString stringWithFormat:@"pspdf_%@", selStr]);
             if ([selStr hasSuffix:@":"]) {
-                PSPDFReplaceMethodWithBlock(UIView.class, selector, newSelector, ^(UIView *_self, CGRect r) {
+                PSPDFReplaceMethodWithBlock(UIView.class, selector, newSelector, ^(__unsafe_unretained UIView *_self, CGRect r) {
                     PSPDFAssertIfNotMainThread();
                     ((void ( *)(id, SEL, CGRect))objc_msgSend)(_self, newSelector, r);
                 });
             }else {
-                PSPDFReplaceMethodWithBlock(UIView.class, selector, newSelector, ^(UIView *_self) {
+                PSPDFReplaceMethodWithBlock(UIView.class, selector, newSelector, ^(__unsafe_unretained UIView *_self) {
                     PSPDFAssertIfNotMainThread();
                     ((void ( *)(id, SEL))objc_msgSend)(_self, newSelector);
                 });

diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -17,21 +17,29 @@
 // You should only use this in debug builds. It doesn't use private API, but I wouldn't ship it.
 #ifdef DEBUG
 
-static void PSPDFSwizzleMethod(Class c, SEL orig, SEL new) {
-    Method origMethod = class_getInstanceMethod(c, orig);
-    Method newMethod = class_getInstanceMethod(c, new);
-    if (class_addMethod(c, orig, method_getImplementation(newMethod), method_getTypeEncoding(newMethod))) {
-        class_replaceMethod(c, new, method_getImplementation(origMethod), method_getTypeEncoding(origMethod));
+BOOL PSPDFReplaceMethodWithBlock(Class c, SEL origSEL, SEL newSEL, id block) {
+    NSCParameterAssert(c && origSEL && newSEL && block);
+    Method origMethod = class_getInstanceMethod(c, origSEL);
+    const char *encoding = method_getTypeEncoding(origMethod);
+
+    // Add the new method.
+    IMP impl = imp_implementationWithBlock(block);
+    if (!class_addMethod(c, newSEL, impl, encoding)) {
+        NSLog(@"Failed to add method: %@ on %@", NSStringFromSelector(newSEL), c);
+        return NO;
     }else {
-        method_exchangeImplementations(origMethod, newMethod);
-    }
-}
+        // Ensure the new selector has the same parameters as the existing selector.
+        Method newMethod = class_getInstanceMethod(c, newSEL);
+        NSCAssert(strcmp(method_getTypeEncoding(origMethod), method_getTypeEncoding(newMethod)) == 0, @"Encoding must be the same.");
 
-void PSPDFReplaceMethod(Class c, SEL orig, SEL newSel, IMP impl) {
-    Method method = class_getInstanceMethod(c, orig);
-    if (!class_addMethod(c, newSel, impl, method_getTypeEncoding(method))) {
-        NSLog(@"Failed to add method: %@ on %@", NSStringFromSelector(newSel), c);
-    }else PSPDFSwizzleMethod(c, orig, newSel);
+        // If original doesn't implement the method we want to swizzle, create it.
+        if (class_addMethod(c, origSEL, method_getImplementation(newMethod), encoding)) {
+            class_replaceMethod(c, newSEL, method_getImplementation(origMethod), encoding);
+        }else {
+            method_exchangeImplementations(origMethod, newMethod);
+        }
+    }
+    return YES;
 }
 
 ///////////////////////////////////////////////////////////////////////////////////////////
@@ -46,18 +54,19 @@ static void PSPDFAssertIfNotMainThread(void) {
 // @note No private API is used here.
 __attribute__((constructor)) static void PSPDFUIKitMainThreadGuard(void) {
     @autoreleasepool {
-        for (NSString *selector in @[PROPERTY(setNeedsLayout), PROPERTY(setNeedsDisplay), PROPERTY(setNeedsDisplayInRect:)]) {
-            SEL newSelector = NSSelectorFromString([NSString stringWithFormat:@"pspdf_%@", selector]);
-            if ([selector hasSuffix:@":"]) {
-                PSPDFReplaceMethod(UIView.class, NSSelectorFromString(selector), newSelector, imp_implementationWithBlock(^(UIView *_self, CGRect r) {
+        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, ^(UIView *_self, CGRect r) {
                     PSPDFAssertIfNotMainThread();
                     ((void ( *)(id, SEL, CGRect))objc_msgSend)(_self, newSelector, r);
-                }));
+                });
             }else {
-                PSPDFReplaceMethod(UIView.class, NSSelectorFromString(selector), newSelector, imp_implementationWithBlock(^(UIView *_self) {
+                PSPDFReplaceMethodWithBlock(UIView.class, selector, newSelector, ^(UIView *_self) {
                     PSPDFAssertIfNotMainThread();
                     ((void ( *)(id, SEL))objc_msgSend)(_self, newSelector);
-                }));
+                });
             }
         }
     }

diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -30,15 +30,15 @@ static void PSPDFSwizzleMethod(Class c, SEL orig, SEL new) {
 void PSPDFReplaceMethod(Class c, SEL orig, SEL newSel, IMP impl) {
     Method method = class_getInstanceMethod(c, orig);
     if (!class_addMethod(c, newSel, impl, method_getTypeEncoding(method))) {
-        PSPDFLogError(@"Failed to add method: %@ on %@", NSStringFromSelector(newSel), c);
+        NSLog(@"Failed to add method: %@ on %@", NSStringFromSelector(newSel), c);
     }else PSPDFSwizzleMethod(c, orig, newSel);
 }
 
 ///////////////////////////////////////////////////////////////////////////////////////////
 #pragma mark - Tracks down calls to UIKit from a Thread other than Main
 
 static 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]);
+    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]);
 }
 
 // This installs a small guard that checks for the most common threading-errors in UIKit.

diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -48,18 +48,16 @@ static void PSPDFAssertIfNotMainThread(void) {
     @autoreleasepool {
         for (NSString *selector in @[PROPERTY(setNeedsLayout), PROPERTY(setNeedsDisplay), PROPERTY(setNeedsDisplayInRect:)]) {
             SEL newSelector = NSSelectorFromString([NSString stringWithFormat:@"pspdf_%@", selector]);
-            for (Class theClass in @[UIView.class, CALayer.class]) {
-                if ([selector hasSuffix:@":"]) {
-                    PSPDFReplaceMethod(theClass, NSSelectorFromString(selector), newSelector, imp_implementationWithBlock(^(UIView *_self, CGRect r) {
-                        PSPDFAssertIfNotMainThread();
-                        ((void ( *)(id, SEL, CGRect))objc_msgSend)(_self, newSelector, r);
-                    }));
-                }else {
-                    PSPDFReplaceMethod(theClass, NSSelectorFromString(selector), newSelector, imp_implementationWithBlock(^(UIView *_self) {
-                        PSPDFAssertIfNotMainThread();
-                        ((void ( *)(id, SEL))objc_msgSend)(_self, newSelector);
-                    }));
-                }
+            if ([selector hasSuffix:@":"]) {
+                PSPDFReplaceMethod(UIView.class, NSSelectorFromString(selector), newSelector, imp_implementationWithBlock(^(UIView *_self, CGRect r) {
+                    PSPDFAssertIfNotMainThread();
+                    ((void ( *)(id, SEL, CGRect))objc_msgSend)(_self, newSelector, r);
+                }));
+            }else {
+                PSPDFReplaceMethod(UIView.class, NSSelectorFromString(selector), newSelector, imp_implementationWithBlock(^(UIView *_self) {
+                    PSPDFAssertIfNotMainThread();
+                    ((void ( *)(id, SEL))objc_msgSend)(_self, newSelector);
+                }));
             }
         }
     }

diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -4,6 +4,16 @@
 
 #define PROPERTY(propName) NSStringFromSelector(@selector(propName))
 
+// A better assert. NSAssert is too runtime dependant, and assert() doesn't log.
+// http://www.mikeash.com/pyblog/friday-qa-2013-05-03-proper-use-of-asserts.html
+// Accepts both:
+// - PSPDFAssert(x > 0);
+// - PSPDFAssert(y > 3, @"Bad value for y");
+#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)
+
 // You should only use this in debug builds. It doesn't use private API, but I wouldn't ship it.
 #ifdef DEBUG
 

diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -2,6 +2,8 @@
 #import <objc/message.h>
 #import <QuartzCore/QuartzCore.h>
 
+#define PROPERTY(propName) NSStringFromSelector(@selector(propName))
+
 // You should only use this in debug builds. It doesn't use private API, but I wouldn't ship it.
 #ifdef DEBUG
 

diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -52,4 +52,5 @@ static void PSPDFAssertIfNotMainThread(void) {
         }
     }
 }
+
 #endif
diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -2,6 +2,9 @@
 #import <objc/message.h>
 #import <QuartzCore/QuartzCore.h>
 
+// You should only use this in debug builds. It doesn't use private API, but I wouldn't ship it.
+#ifdef DEBUG
+
 static void PSPDFSwizzleMethod(Class c, SEL orig, SEL new) {
     Method origMethod = class_getInstanceMethod(c, orig);
     Method newMethod = class_getInstanceMethod(c, new);
@@ -48,4 +51,5 @@ static void PSPDFAssertIfNotMainThread(void) {
             }
         }
     }
-}
+}
+#endif
diff --git a/PSPDFUIKitMainThreadGuard.m b/PSPDFUIKitMainThreadGuard.m
@@ -0,0 +1,51 @@
+#import <objc/runtime.h>
+#import <objc/message.h>
+#import <QuartzCore/QuartzCore.h>
+
+static void PSPDFSwizzleMethod(Class c, SEL orig, SEL new) {
+    Method origMethod = class_getInstanceMethod(c, orig);
+    Method newMethod = class_getInstanceMethod(c, new);
+    if (class_addMethod(c, orig, method_getImplementation(newMethod), method_getTypeEncoding(newMethod))) {
+        class_replaceMethod(c, new, method_getImplementation(origMethod), method_getTypeEncoding(origMethod));
+    }else {
+        method_exchangeImplementations(origMethod, newMethod);
+    }
+}
+
+void PSPDFReplaceMethod(Class c, SEL orig, SEL newSel, IMP impl) {
+    Method method = class_getInstanceMethod(c, orig);
+    if (!class_addMethod(c, newSel, impl, method_getTypeEncoding(method))) {
+        PSPDFLogError(@"Failed to add method: %@ on %@", NSStringFromSelector(newSel), c);
+    }else PSPDFSwizzleMethod(c, orig, newSel);
+}
+
+///////////////////////////////////////////////////////////////////////////////////////////
+#pragma mark - Tracks down calls to UIKit from a Thread other than Main
+
+static 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]);
+}
+
+// This installs a small guard that checks for the most common threading-errors in UIKit.
+// This won't really slow down performance but still only is compiled in DEBUG versions of PSPDFKit.
+// @note No private API is used here.
+__attribute__((constructor)) static void PSPDFUIKitMainThreadGuard(void) {
+    @autoreleasepool {
+        for (NSString *selector in @[PROPERTY(setNeedsLayout), PROPERTY(setNeedsDisplay), PROPERTY(setNeedsDisplayInRect:)]) {
+            SEL newSelector = NSSelectorFromString([NSString stringWithFormat:@"pspdf_%@", selector]);
+            for (Class theClass in @[UIView.class, CALayer.class]) {
+                if ([selector hasSuffix:@":"]) {
+                    PSPDFReplaceMethod(theClass, NSSelectorFromString(selector), newSelector, imp_implementationWithBlock(^(UIView *_self, CGRect r) {
+                        PSPDFAssertIfNotMainThread();
+                        ((void ( *)(id, SEL, CGRect))objc_msgSend)(_self, newSelector, r);
+                    }));
+                }else {
+                    PSPDFReplaceMethod(theClass, NSSelectorFromString(selector), newSelector, imp_implementationWithBlock(^(UIView *_self) {
+                        PSPDFAssertIfNotMainThread();
+                        ((void ( *)(id, SEL))objc_msgSend)(_self, newSelector);
+                    }));
+                }
+            }
+        }
+    }
+}
-Original file line number
+Diff line change
@@ -52,4 +52,5 @@ static void PSPDFAssertIfNotMainThread(void) {
             }
         }
     }
     #endif