Add assert macro that can be used in free functions.

// FREEBIE

src/Util/Asserts.h

@@ -1,7 +1,5 @@
 //
-//  Asserts.h
-//
-//  Copyright (c) 2016 Open Whisper Systems. All rights reserved.
+//  Copyright (c) 2017 Open Whisper Systems. All rights reserved.
 //
 
 #import <Foundation/Foundation.h>
@@ -21,9 +19,18 @@ NSLog(@"Assertion failed: %s", CONVERT_EXPR_TO_STRING(X)); \
 NSAssert(0, @"Assertion failed: %s", CONVERT_EXPR_TO_STRING(X)); \
 }
 
+// OWSAssert() should be used in Obj-C and Swift methods.
+// OWSCAssert() should be used in free functions.
+#define OWSCAssert(X) \
+if (!(X)) { \
+NSLog(@"Assertion failed: %s", CONVERT_EXPR_TO_STRING(X)); \
+NSCAssert(0, @"Assertion failed: %s", CONVERT_EXPR_TO_STRING(X)); \
+}
+
 #else
 
 #define OWSAssert(X)
+#define OWSCAssert(X)
 
 #endif
 

src/Util/OWSDispatch.h

@@ -24,4 +24,6 @@ NS_ASSUME_NONNULL_BEGIN
 
 @end
 
+void AssertIsOnMainThread();
+
 NS_ASSUME_NONNULL_END

src/Util/OWSDispatch.m

@@ -40,4 +40,8 @@ NS_ASSUME_NONNULL_BEGIN
 
 @end
 
+void AssertIsOnMainThread() {
+    OWSCAssert([NSThread isMainThread]);
+}
+
 NS_ASSUME_NONNULL_END