Use weak qualifier replace assign qualifier.

Author: QianGuoqiang

MMMarkdown.xcodeproj/project.pbxproj

@@ -1169,7 +1169,7 @@
 				INFOPLIST_FILE = Source/Info.plist;
 				INSTALL_PATH = "$(LOCAL_LIBRARY_DIR)/Frameworks";
 				LD_RUNPATH_SEARCH_PATHS = "$(inherited) @executable_path/../Frameworks @loader_path/Frameworks";
-				MACOSX_DEPLOYMENT_TARGET = 10.6;
+				MACOSX_DEPLOYMENT_TARGET = 10.7;
 				MTL_ENABLE_DEBUG_INFO = YES;
 				PRODUCT_BUNDLE_IDENTIFIER = "com.diephouse.matt.$(PRODUCT_NAME:rfc1034identifier)";
 				PRODUCT_NAME = MMMarkdown;
@@ -1201,7 +1201,7 @@
 				INFOPLIST_FILE = Source/Info.plist;
 				INSTALL_PATH = "$(LOCAL_LIBRARY_DIR)/Frameworks";
 				LD_RUNPATH_SEARCH_PATHS = "$(inherited) @executable_path/../Frameworks @loader_path/Frameworks";
-				MACOSX_DEPLOYMENT_TARGET = 10.6;
+				MACOSX_DEPLOYMENT_TARGET = 10.7;
 				MTL_ENABLE_DEBUG_INFO = NO;
 				PRODUCT_BUNDLE_IDENTIFIER = "com.diephouse.matt.$(PRODUCT_NAME:rfc1034identifier)";
 				PRODUCT_NAME = MMMarkdown;

Source/MMElement.h

@@ -77,7 +77,7 @@ typedef NS_ENUM(NSInteger, MMTableCellAlignment)
 @property (copy,   nonatomic) NSString      *identifier;
 @property (copy,   nonatomic) NSString      *stringValue;
 
-@property (assign, nonatomic) MMElement *parent;
+@property (weak, nonatomic) MMElement *parent;
 @property (copy,   nonatomic) NSArray<MMElement *>   *children;
 
 @property (copy,   nonatomic) NSString  *language;