Eliminate repeated weak references in MDCThumbTrack

Summary:
Enabled warning to check for repeated use of weak variables.
Updated code to eliminate repeated weak variable references.

Please sanity check my changes for retain cycles.

Reviewers: featherless, #material_components_ios_owners, randallli

Reviewed By: featherless, #material_components_ios_owners, randallli

Projects: #material_components_ios

Differential Revision: http://codereview.cc/D96

Author: Ian Gordon

components/Switch/Switch.xcodeproj/project.pbxproj

@@ -423,6 +423,7 @@
 				CLANG_WARN_EMPTY_BODY = YES;
 				CLANG_WARN_ENUM_CONVERSION = YES;
 				CLANG_WARN_INT_CONVERSION = YES;
+				CLANG_WARN_OBJC_REPEATED_USE_OF_WEAK = YES;
 				CLANG_WARN_OBJC_ROOT_CLASS = YES_ERROR;
 				CLANG_WARN_UNREACHABLE_CODE = YES;
 				CLANG_WARN__DUPLICATE_METHOD_MATCH = YES;
@@ -467,6 +468,7 @@
 				CLANG_WARN_EMPTY_BODY = YES;
 				CLANG_WARN_ENUM_CONVERSION = YES;
 				CLANG_WARN_INT_CONVERSION = YES;
+				CLANG_WARN_OBJC_REPEATED_USE_OF_WEAK = YES;
 				CLANG_WARN_OBJC_ROOT_CLASS = YES_ERROR;
 				CLANG_WARN_UNREACHABLE_CODE = YES;
 				CLANG_WARN__DUPLICATE_METHOD_MATCH = YES;
@@ -499,6 +501,7 @@
 				LD_RUNPATH_SEARCH_PATHS = "$(inherited) @executable_path/Frameworks";
 				PRODUCT_BUNDLE_IDENTIFIER = com.google.Switch;
 				PRODUCT_NAME = "$(TARGET_NAME)";
+				WARNING_CFLAGS = "";
 			};
 			name = Debug;
 		};
@@ -510,6 +513,7 @@
 				LD_RUNPATH_SEARCH_PATHS = "$(inherited) @executable_path/Frameworks";
 				PRODUCT_BUNDLE_IDENTIFIER = com.google.Switch;
 				PRODUCT_NAME = "$(TARGET_NAME)";
+				WARNING_CFLAGS = "";
 			};
 			name = Release;
 		};

components/private/ThumbTrack/src/MDCThumbTrack.m

@@ -611,19 +611,21 @@ - (void)setValueFromThumbPosition:(CGPoint)position isTap:(BOOL)isTap {
   } else {
     value = [self valueForThumbPosition:position];
   }
-  __weak MDCThumbTrack *weakSelf = self;
-  if ([_delegate respondsToSelector:@selector(thumbTrack:willAnimateToValue:)]) {
-    [_delegate thumbTrack:self willAnimateToValue:value];
+  __strong typeof(_delegate) strongDelegate = _delegate;
+  if ([strongDelegate respondsToSelector:@selector(thumbTrack:willAnimateToValue:)]) {
+    [strongDelegate thumbTrack:self willAnimateToValue:value];
   }
+  __weak typeof(self) weakSelf = self;
   [self setValue:value
            animated:YES
       userGenerated:YES
          completion:^{
-           MDCThumbTrack *strongSelf = weakSelf;
+           __strong typeof(weakSelf) strongSelf = weakSelf;
+           __strong typeof(strongSelf->_delegate) strongDelegate = strongSelf->_delegate;
            [strongSelf sendDiscreteChangeAction];
-           if (strongSelf && [strongSelf->_delegate
+           if (strongSelf && [strongDelegate
                                  respondsToSelector:@selector(thumbTrack:didAnimateToValue:)]) {
-             [strongSelf->_delegate thumbTrack:weakSelf didAnimateToValue:value];
+             [strongDelegate thumbTrack:strongSelf didAnimateToValue:value];
            }
          }];
 }