add null_resettable to data sources

Author: morganchen12

FirebaseUI/Database/API/FirebaseCollectionViewDataSource.h

@@ -52,8 +52,9 @@ NS_ASSUME_NONNULL_BEGIN
  * to [Message class] in Obj-C or Message.self in Swift, then objects of type
  * Message will be
  * returned instead of type FIRDataSnapshot.
+ * Defaults to FIRDataSnapshot.
  */
-@property(strong, nonatomic, nullable) Class modelClass;
+@property(strong, nonatomic, null_resettable) Class modelClass;
 
 /**
  * The cell class to coerce UICollectionViewCells to (if desired). For instance,
@@ -62,8 +63,9 @@ NS_ASSUME_NONNULL_BEGIN
  * in Swift, then
  * objects of type CustomCollectionViewCell will be returned instead of type
  * UICollectionViewCell.
+ * Defaults to UICollectionViewCell.
  */
-@property(strong, nonatomic, nullable) Class cellClass;
+@property(strong, nonatomic, null_resettable) Class cellClass;
 
 /**
  * The reuse identifier for cells in the UICollectionView.

FirebaseUI/Database/API/FirebaseTableViewDataSource.h

@@ -51,8 +51,9 @@ NS_ASSUME_NONNULL_BEGIN
  * to [Message class] in Obj-C or Message.self in Swift, then objects of type
  * Message will be
  * returned instead of type FIRDataSnapshot.
+ * Defaults to FIRDataSnapshot.
  */
-@property(strong, nonatomic) Class modelClass;
+@property(strong, nonatomic, null_resettable) Class modelClass;
 
 /**
  * The reuse identifier for cells in the UITableView.

FirebaseUI/Database/Implementation/FirebaseCollectionViewDataSource.m

@@ -294,4 +294,22 @@ - (void)populateCellWithBlock:(void (^)(__kindof UICollectionViewCell *cell,
   self.populateCell = callback;
 }
 
+#pragma mark - Accessors
+
+- (void)setModelClass:(Class)modelClass {
+  if (modelClass == nil) {
+    _modelClass = [FIRDataSnapshot class];
+  } else {
+    _modelClass = modelClass;
+  }
+}
+
+- (void)setCellClass:(Class)cellClass {
+  if (cellClass == nil) {
+    _cellClass = [UICollectionViewCell class];
+  } else {
+    _cellClass = cellClass;
+  }
+}
+
 @end

FirebaseUI/Database/Implementation/FirebaseTableViewDataSource.m

@@ -295,4 +295,14 @@ - (void)populateCellWithBlock:(void (^)(__kindof UITableViewCell *cell,
   self.populateCell = callback;
 }
 
+#pragma mark - Accessors
+
+- (void)setModelClass:(Class)modelClass {
+  if (modelClass == nil) {
+    _modelClass = [FIRDataSnapshot class];
+  } else {
+    _modelClass = modelClass;
+  }
+}
+
 @end