Many apps use Core Data for persistence and also need to import data from a server. Imported items typically have a unique identifier which can be used for identifying them. The count of imported items can be high therefore it is preferred to batch insert the items instead of adding them one by one. Core Data framework has a specialized request for this called NSBatchInsertRequest which is available since iOS 13. If we combine batch insert with Core Data constraints then we can achieve a flow where new items are only created when the store does not have an item for the unique identifier. All the other items already available in the persistent store are updated (instead of deleting the old item and reinserting it). In this blog post let’s take a look on how it works with a sample app which displays a list of Product entities with a name and a unique serial code attributes.
Product entity with “name” and “serialCode” attributes.
Constraints on the entity can be set in the model editor. For making sure that only one Product with a serial code of X exists in the persistent store then we will need to add a constraint on the serialCode attribute. Core Data framework will then make sure that only one entity with unique serial code exists in the persistent store. Neat, no need to query the store first for existing products and manually checking for possible duplicates.
CoreData constraint set to Product entity.
With a constraint set up, let’s take a look on the batch insert. Apple added NSBatchInsertRequest to Core Data framework in iOS 13. As we added a constraint then we need to tell Core Data what to do if there is already an item for the unique serial code. If we set NSManagedObjectContext‘s merge policy to NSMergeByPropertyObjectTrumpMergePolicy before executing the batch insert then Core Data goes and updates existing items with incoming attribute values fetched from a server. If there is not an item in the store with serial code then a new item is inserted. In summary, we get a behaviour where existing items are updated and missing items are inserted when importing items from a server. The flow of fetching data from a server, running batch insert on a background context and then refreshing fetched results controller can be seen below.
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Import function in a view model which fetches a list of products and inserts into a persistent store.
Summary
NSBatchInsertRequest is a welcoming change which makes it easy to insert and update existing items already in the persistent store. Setting up a constraint on a unique identifier and setting merge policy on a context enables us to handle SQL upserts without much code.
This time we are going to look into how to embed custom view in UIScrollView and setting up autolayout constraints for different scenarios.
Setting up view controller with scroll view
We’ll gonna create a new view controller what contains UIScrollView as it’s subview. UIScrollView will cover the whole view area. Everything is set up in code as it is easier to represent the steps needed. It is easy to use those steps and setting up constraints in interface builder.
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Most common case is to have scroll view with vertically scrollable content. Embedded view’s leading, trailing, top and bottom layout anchors should be equal to scroll view’s content layout margin’s corresponding anchors. For making the view only scrollable in vertical axis, embedded view’s width should match with scroll view’s width. This is achieved by adding an extra width constraint to the embedded view. Width constraint should make the view width equal to scroll view’s width. When setting view’s up in interface builder, make sure to add at least one subview to the embedded view what has intrinsic content size (label, button etc) (also add constraints). Otherwise interface builder does not know what size the embedded view has.
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Horizontally scrollable view follows the same setup except instead of width constraint, we’ll add height constraint instead. When setting up the height constraint, scroll view’s safeAreaLayoutGuide’s heightAnchor should be used. This layout guide ignores the portion of the view what is covered by navigation bars, tab bars, toolbars, and other views.
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Case 3: Vertically and horizontally scrollable view
For making the embedded view to be scrollable in horizontal and vertical axis, we’ll only add leading, trailing, top and bottom constraints to scroll view’s contentLayoutGuide’s corresponding anchors. Embedded view should define intrinsicContentSize as this size is used by scroll view for knowing what size the view wants to be. This kind of layout is probably usable only for fixed size view’s. For example, building a custom grid view where the row and column count define the size of the view.
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Vertically and horizontally scrollable embedded view
Summary
We took a look on how to add constraints to embedded view in scroll view for getting a specific scrolling behaviour. In one sentence: leading, trailing, top, and bottom constraints to contentLayoutGuide and adding width or height constraint when necessary.
Augmented Code 