Completed writeup of data structure review

12-healthkit/12-healthkit.md

@@ -27,4 +27,104 @@ available in the github repo at
 [github.com/ShinobiControls/iOS8-day-by-day](https://github.com/ShinobiControls/iOS8-day-by-day).
 
 
+## Data Structure Overview
+
+The structure of data in HealthKit represents an ontology of pretty much
+everything related to personal well-being. This includes obvious measurements
+such as body mass, but also extends to more specialized readings such as blood
+oxygen saturation and dietary zinc. The underlying database is a set of time-
+series - consisting of independent time-indexed samples.
+
+In terms of code these samples are represented by the `HKSample` class, which
+itself is a subclass of the immutable `HKObject` class. `HKObject` is the
+fundamental type used in HealthKit - and includes properties such as `UIID`,
+`source` and `metadata`. Since these objects are immutable, they are created
+using initializers on the `HKQuantitySample` and `HKCategorySample` subclasses.
+These are fairly self explanatory in their distinction between representing a
+categorical data type and a quantifiable type (such as body mass). They each
+have some kind of type property to specify what the value represents
+(`categoryType` and `quantityType` respectively). The following demonstrates
+creating a quantity sample for body mass:
+
+    let weightType = HKObjectType.quantityTypeForIdentifier(HKQuantityTypeIdentifierBodyMass)
+    let weightValue = HKQuantity(unit: HKUnit(fromString: "kg"), doubleValue: massNumber)
+    let metadata = [ HKMetadataKeyWasUserEntered : true ]
+    let sample = HKQuantitySample(type: weightType, quantity: weightValue,
+                    startDate: datePicker.date, endDate: datePicker.date, metadata: metadata)  
+
+- `HKObjectType` is a class which represents the aforementioned type property.
+It has methods for creating types such as`quantityTypeForIdentifier()` and
+`categoryTypeForIdentifier()`, which take a framework-defined string constant
+to specify exactly which type it represents (e.g. `HKQuantityTypeIdentifierBodyMass`).
+- A quantity sample obviously has an associated value, and this is provided by
+the `HKQuantity` class. This requires a double value and an `HKUnit` object
+which represents the units the double was measured in.
+- You can provide a metadata dictionary including details such as the
+measurement device - here the `HKMetadataKeyWasUserEntered` key shows that the
+data has been manually entered.
+- Finally the `HKQuantitySample` is constructed using the type, quantity and
+metadata objects you've created, along with a start and end date.
+
+The `HKUnit` class is an incredibly powerful concept, in that it will cause
+HealthKit to convert between units (provided they represent the same physical
+quantity) implicitly. This goes both for simple units (such as those
+representing mass, volume and temperature) and for compound units (such as those
+representing density). The powerful string parsing of the `HKUnit(fromString:)`
+initializer is an excellent way to create these objects.
+
+To see the selection of different values that can be recorded, check out the
+__HKTypeIdentifiers__ file. The following represents a small selection:
+
+- Body mass
+- Body fat percentage
+- Height
+- Step count
+- Blood glucose
+- Blood alcohol content
+- Dietary energy consumed
+- Body temperature
+- Inhaler usage
+
+In addition to quantity and categorical time-series types, you can also store
+characteristic types, that is those which are incredibly unlikely (or don't) to
+change over time, for example date of birth, sex and blood type. You would
+expect that a user would set these in the health app and then they are available
+to you when reading data from the store.
+
+As well as personal physical measurements, you can also store details of workout
+sessions through the `HKWorkout` subclass of `HKSample`. This has properties for
+recording the type of workout, the duration, the energy burned and the distance.
+Since `HKWorkout` is another subclass of `HKObject` it is also immutable, so
+these properties are all populated at initialization time. The energy burned and
+distance are again examples of `HKQuantity` objects, and the activity type is an
+enum (`HKWorkoutActivityType`) which includes over 50 types of exercise,
+including:
+
+- Archery
+- CrossTraining
+- Dance
+- Fishing
+- Hocket
+- SkatingSports
+- SnowSports
+- StairClimbing
+- Swimming
+- Yoga
+
+You'll notice that through out this run through of the data model, you've been
+dealing solely with `HKObject` subclasses. This is very much like how CoreData
+uses `NSManagedObject`, and allows the interface to the data store to be
+consistent, irrespective of whether you're storing a workout or a blood pressure
+reading. We'll take a look at this interface soon, but first there's a rather
+important issue to take a look at - that of permissions.
+
+## Permissions
+
+
+## Writing Data
+
+
+## Reading Data
+
+
 ## Conclusion