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Update IGListDiffable and Equality.md

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Summary:
Per #509
Closes https://github.com/Instagram/IGListKit/pull/513

Differential Revision: D4613737

Pulled By: jessesquires

fbshipit-source-id: f77960819a047a0bd504dbf7ebf0558a569801cf
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Jesse Squires 2017-02-24 13:32:27 -08:00 committed by Facebook Github Bot
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Guides/IGListDiffable and Equality.md
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# IGListDiffable and Equality
This guide details how to write good `isEqual:` methods.
This guide explains the `IGListDiffable` protocol and how to write good `-isEqual:` methods.
## Background
`IGListKit` requires that models implement the method `isEqualToDiffableObject:` which should perform the same type of check as `isEqual:`, but without impacting performance characteristics like in Objective-C containers such as `NSDictionary` and `NSSet`.
The [`IGListDiffable` protocol](https://instagram.github.io/IGListKit/Protocols/IGListDiffable.html) requires clients to implement two methods, `-diffIdentifier` and `-isEqualToDiffableObject:`.
The method `-isEqualToDiffableObject:` should perform the same type of check as `-isEqual:`, but without impacting performance characteristics, like in Objective-C containers such as `NSDictionary` and `NSSet`.
Why are both of these methods required for diffing? The point of having the two methods has to do with **identity** and **equality**, where the diff identifier uniquely identifies data (common scenario is primary key in databases). Equality comes into play when comparing the values of two uniquely identical objects (driving reloading).
See also: [#509](https://github.com/Instagram/IGListKit/issues/509)
## `IGListDiffable` bare minimum
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## Writing better Equality methods
Even though `IGListKit` uses the method `isEqualToDiffableObject:`, the concepts of writing a good equality check apply in general. Here are the basics to writing good `-isEqual:` and `-hash` functions. Note this is all Objective-C but applies to Swift also.
Even though `IGListKit` uses the method `-isEqualToDiffableObject:`, the concepts of writing a good equality check apply in general. Here are the basics to writing good `-isEqual:` and `-hash` functions. Note this is all Objective-C but applies to Swift also.
- If you override `-isEqual:` you **must** override `-hash`. Check out this [article by Mike Ash](https://www.mikeash.com/pyblog/friday-qa-2010-06-18-implementing-equality-and-hashing.html) for details.
- Always compare the pointer first. This saves a lot of wasteful `objc_msgSend(...)` calls and value comparisons if checking the same instance.
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## Using both `IGListDiffable` and `-isEqual:`
Making your objects work universally with Objective-C containers and `IGListKit` is easy once you've implemented `isEqual:` and `-hash`.
Making your objects work universally with Objective-C containers and `IGListKit` is easy once you've implemented `-isEqual:` and `-hash`.
```objc
@interface User <IGListDiffable>