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IGListDiffable and Equality Reference
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< h1 id = 'iglistdiffable-and-equality' class = 'heading' > IGListDiffable and Equality< / h1 >
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< p > This guide explains the < code > < a href = "Protocols/IGListDiffable.html" > IGListDiffable< / a > < / code > protocol and how to write good < code > -isEqual:< / code > methods. < / p >
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< h2 id = 'background' class = 'heading' > Background< / h2 >
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< p > The < a href = "https://instagram.github.io/IGListKit/Protocols/IGListDiffable.html" > < code > < a href = "Protocols/IGListDiffable.html" > IGListDiffable< / a > < / code > protocol< / a > requires clients to implement two methods, < code > -diffIdentifier< / code > and < code > -isEqualToDiffableObject:< / code > .< / p >
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< p > The method < code > -isEqualToDiffableObject:< / code > should perform the same type of check as < code > -isEqual:< / code > , but without impacting performance characteristics, like in Objective-C containers such as < code > NSDictionary< / code > and < code > NSSet< / code > .< / p >
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< p > Why are both of these methods required for diffing? The point of having the two methods has to do with < strong > identity< / strong > and < strong > equality< / strong > , 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).< / p >
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< p > See also: < a href = "https://github.com/Instagram/IGListKit/issues/509" > #509< / a > < / p >
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< h2 id = 'code-iglistdiffable-code-bare-minimum' class = 'heading' > < code > < a href = "Protocols/IGListDiffable.html" > IGListDiffable< / a > < / code > bare minimum< / h2 >
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< p > The quickest way to get started with diffable models is use the < em > object itself< / em > as the identifier, and use the superclass’ s < code > -[NSObject isEqual:]< / code > implementation for equality:< / p >
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< pre class = "highlight objective_c" > < code > < span class = "k" > -< / span > < span class = "p" > (< / span > < span class = "n" > id< / span > < span class = "o" > < < / span > < span class = "n" > NSObject< / span > < span class = "o" > > < / span > < span class = "p" > )< / span > < span class = "n" > diffIdentifier< / span > < span class = "p" > {< / span >
< span class = "k" > return< / span > < span class = "n" > self< / span > < span class = "p" > ;< / span >
< span class = "p" > }< / span >
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< span class = "k" > -< / span > < span class = "p" > (< / span > < span class = "n" > BOOL< / span > < span class = "p" > )< / span > < span class = "nf" > isEqualToDiffableObject< / span > < span class = "p" > :(< / span > < span class = "n" > id< / span > < span class = "o" > < < / span > < span class = "n" > IGListDiffable< / span > < span class = "o" > > < / span > < span class = "p" > )< / span > < span class = "nv" > object< / span > < span class = "p" > {< / span >
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< span class = "k" > return< / span > < span class = "p" > [< / span > < span class = "n" > self< / span > < span class = "nf" > isEqual< / span > < span class = "p" > :< / span > < span class = "n" > object< / span > < span class = "p" > ];< / span >
< span class = "p" > }< / span >
< / code > < / pre >
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< h2 id = 'writing-better-equality-methods' class = 'heading' > Writing better Equality methods< / h2 >
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< p > Even though < code > IGListKit< / code > uses the method < code > -isEqualToDiffableObject:< / code > , the concepts of writing a good equality check apply in general. Here are the basics to writing good < code > -isEqual:< / code > and < code > -hash< / code > functions. Note this is all Objective-C but applies to Swift also.< / p >
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< ul >
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< li > If you override < code > -isEqual:< / code > you < strong > must< / strong > override < code > -hash< / code > . Check out this < a href = "https://www.mikeash.com/pyblog/friday-qa-2010-06-18-implementing-equality-and-hashing.html" > article by Mike Ash< / a > for details.< / li >
< li > Always compare the pointer first. This saves a lot of wasteful < code > objc_msgSend(...)< / code > calls and value comparisons if checking the same instance.< / li >
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< li > When comparing object values, always check for < code > nil< / code > before < code > -isEqual:< / code > . For example, < code > [nil isEqual:nil]< / code > counterintuitively returns < code > NO< / code > . Instead, do < code > left == right || [left isEqual:right]< / code > .< / li >
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< li > Always compare the < strong > cheapest values first< / strong > . For example, doing < code > [self.array isEqual:other.array] & & self.intVal == other.intVal< / code > is extremely wasteful if the < code > intVal< / code > values are different. Use lazy evaluation!< / li >
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< / ul >
< p > As an example, if I had a < code > User< / code > model with the following interface:< / p >
< pre class = "highlight objective_c" > < code > < span class = "k" > @interface< / span > < span class = "nc" > User< / span > < span class = "p" > :< / span > < span class = "nc" > NSObject< / span >
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< span class = "k" > @property< / span > < span class = "n" > NSInteger< / span > < span class = "n" > identifier< / span > < span class = "p" > ;< / span >
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< span class = "k" > @property< / span > < span class = "n" > NSString< / span > < span class = "o" > *< / span > < span class = "n" > name< / span > < span class = "p" > ;< / span >
< span class = "k" > @property< / span > < span class = "n" > NSArray< / span > < span class = "o" > *< / span > < span class = "n" > posts< / span > < span class = "p" > ;< / span >
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< span class = "k" > @end< / span >
< / code > < / pre >
< p > You would implement its equality methods like so:< / p >
< pre class = "highlight objective_c" > < code > < span class = "k" > @implementation< / span > < span class = "nc" > User< / span >
< span class = "k" > -< / span > < span class = "p" > (< / span > < span class = "n" > NSUInteger< / span > < span class = "p" > )< / span > < span class = "n" > hash< / span > < span class = "p" > {< / span >
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< span class = "k" > return< / span > < span class = "n" > self< / span > < span class = "p" > .< / span > < span class = "n" > identifier< / span > < span class = "p" > ;< / span >
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< span class = "p" > }< / span >
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< span class = "k" > -< / span > < span class = "p" > (< / span > < span class = "n" > BOOL< / span > < span class = "p" > )< / span > < span class = "nf" > isEqual< / span > < span class = "p" > :(< / span > < span class = "n" > id< / span > < span class = "p" > )< / span > < span class = "nv" > object< / span > < span class = "p" > {< / span >
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< span class = "k" > if< / span > < span class = "p" > (< / span > < span class = "n" > self< / span > < span class = "o" > ==< / span > < span class = "n" > object< / span > < span class = "p" > )< / span > < span class = "p" > {< / span >
< span class = "k" > return< / span > < span class = "nb" > YES< / span > < span class = "p" > ;< / span >
< span class = "p" > }< / span >
< span class = "k" > if< / span > < span class = "p" > (< / span > < span class = "o" > !< / span > < span class = "p" > [< / span > < span class = "n" > object< / span > < span class = "nf" > isKindOfClass< / span > < span class = "p" > :[< / span > < span class = "n" > User< / span > < span class = "nf" > class< / span > < span class = "p" > ]])< / span > < span class = "p" > {< / span >
< span class = "k" > return< / span > < span class = "nb" > NO< / span > < span class = "p" > ;< / span >
< span class = "p" > }< / span >
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< span class = "n" > User< / span > < span class = "o" > *< / span > < span class = "n" > right< / span > < span class = "o" > =< / span > < span class = "n" > object< / span > < span class = "p" > ;< / span >
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< span class = "k" > return< / span > < span class = "n" > self< / span > < span class = "p" > .< / span > < span class = "n" > identifier< / span > < span class = "o" > ==< / span > < span class = "n" > right< / span > < span class = "p" > .< / span > < span class = "n" > identifier< / span >
< span class = "o" > & & < / span > < span class = "p" > (< / span > < span class = "n" > self< / span > < span class = "p" > .< / span > < span class = "n" > name< / span > < span class = "o" > ==< / span > < span class = "n" > right< / span > < span class = "p" > .< / span > < span class = "n" > name< / span > < span class = "o" > ||< / span > < span class = "p" > [< / span > < span class = "n" > self< / span > < span class = "p" > .< / span > < span class = "n" > name< / span > < span class = "nf" > isEqual< / span > < span class = "p" > :< / span > < span class = "n" > right< / span > < span class = "p" > .< / span > < span class = "n" > name< / span > < span class = "p" > ])< / span >
< span class = "o" > & & < / span > < span class = "p" > (< / span > < span class = "n" > self< / span > < span class = "p" > .< / span > < span class = "n" > posts< / span > < span class = "o" > ==< / span > < span class = "n" > right< / span > < span class = "p" > .< / span > < span class = "n" > posts< / span > < span class = "o" > ||< / span > < span class = "p" > [< / span > < span class = "n" > self< / span > < span class = "p" > .< / span > < span class = "n" > posts< / span > < span class = "nf" > isEqualToArray< / span > < span class = "p" > :< / span > < span class = "n" > right< / span > < span class = "p" > .< / span > < span class = "n" > posts< / span > < span class = "p" > ]);< / span >
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< span class = "p" > }< / span >
< span class = "k" > @end< / span >
< / code > < / pre >
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< h2 id = 'using-both-code-iglistdiffable-code-and-code-isequal-code' class = 'heading' > Using both < code > < a href = "Protocols/IGListDiffable.html" > IGListDiffable< / a > < / code > and < code > -isEqual:< / code > < / h2 >
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< p > Making your objects work universally with Objective-C containers and < code > IGListKit< / code > is easy once you’ ve implemented < code > -isEqual:< / code > and < code > -hash< / code > .< / p >
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< pre class = "highlight objective_c" > < code > < span class = "k" > @interface< / span > < span class = "nc" > User< / span > < span class = "o" > < < / span > < span class = "n" > IGListDiffable< / span > < span class = "o" > > < / span >
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< span class = "c1" > // properties...< / span >
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< span class = "k" > @end< / span >
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< span class = "k" > @implementation< / span > < span class = "nc" > User< / span >
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< span class = "k" > -< / span > < span class = "p" > (< / span > < span class = "n" > id< / span > < span class = "o" > < < / span > < span class = "n" > NSObject< / span > < span class = "o" > > < / span > < span class = "p" > )< / span > < span class = "n" > diffIdentifier< / span > < span class = "p" > {< / span >
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< span class = "k" > return< / span > < span class = "err" > @< / span > < span class = "p" > (< / span > < span class = "n" > self< / span > < span class = "p" > .< / span > < span class = "n" > identifier< / span > < span class = "p" > );< / span >
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< span class = "p" > }< / span >
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< span class = "k" > -< / span > < span class = "p" > (< / span > < span class = "n" > BOOL< / span > < span class = "p" > )< / span > < span class = "nf" > isEqualToDiffableObject< / span > < span class = "p" > :(< / span > < span class = "n" > id< / span > < span class = "o" > < < / span > < span class = "n" > IGListDiffable< / span > < span class = "o" > > < / span > < span class = "p" > )< / span > < span class = "nv" > object< / span > < span class = "p" > {< / span >
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< span class = "k" > return< / span > < span class = "p" > [< / span > < span class = "n" > self< / span > < span class = "nf" > isEqual< / span > < span class = "p" > :< / span > < span class = "n" > object< / span > < span class = "p" > ];< / span >
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< span class = "p" > }< / span >
< span class = "k" > @end< / span >
< / code > < / pre >
< / section >
< / section >
< section id = "footer" >
2023-04-07 07:23:43 +00:00
< p > © 2023 < a class = "link" href = "https://twitter.com/MetaOpenSource" target = "_blank" rel = "external noopener" > Instagram< / a > . All rights reserved. (Last updated: 2023-04-07)< / p >
< p > Generated by < a class = "link" href = "https://github.com/realm/jazzy" target = "_blank" rel = "external noopener" > jazzy ♪♫ v0.14.3< / a > , a < a class = "link" href = "https://realm.io" target = "_blank" rel = "external noopener" > Realm< / a > project.< / p >
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< / section >
< / article >
< / div >
< / body >
< / html >
