Swift 4设计模式教程：由Joshua Greene & Jay Strawn讲解

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"《高清彩版 Ray Wenderlich 设计模式教程》是一本由 Joshua Greene 和 Jay Strawn 合著的专业级iOS设计模式学习指南。这本书是针对Swift 4环境编写的，专为iOS开发者提供深入理解并实践设计模式的实用教程。作者在书中详细阐述了各种常见的设计模式，如工厂模式、单例模式、观察者模式、策略模式等，旨在帮助读者提升软件架构的设计能力和代码组织的灵活性。该书版权属于Razeware LLC，强调所有内容受法律保护，未经版权所有者事先书面许可，不得复制或分发任何部分，包括文字、图片和源代码。这表明它是一部严谨的学术资源，尊重知识产权。书中还包含了免责声明，指出读者在使用本书提供的软件或源代码时应自行承担风险，作者和版权持有人不对任何合同责任、侵权行为或间接损失负责，确保用户在了解其潜在风险的前提下使用。此外，书中还提到了商标使用的规定，所有在书中出现的商标和注册商标都是各自所有者的财产，体现了对知识产权的尊重。最后，作者向自己的女儿们Marsha、Madeline和Amelia表达了感谢，感谢她们的耐心陪伴和支持。《高清彩版 Ray Wenderlich 设计模式教程》是一本富有深度且实用的iOS设计模式教材，适合有一定编程基础的开发者深入学习和实践，以提升他们的软件设计水平和应对复杂项目的能力。阅读这本书，开发者将能够理解和掌握如何在Swift 4环境中优雅地应用各种设计模式，从而提升项目的可维护性和扩展性。"

Chapter1:WhatareDesign

Patterns?

“Extra,extra!Readallaboutit!”

“Fearedbynewcomers.Lovedbyarchitects.Readtheinsidestory

aboutdesignpatterns.Thetruthmaysurpriseyou!”

Didyouknowdesignpatternscanmakeyouabetterdeveloper?“Of

course,”yousay—youarereadingthisbook,afterall!

Didyouknowdesignpatternscanhelpyoumakemoremoney?It’s

true.Youcansavetime,worklessandultimatelycreatemoregreat

thingsbyusingdesignpatternscorrectly.

Anddidyouknowdesignpatternscanhelpyoufightvampires?OK,

maybenot—designpatternsaren’tsilverbullets,afterall.

However,designpatternsareincrediblyuseful,nomatterwhat

languageorplatformyoudevelopfor,andeverydevelopershould

absolutelyknowaboutthem.Theyshouldalsoknowhowandwhento

applythem.That'swhatyou'regoingtolearninthisbook!

Areal-worldexample

Theintroductiontoldyouthatdesignpatternsarereusable,template

solutionstocommondevelopmentproblems.Designpatternsaren’t

concreteimplementations,butrather,serveasstartingpointsfor

writingcode.Theydescribegenericsolutionstoproblemsthatmany

experienceddevelopershaveencounteredmanytimesbefore.

Whatdoesthismeanexactly…?Considerthisnon-development,real-

worldscenario:

You’retheproudownerofagardeningcompany,andyourbusinessis

really,er,blooming.You’veonlydoneafewsmallprojectsuptonow-

atreeplantedhere,andmaybeafewflowersthere.However,youjust

landedabigclientwhowantsseveraldozentreesandflowersplanted

ontheirproperty.

Yourstandardprocedurehasbeenforyouremployeestocarryeach

flowerortreesaplingintoplaceindividually.Oncetheflowerhas

beentemporarilyplacedintheflowerbed,yourcustomerinspectsand

approvesthearrangementbeforeyouplanteverythingintheground.

You’reworriedit’sgoingtotakeforevertocarryeachflowerandtree

intoplaceforthislargeproject.Andyouevenneedafewpeopleto

carrysomeofthebiggertrees.Whileyoucouldhirelotsoftemporary

employees,youwouldn’tmakeaprofitonthejob.There’sgottobea

betterway!

Youdecidetoaskothergardenerswhattheydo,andyoufindoutthey

usewheelbarrowsandcarts.Whatagreatidea!Youtellyour

employeestouseacarttomovemultipleflowersatthesametime

intoplaceandawheelbarrowtomovetheheavytrees.Inthe

meantime,youusealoungechairchairtowatchyourworkersgoto

it…isn’tmanagementgreat?

Sonowyouknowallaboutdesignpatterns!Wait,youneedmore

details?Okay,let’sbreakitdown…

Exampleexplanation

The“designpattern”hereistheuseofwheelbarrowsandcarts.These

arecommon,bestpracticetoolsingardening.Similarly,software

designpatternsformasetofbestpracticesindevelopment.Youcould

havechosennottousewheelbarrowsandcarts,butakintoavoiding

softwaredesignpatterns,youassumemoreriskbymakingtheproject

moretime-andlabor-intensive.

Backtothepointof“askingothergardenerswhattheydo.”Most

designpatternshavebeenaroundforalongtime—havingstartedlife

inthe1970sand1980s—andtheycontinuetoworkwelltothisday.

Thislongevityispartlyduetothefacttheirusehasbeenvalidatedin

manyprojectsoverthedecades,butit’salsobecausetheyaren’t

concretesolutions.

Inthegardeningscenario,youdecidedthatcartswillbeusedtomove

flowersandwheelbarrowswillbeusedtomovetrees.Theseare

implementationdetails:youcouldhaveusedcartstomoveboth

flowersandtrees,onlyusedwheelbarrows,oranyothercombination

thatmadethejobeasier.

Designpatternsaregeneric,go-tosolutionsforsolvingcommon

problems,likeusingwheelbarrowsandcarts.Theyarestartingpoints

forconcreteimplementations,likeusingcartsforflowersand

wheelbarrowsfortrees.

Timetoleavethegardenbehindandheadbacktotheworldof

softwaredesignpatterns.

Typesofdesignpatterns

Therearethreemaintypesofdesignpatterns:

1. Structuraldesignpattern:Describeshowobjectsarecomposed

andcombinedtoformlargerstructures.Examplesofstructural

designpatternsincludeModel-View-Controller(MVC),Model-

View-ViewModel(MVVM)andFacade.

2. Behavioraldesignpattern:Describeshowobjectscommunicate

witheachother.Examplesofbehavioraldesignpatternsare

Delegation,StrategyandObserver.

3. Creationaldesignpattern:Describeshowtocreateorinstantiate

objects.ExamplesofcreationalpatternsareBuilder,Singleton

andPrototype.

Youmaybewonderingifknowingadesignpattern’stypereally

matters.Well,yes…andno.

It’snotusefultomemorizeallpatternsbytype.Mostdevelopersdon’t

dothis.However,ifyou’renotsurewhetheraparticularpatternwill

work,it’ssometimesusefultoconsiderotherpatternsofthesame

type.Youjustmightfindonethatworksbetterforyourparticular

problem.

Note:There’sanongoingdebateonwhethersomepatterns,

includingMVVMandMVC,areactuallyarchitecturalpatterns,

whichspananentireapporsubsystemarchitecture.Hence,they

arebroaderthandesignpatterns,whichonlyspancomponentsor

piecesofanapp.Architecturalpatternscanevenuseor

encompassseveraldesignpatterns.

Forthepurposesofthisbook,acomprehensivediscussionof

architecturalpatternsisoutofscope.We’vechosentolabel

MVVMandMVCasstructuraldesignpatternsbecausetheycan

beusedalongsideotherdesignpatternsinacomponentfashion.

TheyarealsoverycommonlyusediniOSprojects,andwewanted

toensurewecoveredthem.

Ifsomeonesaystheseareactuallyarchitecturalpatterns,we

don’tnecessarilydisagree,astheycanalsobeusedthatway.

Criticismsofdesignpatterns

Asindicatedearlier,“therearenosilverbulletsinsoftware

development,”anddesignpatternsarenoexceptiontothis.This

meansthatjustknowingandemployingdesignpatternswillnot

guaranteethatyouhaveawellarchitectedpieceofsoftware.Thereare

hundredofdesignpatterns,andsoknowingwhenandhowtoemploy

eachoneisimportant.

Herearesomecommoncriticismsofdesignpatterns:

Ifyouoverusedesignpatterns,yourprojectcanbecome

overlycomplex.

Youneedtobecarefulaboutoverusinganytool,includingdesign

patterns.Youcanminimizethisissuebyclearlyandcorrectlydefining

theproblemtobesolvedbeforeaddingadesignpatterntoyour

project.

Manydesignpatternsaremaderedundantbymodern

programminglanguages.

It’struethatmodernprogramminglanguageslikeSwiftmakesome

designpatternsirrelevantortrivialtoimplement.However,just

becausesomepatternsareprovidedviaaprogramminglanguage

doesn’tmeanallpatternswillbe.

Designpatternsarealazysubstituteforlearningobject-

orientedprinciples.

Whynotlearnboth?Astrongunderstandingofobject-oriented

principleswillcertainlyhelpyouinyourdevelopment.However,if

youalreadyknowadesignpatternworkswellforaparticularproblem,

whyshouldyoureinventthesolutionfromscratch?

But,but…checkoutthisthreadonTwitter,whichdefinitely

showsthatdesignpatternsareworthless!

Regardlessoftheparticularcriticism,designpatternshavebeen

aroundforalongtime,andthey’vebeenusedinalotofapps.Soat

somepoint,you’regoingtoencounterthem.

Wethinkit’sbesttohaveanunderstandingofwhattheyarebefore

yourunintothem,insteadoftryingtowingitonthefly,whichinour

experienceisusuallylateonaSundaynight,thedaybeforethe

releasedeadline,rightafterdiscoveringacriticalbug.

Benefitsofdesignpatterns

We’vementionedmanybenefitsofdesignpatternsalready,butwe

wantedtopointoutafewmore:

Designpatternscreateacommonlanguage.

Insteadofdescribingaparticularsolutionindetail,youcansimply

statewhichdesignpatternyouthinkwouldworkbest.This

streamlinescommunicationbetweendevelopers.

Designpatternsfast-trackdeveloperonboarding.

It’smucheasiertoonboardanewdeveloperonaprojectthatuses

designpatternsthanaprojectwithcompletelycustomlogic.

Designpatternsmakeyouabetterperson.

Well,thisonemaystillbeupfordebate.Butsomedegreeofself-

improvementisneverwasted!However,thereisagrainoftruthto

this,asthenextdevelopertomaintainyourprojectwillcertainly

thinkyou’reabetterpersonforleavinganice,design-pattern-filled

projectinsteadofaspaghetti-codedmess!

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