来谈谈：英文教程:CSS的Float

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网页制造poluoluo文章简介：英文教程:CSS的Float.
Whatis“Float”?

FloatisaCSSpositioningproperty.Tounderstanditspurposeandorigin,wecanlooktoprintdesign.Inaprintlayout,imagesmaybesetintothepagesuchthattextwrapsaroundthemasneeded.Thisiscommonlyandappropriatelycalled“textwrap”.Hereisanexampleofthat.

Inpagelayoutprograms,theboxesthatholdthetextcanbetoldtohonorthetextwrap,ortoignoreit.Ignoringthetextwrapwillallowthewordstoflowrightovertheimagelikeitwasn’teventhere.Thisisthedifferencebetweenthatimagebeingpartoftheflowofthepage(ornot).Webdesignisverysimilar.

Inwebdesign,pageelementswiththeCSSfloatpropertyappliedtothemarejustliketheimagesintheprintlayoutwherethetextflowsaroundthem.Floatedelementsremainapartoftheflowofthewebpage.Thisisdistinctlydifferentthanpageelementsthatuseabsolutepositioning.Absolutelypositionedpageelementsareremovedfromtheflowofthewebpage,likewhenthetextboxintheprintlayoutwastoldtoignorethepagewrap.Absolutelypositionedpageelementswillnotaffectthepositionofotherelementsandotherelementswillnotaffectthem,whethertheytoucheachotherornot.

SettingthefloatonanelementwithCSShappenslikethis:

1. #sidebar{float:right;}

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Therearefourvalidvaluesforthefloatproperty.LeftandRightfloatelementsthosedirectionsrespectively.None(thedefault)ensurestheelementwillnotfloatandInheritwhichwillassumethefloatvaluefromthatelementsparentelement.
Whatarefloatsusedfor?

Asidefromthesimpleexampleofwrappingtextaroundimages,floatscanbeusedtocreateentireweblayouts.

Floatsarealsohelpfulforlayoutinsmallerinstances.Takeforexamplethislittleareaofawebpage.Ifweusefloatforourlittleavatarimage,whenthatimagechangessizethetextintheboxwillreflowtoaccomodate:

Thissamelayoutcouldbeaccomplishedusingrelativepositioningoncontainerandabsolutepositioningontheavataraswell.Indoingitthisway,thetextwouldbeunafftedbytheavatarandnotbeabletoreflowonasizechange.

ClearingtheFloat

Float’ssisterpropertyisclear.Anelementthathastheclearpropertysetonitwillnotmoveupadjacenttothefloatlikethefloatdesires,butwillmoveitselfdownpastthefloat.Againanillustrationprobablydoesmoregoodthanwordsdo.

Intheaboveexample,thesidebarisfloatedtotherightandisshorterthanthemaincontentarea.Thefooterthenisrequiredtojumpupintothatavailablespaceasisrequiredbythefloat.Tofixthisproblem,thefootercanbeclearedtoensureitstaysbeneathbothfloatedcolumns.

1. #footer{clear:both;}

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Clearhasfourvalidvaluesaswell.Bothismostcommonlyused,whichclearsfloatscomingfromeitherdirection.LeftandRightcanbeusedtoonlyclearthefloatfromonedirectionrespectively.Noneisthedefault,whichistypicallyunnecessaryunlessremovingaclearvaluefromacascade.Inheritwouldbethefifth,butisstranglynotsupportedinInternetExplorer.Clearingonlytheleftorrightfloat,whilelesscommonlyseeninthewild,definitelyhasitsuses.

TheGreatCollapse

Oneofthemorebewilderingthingsaboutworkingwithfloatsishowtheycanaffecttheelementthatcontainsthem(their“parent”element).Ifthisparentelementcontainednothingbutfloatedelements,theheightofitwouldliterallycollapsetonothing.Thisisn’talwaysobviousiftheparentdoesn’tcontainanyvisuallynoticeablebackground,butitisimportanttobeawareof.

Asanti-intuitiveascollapsingseemstobe,thealternativeisworse.Considerthisscenario:

Iftheblockelementontopwheretohaveautomaticallyexpandedtoaccomodatethefloatedelement,wewouldhaveanunnaturalspacingbreakintheflowoftextbetweenparagraphs,withnopracticalwayoffixingit.Ifthiswerethecase,usdesignerswouldbecomplainingmuchharderaboutthisbehaviorthanwedoaboutcollapsing.
Collapsingalmostalwaysneedstobedealtwithtopreventstrangelayoutandcross-browserproblems.Wefixitbyclearingthefloatafterthefloatedelementsinthecontainerbutbeforethecloseofthecontainer.
TechniquesforClearingFloats

Ifyouareinasituationwhereyoualwaysknowwhatthesucceedingelementisgoingtobe,youcanapplytheclear:both;valuetothatelementandgoaboutyourbusiness.Thisisidealasitrequiresnofancyhacksandnoadditionalelementsmakingitperfectlysemantic.Ofcoursethingsdon’ttypicallyworkoutthatwayandweneedtohavemorefloat-clearingtoolsinourtoolbox.

• TheEmptyDivMethodis,quiteliterally,anemptydiv.<divstyle="clear:both;"><div>.Sometimesyou’llseea<br/>elementorsomeotherrandomelementused,butdivisthemostcommonbecauseithasnobrowerdefaultstyling,doesn’thaveanyspecialfunction,andisunlikelytobegenericallystyledwithCSS.Thismethodisscornedbysemanticpuristssinceitspresencehasnocontexualmeaningatalltothepageandistherepurelyforpresentation.Ofcourseinthestrictestsensetheyareright,butitgetsthejobdonerightanddoesn’thurtanybody.
  
• TheOverflowMethodreliesonsettingtheoverflowCSSpropertyonaparentelement.Ifthispropertyissettoautoorhiddenontheparentelement,theparentwillexpandtocontainthefloats,effectivelyclearingitforsucceedingelements.Thismethodcanbebeautifullysemanticasitmaynotrequireanadditionalelements.Howeverifyoufindyourselfaddinganewdivjusttoapplythis,itisequallyasunsemanticastheemptydivmethodandlessadaptable.Alsobearinmindthattheoverflowpropertyisn’tspecificallyforclearingfloats.Becarefulnottohidecontentortriggerunwantedscrollbars.
  
• TheEasyClearingMethodusesacleverCSSpseduoselector(:after)toclearfloats.Ratherthansettingtheoverflowontheparent,youapplyanadditionalclasslike“clearfix”toit.ThenapplythisCSS:
  1. .clearfix:after{content:".";visibility:hidden;display:block;height:0;clear:both;}

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  Thiswillapplyasmallbitofcontent,hiddenfromview,aftertheparentelementwhichclearsthefloat.Thisisn’tquitethewholestory,asadditionalcodeneedstobeusedtoaccomodateforolderbrowsers.
  
  
  Differentscenarioscallfordifferentfloatclearningmethods.Takeforexampleagridofblocks,eachofdifferenttypes.
  
  Tobettervisuallyconnectthesimilarblocks,wewanttostartanewrowasweplease,inthiscasewhenthecolorchanges.Wecoulduseeithertheoverfloworeasyclearningmethodifeachofthecolorgroupshadaparentelement.Or,weusetheemptydivmethodinbetweeneachgroup.Threewrappingdivsthatdidn’tpreviouslyexistorthreeafterdivsthatdidn’tpreviouslyexist.I’llletyoudecidewhichisbetter.
  
  ProblemswithFloats
  
  Floatsoftengetbeatonforbeingfragile.ThemajorityofthisfragilitycomesfromIE6andtheslewoffloat-relatedbugsithas.AsmoreandmoredesignersaredroppingsupportforIE6,youmaynotcare,butforthefolksthatdocarehereisaquickrundown.
  
  
  
  
  • Pushdownisasymptomofanelementinsideafloateditembeingwiderthanthefloatitself(typicallyanimage).Mostbrowserswillrendertheimageoutsidethefloat,butnothavethepartstickingoutaffectotherlayout.IEwillexpandthefloattocontaintheimage,oftendrasticallyaffectinglayout.Acommonexampleisanimagestickingoutofthemaincontentpushthesidebardownbelow.
    Quickfix:Makesureyoudon’thaveanyimagesthatdothis,useoverflow:hiddentocutoffexcess.
    
    
  • DoubleMarginBug&ndash;AnotherthingtorememberwhendealingwithIE6isthatifyouapplyamargininthesamedirectionasthefloat,itwilldoublethemargin.Quickfix:setdisplay:inlineonthefloat,anddon’tworryitwillremainablock-levelelement.
    
  • The3pxJogiswhentextthatisupnexttoafloatedelementismysteriouslykickedawayby3pxlikeaweirdforcefieldaroundthefloat.Quickfix:setawidthorheightontheaffectedtext.
    
  • InIE7,theBottomMarginBugiswhenifafloatedparenthasfloatedchildreninsideit,bottommarginonthosechildrenisignoredbytheparent.Quickfix:usingbottompaddingontheparentinstead.
    
  Alternatives
  
  Ifyouneedtextwrappingaroundimages,therereallyaren’tanyalternativesforfloat.Speakingofwhich,checkoutthisratherclevertechniqueforwrappingtextaroundirregularshapes.Butforpagelayout,theredefinitelyarechoices.EricSolrighthereonAListAparthasanarticleonFauxAbsolutePositioning,whichisaveryinterestingtechniquethatinmanywayscombinestheflexibilityoffloatswiththestrengthofabsolutepositioning.CSS3hastheTemplateLayoutModulethat,whenwidelyadopted,willprovetobethepagelayouttechniqueofchoice.
  
  
  
  

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