Why this line xmlns android http schemas android com apk res android must be the first in the layout xml file

Question

Why is this line needed in xml layout file   xmlns android  http   schemas android com apk res android

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In XML  element names are defined by the developer  This often results in a conflict when trying to mix XML documents from different XML applications  A user or an XML application will not know how to handle these differences  Name conflicts in XML can easily be avoided using a name prefix  When using prefixes in XML  a namespace for the prefix must be defined The namespace can be defined by an xmlns attribute in the start tag of an element The namespace declaration has the following syntax  xmlns prefix  URI

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xmlns android This is start tag for define android namespace in Android  This is standerd convention define by android google developer  when you are using and layout default or custom  then must use this namespace   Defines the Android namespace  This attribute should always be set to  quot http   schemas android com apk res android quot    From the  lt manifest gt  element documentation

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To understand why xmlns android    http   schemas android com apk res android    must be the first in the layout xml file We shall understand the components using an example  Sample     lt FrameLayout xmlns android  http   schemas android com apk res android      android layout width  match parent      android layout height  match parent      android id    id container   gt       lt  FrameLayout gt      Uniform Resource Indicator URI      In computing  a uniform resource identifier  URI  is a string of characters used to identify a name of a resource  Such identification enables interaction with representations of the resource over a network  typically the World Wide Web  using specific protocols    Ex http   schemas android com apk res android id is the URI here    XML Namespace    XML namespaces are used for providing uniquely named elements and attributes in an XML document  xmlns android describes the android namespace  Its used like this because this is a design choice by google to handle the errors at compile time  Also suppose we write our own textview widget with different features compared to android textview  android namespace helps to distinguish between our custom textview widget and android textview widget

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In XML  xmlns declares a Namespace  In fact  when you do    lt LinearLayout android id gt   lt  LinearLayout gt    Instead of calling android id  the xml will use http   schemas android com apk res android id to be unique  Generally this page doesn t exist  it s a URI  not a URL   but sometimes it is a URL that explains the used namespace   The namespace has pretty much the same uses as the package name in a Java application   Here is an explanation      Uniform Resource Identifier  URI       A Uniform Resource Identifier  URI  is a string of characters which   identifies an Internet Resource       The most common URI is the Uniform Resource Locator  URL  which   identifies an Internet domain address  Another  not so common type of   URI is the Universal Resource Name  URN     In our examples we will only use URLs

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Xmlns means xml namespace  It is created to avoid naming conflicts in the xml   s  In order to avoid naming conflicts by any other way we need to provide each element with a prefix  to avoid repetitive usage of the prefix in each xml tag we use xmlns at the root of the xml  Hence we have the tag xmlns android    http   schemas android com apk res android    Now android here simply means we are assigning the namespace    http   schemas android com apk res android    to it  This namespace is not a URL but a URI also known as URN universal resource name  which is rarely used in place of URI  Due to this android will be responsible to identify the android related elements in the xml document which would be android xxxxxxx etc  Without this namespace android xxxxxxx will be not recognized    To put in layman   s term    without xmlns android    http   schemas android com apk res android    android related tags wont be recognized in the xml document of our layout

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I think it makes clear with the namespace  as we can create our own attributes and if the user specified attribute is the same as the Android one it avoid the conflict of the namespace

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xmlns android  http   schemas android com apk res android    This is form of  xmlns android      id   Now to refernce it we use for example  android layout width  wrap content  android text  Hello World     Another xmlns is   xmlns app  http   schemas android com apk res-auto    which is in form of xmlns app       id  and its use is given below   app layout constraintBottom toBottomOf  parent   app layout constraintLeft toLeftOf  parent

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xmlns android Defines the Android namespace  This attribute should always be set to  quot http   schemas android com apk res android quot    refer https   developer android com guide topics manifest manifest-element nspace

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This is just the XML Name Space declaration  We use this Name Space in order to specify that the attributes listed below  belongs to Android  Thus they starts with  android    You can actually create your own custom attributes  So to prevent the name conflicts where 2 attributes are named the same thing  but behave differently  we add the prefix  android   to signify that these are Android attributes   Thus  this Name Space declaration must be included in the opening tag of the root view of your XML file

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xmlns refers to the XML namespace  When using prefixes in XML  a so-called namespace for the prefix must be defined  The namespace is defined by the xmlns attribute in the start tag of an element  The namespace declaration has the following syntax  xmlns prefix  URI    Note  The namespace URI is not used by the parser to look up information   The purpose is to give the namespace a unique name  However  often companies use the namespace as a pointer to a web page containing namespace information

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xmlns android  http   schemas android com apk res android     xmlns   is xml name space   and the URL    http   schemas android com apk res android  is nothing but   XSD which is  XML schema definition    which is used define rules for XML file    Example      lt LinearLayout  xmlns android  http   schemas android com apk res android  android orientation  vertical   android layout width  match parent  android layout height  match parent  gt    lt EditText    android layout width  match parent     android layout height  wrap content     android layout marginBottom  4dp     android hint  User Name      gt   lt  LinearLayout gt     Let me explain What Kind of Rules        In above XML file we already define layout width for our layout now IF you will define same attribute second time you will get an error    EditText is there but if you want add another EditText no problem      Such Kind of Rules are define in XML XSD    http   schemas android com apk res android   little bit late but I hope this helps you

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It is a XML name space declaration in order to specify that the attributes that are within the view group that it s decelerated in are android related

[android] Why this line xmlns:android="http://schemas.android.com/apk/res/android" must be the first in the layout xml file?

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