Declaring a custom android UI element using XML

Question

How do I declare an Android UI element using XML

User · Answer

Addition to most voted answer   obtainStyledAttributes    I want to add some words about obtainStyledAttributes   usage  when we create custom view using android xxx prdefined attributes  Especially when we use TextAppearance  As was mentioned in  2  Creating constructors   custom view gets AttributeSet on its creation  Main usage we can see in TextView source code  API 16    final Resources Theme theme   context getTheme        TextAppearance is inspected first  but let observe it later  TypedArray a   theme obtainStyledAttributes              attrs  com android internal R styleable TextView  defStyle  0    int n   a getIndexCount    for  int i   0  i  lt  n  i           int attr   a getIndex i          huge switch with pattern value a getXXX attr   lt   gt  a getXXX a getIndex i     a recycle      What we can see here  obtainStyledAttributes AttributeSet set  int   attrs  int defStyleAttr  int defStyleRes  Attribute set is processed by theme according to documentation  Attribute values are compiled step by step  First attributes are filled from theme  then values are replaced by values from style  and finally exact values from XML for special view instance replace others  Array of requested attributes - com android internal R styleable TextView It is an ordinary array of constants  If we are requesting standard attributes  we can build this array manually   What is not mentioned in documentation - order of result TypedArray elements  When custom view is declared in attrs xml  special constants for attribute indexes are generated  And we can extract values this way  a getString R styleable MyCustomView android text   But for manual int   there are no constants  I suppose  that getXXXValue arrayIndex  will work fine   And other question is   How we can replace internal constants  and request standard attributes   We can use android R attr   values   So if we want to use standard TextAppearance attribute in custom view and read its values in constructor  we can modify code from TextView this way   ColorStateList textColorApp   null  int textSize   15  int typefaceIndex   -1  int styleIndex   -1   Resources Theme theme   context getTheme     TypedArray a   theme obtainStyledAttributes attrs  R styleable CustomLabel  defStyle  0   TypedArray appearance   null  int apResourceId   a getResourceId R styleable CustomLabel android textAppearance  -1   a recycle    if  apResourceId    -1        appearance            theme obtainStyledAttributes apResourceId  new int     android R attr textColor  android R attr textSize               android R attr typeface  android R attr textStyle       if  appearance    null        textColorApp   appearance getColorStateList 0       textSize   appearance getDimensionPixelSize 1  textSize       typefaceIndex   appearance getInt 2  -1       styleIndex   appearance getInt 3  -1        appearance recycle        Where CustomLabel is defined    lt declare-styleable name  CustomLabel  gt       lt  -- Label text  -- gt       lt attr name  android text    gt       lt  -- Label text color  -- gt       lt attr name  android textColor    gt       lt  -- Combined text appearance properties  -- gt       lt attr name  android textAppearance    gt   lt  declare-styleable gt    Maybe  I m mistaken some way  but Android documentation on obtainStyledAttributes   is very poor   Extending standard UI component  At the same time we can just extend standard UI component  using all its declared attributes  This approach is not so good  because TextView for instance declares a lot of properties  And it will be impossible to implement full functionality in overriden onMeasure   and onDraw     But we can sacrifice theoretical wide reusage of custom component  Say  I know exactly what features I will use   and don t share code with anybody   Then we can implement constructor CustomComponent Context  AttributeSet  defStyle    After calling super      we will have all attributes parsed and available through getter methods

User · Answer

You can include any layout file in other layout file as-                lt RelativeLayout                 android layout width  wrap content                  android layout height  wrap content                  android layout marginLeft  10dp                  android layout marginRight  30dp   gt                    lt include                     android id    id frnd img file                      android layout width  wrap content                      android layout height  wrap content                      layout   layout include imagefile   gt                    lt include                     android id    id frnd video file                      android layout width  wrap content                      android layout height  wrap content                      layout   layout include video lay    gt                    lt ImageView                     android id    id downloadbtn                      android layout width  30dp                      android layout height  30dp                      android layout centerInParent  true                      android src   drawable plus   gt               lt  RelativeLayout gt    here the layout files in include tag are other  xml layout files in the same res folder

User · Answer

The Android Developer Guide has a section called Building Custom Components  Unfortunately  the discussion of XML attributes only covers declaring the control inside the layout file and not actually handling the values inside the class initialisation  The steps are as follows   1  Declare attributes in values attrs xml   lt  xml version  1 0  encoding  utf-8   gt   lt resources gt       lt declare-styleable name  MyCustomView  gt           lt attr name  android text   gt           lt attr name  android textColor   gt                       lt attr name  extraInformation  format  string    gt       lt  declare-styleable gt   lt  resources gt    Notice the use of an unqualified name in the declare-styleable tag  Non-standard android  attributes like extraInformation need to have their type declared  Tags declared in the  superclass will be available in subclasses without having to be redeclared   2  Create constructors  Since there are two constructors that use an AttributeSet for initialisation  it is convenient to create a separate initialisation method for the constructors to call    private void init AttributeSet attrs         TypedArray a getContext   obtainStyledAttributes           attrs           R styleable MyCustomView          Use a     Log i  test  a getString           R styleable MyCustomView android text        Log i  test     a getColor           R styleable MyCustomView android textColor  Color BLACK        Log i  test  a getString           R styleable MyCustomView extraInformation           Don t forget this     a recycle        R styleable MyCustomView is an autogenerated int   resource where each element is the ID of an attribute  Attributes are generated for each property in the XML by appending the attribute name to the element name  For example  R styleable MyCustomView android text contains the android text attribute for MyCustomView  Attributes can then be retrieved from the TypedArray using various get functions  If the attribute is not defined in the defined in the XML  then null is returned  Except  of course  if the return type is a primitive  in which case the second argument is returned   If you don t want to retrieve all of the attributes  it is possible to create this array manually The ID for standard android attributes are included in android R attr  while attributes for this project are in R attr   int attrsWanted   new int   android R attr text  R attr textColor     Please note that you should not use anything in android R styleable  as per this thread it may change in the future  It is still in the documentation as being to view all these constants in the one place is useful   3  Use it in a layout files such as layout main xml  Include the namespace declaration xmlns app  http   schemas android com apk res-auto  in the top level xml element  Namespaces provide a method to avoid the conflicts that sometimes occur when different schemas use the same element names  see this article for more info   The URL is simply a manner of uniquely identifying schemas - nothing actually needs to be hosted at that URL  If this doesn t appear to be doing anything  it is because you don t actually need to add the namespace prefix unless you need to resolve a conflict    lt com mycompany projectname MyCustomView     android layout width  wrap content      android layout height  wrap content      android background   android color transparent      android text  Test text      android textColor   FFFFFF      app extraInformation  My extra information    gt     Reference the custom view using the fully qualified name   Android LabelView Sample  If you want a complete example  look at the android label view sample   LabelView java   TypedArray a context obtainStyledAttributes attrs  R styleable LabelView    CharSequences a getString R styleable LabelView text     attrs xml   lt declare-styleable name  LabelView  gt       lt attr name  text format  string   gt       lt attr name  textColor format  color   gt       lt attr name  textSize format  dimension   gt   lt  declare-styleable gt    custom view 1 xml   lt com example android apis view LabelView     android background   drawable blue      android layout width  fill parent      android layout height  wrap content      app text  Blue  app textSize  20dp   gt    This is contained in a LinearLayout with a namespace attribute  xmlns app  http   schemas android com apk res-auto   Links   StackOverflow Thread  Retrieving an XML attribute for custom control How do I use obtainStyledAttributes with internal themes of Android Defining custom attributes   list of supported attribute formats

User · Answer

Thanks a lot for the first answer   As for me  I had just one problem with it  When inflating my view  i had a bug   java lang NoSuchMethodException   MyView Context  Attributes   I resolved it by creating a new constructor    public MyView Context context  AttributeSet attrs         super context  attrs           some code     Hope this will help

User · Answer

Great reference  Thanks  An addition to it   If you happen to have a library project included which has declared custom attributes for a custom view  you have to declare your project namespace  not the library one s  Eg   Given that the library has the package  com example library customview  and the working project has the package  com example customview   then   Will not work  shows the error   error  No resource identifier found for attribute  newAttr  in package        com example library customview        lt com library CustomView         xmlns android  http   schemas android com apk res android          xmlns app  http   schemas android com apk res com example library customview          android id    id myView          app newAttr  value    gt    Will work    lt com library CustomView         xmlns android  http   schemas android com apk res android          xmlns app  http   schemas android com apk res com example customview          android id    id myView          app newAttr  value    gt

User · Answer

It seems that Google has updated its developer page and added various trainings there    One of them deals with the creation of custom views and can be found here

[xml] Declaring a custom android UI element using XML

obtainStyledAttributes()

Extending standard UI component

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