Defining custom attrs

Question

I need to implement my own attributes like in com android R attr  Found nothing in official documentation so I need information about how to define these attrs and how to use them from my code

User · Answer

if you omit the format attribute from the attr element  you can use it to reference a class from XML layouts    example from attrs xml  Android Studio understands that the class is being referenced from XML   i e    Refactor  gt  Rename works Find Usages works and so on        don t specify a format attribute in     src main res values attrs xml   lt  xml version  1 0  encoding  utf-8   gt   lt resources gt        lt declare-styleable name  MyCustomView  gt                        lt attr name  give me a class   gt                    lt  declare-styleable gt    lt  resources gt    use it in some layout file     src main res layout activity  main menu xml   lt  xml version  1 0  encoding  utf-8   gt   lt SomeLayout     xmlns app  http   schemas android com apk res-auto  gt        lt  -- make sure to use   dollar signs for nested classes -- gt       lt MyCustomView         app give me a class  class type name Outer Nested  gt        lt MyCustomView         app give me a class  class type name AnotherClass  gt    lt  SomeLayout gt    parse the class in your view initialization code     src main java     MyCustomView kt  class MyCustomView          context Context          attrs AttributeSet       View context attrs           parse XML attributes              private val giveMeAClass SomeCustomInterface     init               context theme obtainStyledAttributes attrs R styleable ColorPreference 0 0  apply                         try                                  very important to use the class loader from the passed-in context                 giveMeAClass   context  class java classLoader                            loadClass getString R styleable MyCustomView give me a class                            newInstance      instantiate using 0-args constructor                          let  it as SomeCustomInterface                            finally                               recycle

User · Answer

Qberticus s answer is good  but one useful detail is missing  If you are implementing these in a library replace   xmlns whatever  http   schemas android com apk res org example mypackage    with   xmlns whatever  http   schemas android com apk res-auto    Otherwise the application that uses the library will have runtime errors

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The answer above covers everything in great detail  apart from a couple of things   First  if there are no styles  then the  Context context  AttributeSet attrs  method signature will be used to instantiate the preference  In this case just use context obtainStyledAttributes attrs  R styleable MyCustomView  to get the TypedArray   Secondly it does not cover how to deal with plaurals resources  quantity strings   These cannot be dealt with using TypedArray  Here is a code snippet from my SeekBarPreference that sets the summary of the preference formatting its value according to the value of the preference  If the xml for the preference sets android summary to a text string or a string resouce the value of the preference is formatted into the string  it should have  d in it  to pick up the value   If android summary is set to a plaurals resource  then that is used to format the result      Use your own name space if not using an android resource  final static private String ANDROID NS         http   schemas android com apk res android   private int pluralResource  private Resources resources  private String summary   public SeekBarPreference Context context  AttributeSet attrs                   TypedArray attributes   context obtainStyledAttributes          attrs  R styleable SeekBarPreference       pluralResource    attrs getAttributeResourceValue ANDROID NS   summary   0       if  pluralResource     0            if    resources getResourceTypeName pluralResource  equals  plurals                  pluralResource   0                      if  pluralResource     0            summary   attributes getString              R styleable SeekBarPreference android summary             attributes recycle        Override public CharSequence getSummary         int value   getPersistedInt defaultValue       if  pluralResource    0            return resources getQuantityString pluralResource  value  value             return  summary    null    null   String format summary  value          This is just given as an example  however  if you want are tempted to set the summary on the preference screen  then you need to call notifyChanged   in the preference s onDialogClosed method

User · Answer

Currently the best documentation is the source  You can take a look at it here  attrs xml     You can define attributes in the top  lt resources gt  element or inside of a  lt declare-styleable gt  element  If I m going to use an attr in more than one place I put it in the root element  Note  all attributes share the same global namespace  That means that even if you create a new attribute inside of a  lt declare-styleable gt  element it can be used outside of it and you cannot create another attribute with the same name of a different type   An  lt attr gt  element has two xml attributes name and format  name lets you call it something and this is how you end up referring to it in code  e g   R attr my attribute  The format attribute can have different values depending on the  type  of attribute you want     reference - if it references another resource id  e g    color my color     layout my layout   color boolean dimension float integer string fraction enum - normally implicitly defined flag - normally implicitly defined   You can set the format to multiple types by using    e g   format  reference color    enum attributes can be defined as follows    lt attr name  my enum attr  gt     lt enum name  value1  value  1    gt     lt enum name  value2  value  2    gt   lt  attr gt    flag attributes are similar except the values need to be defined so they can be bit ored together    lt attr name  my flag attr  gt     lt flag name  fuzzy  value  0x01    gt     lt flag name  cold  value  0x02    gt   lt  attr gt    In addition to attributes there is the  lt declare-styleable gt  element  This allows you to define attributes a custom view can use  You do this by specifying an  lt attr gt  element  if it was previously defined you do not specify the format  If you wish to reuse an android attr  for example  android gravity  then you can do that in the name  as follows   An example of a custom view  lt declare-styleable gt     lt declare-styleable name  MyCustomView  gt     lt attr name  my custom attribute    gt     lt attr name  android gravity    gt   lt  declare-styleable gt    When defining your custom attributes in XML on your custom view you need to do a few things  First you must declare a namespace to find your attributes  You do this on the root layout element  Normally there is only xmlns android  http   schemas android com apk res android   You must now also add xmlns whatever  http   schemas android com apk res-auto    Example    lt  xml version  1 0  encoding  utf-8   gt   lt LinearLayout   xmlns android  http   schemas android com apk res android    xmlns whatever  http   schemas android com apk res-auto    android orientation  vertical    android layout width  fill parent    android layout height  fill parent  gt        lt org example mypackage MyCustomView       android layout width  fill parent        android layout height  wrap content        android gravity  center        whatever my custom attribute  Hello  world     gt   lt  LinearLayout gt    Finally  to access that custom attribute you normally do so in the constructor of your custom view as follows   public MyCustomView Context context  AttributeSet attrs  int defStyle      super context  attrs  defStyle      TypedArray a   context obtainStyledAttributes attrs  R styleable MyCustomView  defStyle  0      String str   a getString R styleable MyCustomView my custom attribute        do something with str    a recycle        The end

User · Answer

The traditional approach is full of boilerplate code and clumsy resource handling  That s why I made the Spyglass framework  To demonstrate how it works  here s an example showing how to make a custom view that displays a String title   Step 1  Create a custom view class   public class CustomView extends FrameLayout       private TextView titleView       public CustomView Context context            super context           init null  0  0              public CustomView Context context  AttributeSet attrs            super context  attrs           init attrs  0  0              public CustomView Context context  AttributeSet attrs  int defStyleAttr            super context  attrs  defStyleAttr           init attrs  defStyleAttr  0               RequiresApi 21      public CustomView              Context context               AttributeSet attrs              int defStyleAttr              int defStyleRes             super context  attrs  defStyleAttr  defStyleRes           init attrs  defStyleAttr  defStyleRes              public void setTitle String title            titleView setText title              private void init AttributeSet attrs  int defStyleAttr  int defStyleRes            inflate getContext    R layout custom view  this            titleView   findViewById R id title view             Step 2  Define a string attribute in the values attrs xml resource file    lt resources gt       lt declare-styleable name  CustomView  gt           lt attr name  title  format  string   gt       lt  declare-styleable gt   lt  resources gt    Step 3  Apply the  StringHandler annotation to the setTitle method to tell the Spyglass framework to route the attribute value to this method when the view is inflated    HandlesString attributeId   R styleable CustomView title  public void setTitle String title        titleView setText title       Now that your class has a Spyglass annotation  the Spyglass framework will detect it at compile-time and automatically generate the CustomView SpyglassCompanion class   Step 4  Use the generated class in the custom view s init method   private void init AttributeSet attrs  int defStyleAttr  int defStyleRes        inflate getContext    R layout custom view  this        titleView   findViewById R id title view        CustomView SpyglassCompanion              builder                withTarget this               withContext getContext                 withAttributeSet attrs               withDefaultStyleAttribute defStyleAttr               withDefaultStyleResource defStyleRes               build                callTargetMethodsNow        That s it  Now when you instantiate the class from XML  the Spyglass companion interprets the attributes and makes the required method call  For example  if we inflate the following layout then setTitle will be called with  Hello  World   as the argument    lt FrameLayout     xmlns android  http   schemas android com apk res android      xmlns app  http   schemas android com apk res-auto      android width  match parent      android height  match parent  gt        lt com example CustomView         android width  match parent          android height  match parent          app title  Hello  World    gt   lt  FrameLayout gt    The framework isn t limited to string resources has lots of different annotations for handling other resource types  It also has annotations for defining default values and for passing in placeholder values if your methods have multiple parameters   Have a look at the Github repo for more information and examples

[android] Defining custom attrs

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