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.

Answer 1

Currently the best documentation is the source. You can take a look at it here (attrs.xml).

You can define attributes in the top <resources> element or inside of a <declare-styleable> 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 <declare-styleable> element it can be used outside of it and you cannot create another attribute with the same name of a different type.

An <attr> 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:

<attr name="my_enum_attr">   <enum name="value1" value="1" />   <enum name="value2" value="2" /> </attr> 

flag attributes are similar except the values need to be defined so they can be bit ored together:

<attr name="my_flag_attr">   <flag name="fuzzy" value="0x01" />   <flag name="cold" value="0x02" /> </attr> 

In addition to attributes there is the <declare-styleable> element. This allows you to define attributes a custom view can use. You do this by specifying an <attr> 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 <declare-styleable>:

<declare-styleable name="MyCustomView">   <attr name="my_custom_attribute" />   <attr name="android:gravity" /> </declare-styleable> 

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:

<?xml version="1.0" encoding="utf-8"?> <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">      <org.example.mypackage.MyCustomView       android:layout_width="fill_parent"       android:layout_height="wrap_content"       android:gravity="center"       whatever:my_custom_attribute="Hello, world!" /> </LinearLayout> 

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. :)

Answer 2

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.