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copied!<p>The Android Developer Guide has a section called <a href="http://developer.android.com/guide/topics/ui/custom-components.html" rel="noreferrer">Building Custom Components</a>. Unfortunately, <a href="http://developer.android.com/guide/topics/ui/custom-components.html#modifying" rel="noreferrer">the discussion of XML attributes</a> only covers declaring the control inside the layout file and not actually handling the values inside the class initialisation. The steps are as follows:</p> <h3>1. Declare attributes in <code>values\attrs.xml</code></h3> <pre class="lang-xml prettyprint-override"><code><?xml version="1.0" encoding="utf-8"?> <resources> <declare-styleable name="MyCustomView"> <attr name="android:text"/> <attr name="android:textColor"/> <attr name="extraInformation" format="string" /> </declare-styleable> </resources> </code></pre> <p>Notice the use of an unqualified name in the <code>declare-styleable</code> tag. Non-standard android attributes like <code>extraInformation</code> need to have their type declared. Tags declared in the superclass will be available in subclasses without having to be redeclared.</p> <h3>2. Create constructors</h3> <p>Since there are two constructors that use an <code>AttributeSet</code> for initialisation, it is convenient to create a separate initialisation method for the constructors to call. </p> <pre class="lang-java prettyprint-override"><code>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(); } </code></pre> <p><code>R.styleable.MyCustomView</code> is an autogenerated <code>int[]</code> 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, <code>R.styleable.MyCustomView_android_text</code> contains the <code>android_text</code> attribute for <code>MyCustomView</code>. Attributes can then be retrieved from the <code>TypedArray</code> using various <code>get</code> functions. If the attribute is not defined in the defined in the XML, then <code>null</code> is returned. Except, of course, if the return type is a primitive, in which case the second argument is returned.</p> <p>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 <code>android.R.attr</code>, while attributes for this project are in <code>R.attr</code>.</p> <pre class="lang-java prettyprint-override"><code>int attrsWanted[]=new int[]{android.R.attr.text, R.attr.textColor}; </code></pre> <p>Please note that you should <strong>not</strong> use anything in <code>android.R.styleable</code>, as per <a href="http://groups.google.com/group/android-developers/tree/browse_frm/thread/6554c6688f3ca6d9/8d018aa3f5c2beb9?rnum=1&_done=%2Fgroup%2Fandroid-developers%2Fbrowse_frm%2Fthread%2F6554c6688f3ca6d9%3Ftvc%3D1%26#doc_72f29c1df624bbed" rel="noreferrer">this thread</a> 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.</p> <h3>3. Use it in a layout files such as <code>layout\main.xml</code></h3> <p>Include the namespace declaration <code>xmlns:app="http://schemas.android.com/apk/res-auto"</code> 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 <a href="https://stackoverflow.com/questions/128389/what-are-xml-namespaces-forhttps://stackoverflow.com/questions/128389/what-are-xml-namespaces-for">this article</a> for more info). The URL is simply a manner of uniquely identifying schemas - <a href="https://stackoverflow.com/questions/5758041/why-are-urls-in-xml-namespaces">nothing actually needs to be hosted at that URL</a>. 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.</p> <pre class="lang-xml prettyprint-override"><code><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" /> </code></pre> <p>Reference the custom view using the fully qualified name.</p> <h3>Android LabelView Sample</h3> <p>If you want a complete example, look at the android label view sample.</p> <p><a href="http://developer.android.com/resources/samples/ApiDemos/src/com/example/android/apis/view/LabelView.html" rel="noreferrer">LabelView.java</a></p> <pre class="lang-java prettyprint-override"><code> TypedArray a=context.obtainStyledAttributes(attrs, R.styleable.LabelView); CharSequences=a.getString(R.styleable.LabelView_text); </code></pre> <p><a href="http://developer.android.com/resources/samples/ApiDemos/res/values/attrs.html" rel="noreferrer">attrs.xml</a></p> <pre class="lang-xml prettyprint-override"><code><declare-styleable name="LabelView"> <attr name="text"format="string"/> <attr name="textColor"format="color"/> <attr name="textSize"format="dimension"/> </declare-styleable> </code></pre> <p><a href="http://developer.android.com/resources/samples/ApiDemos/res/layout/custom_view_1.html" rel="noreferrer">custom_view_1.xml</a></p> <pre><code><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"/> </code></pre> <p>This is contained in a <code>LinearLayout</code> with a namespace attribute: <code>xmlns:app="http://schemas.android.com/apk/res-auto"</code></p> <h3>Links</h3> <ul> <li><a href="https://stackoverflow.com/questions/2460074/how-to-retrieve-xml-attribute-for-custom-control">StackOverflow Thread: Retrieving an XML attribute for custom control</a></li> <li><a href="https://stackoverflow.com/questions/2127177/how-do-i-use-obtainstyledattributesint-with-internal-themes-of-android">How do I use obtainStyledAttributes with internal themes of Android</a></li> <li><a href="https://stackoverflow.com/questions/3441396/defining-custom-attrs">Defining custom attributes + list of supported attribute formats</a></li> </ul>

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