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

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 element or inside of a 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 element it can be used outside of it and you cannot create another attribute with the same name of a different type.

An 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:

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

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

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:

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. :)