pretty

Pretty is a library that enables one to enhance Android layout inflation without boilerplate and knowledge about LayoutInflater internals. Pretty hooks into the Android layout inflation so you could add new attributes to existing View subtypes.

Example

Lets take a standard example - using a custom font with TextViews. If you have used a custom font chances are that you have something like this in your codebase:

TextView text1 = (TextView) findViewById(R.id.text1);
Typeface tf = Typeface.createFromAsset(getResources().getAssets(), "my-font.ttf");
text1.setTypeface(tf);

This approach doesn't really scale that well once you have a bunch of complex layouts. This usually results in either creating a TextView subclass or some trickery with walking the view tree and replacing the typeface for all TextView instances.

Also one shouldn't mix presentation definition (the font) with logic (the Java code) if at all possible. Wouldn't it be nice if you could just point to your custom font right there in the layout? E.g.:

<TextView android:id="@+id/text1"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:typeface_asset="my-font.ttf" />

With some LayoutInflater trickery it is actually possible. Pretty does all the trickery for you so you can focus on the code that matters. Lets write a pretty decorator that enables us to use a custom attribute to load the typeface.

First off we need to define our custom attribute in res/values/attrs.xml:

<?xml version="1.0" encoding="utf-8"?>
<resources>
  <attr name="typeface_asset" format="string" />
</resources>

After having a custom attribute we can reference it in java code through the R.attr class and in our layouts using the res-auto package namespace. Now we can write a decorator that uses the attribute to change the typeface for any TextView instances.

We could write a general Decor implementation but there is a base class for writing decorators that work explicitly with attributes - AttrsDecor<T extends View>. Lets use it to implement our typeface_asset attribute:

public class FontDecor extends AttrsDecor<TextView> {
 @Override
 protected int[] attrs() {
    // return the attrs this decorator applies to
  return new int[] { R.attr.typeface_asset };
 }

 @Override
 protected Class<TextView> clazz() {
    // we want to modify all the TextView instances. This includes descendants,
    // like Button or EditText.
  return TextView.class;
 }

 @Override
 protected void apply(TextView view, int attr, TypedValue value) {
    // this method is called if we have a TextView with any of the attributes
    // we previously declared. Usually one would have a switch here but as this
    // decorator only supports a single attr we don't need to do that as we know
    // it can only be R.attr.typeface_asset.
    // So lets just replace the typeface here.
  view.setTypeface(Typeface.createFromAsset(view.getResources().getAssets(), value.string.toString()));
 }
}

Now we can use our attribute in a layout:

<LinearLayout
  xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:auto="http://schemas.android.com/apk/res-auto"
  android:orientation="vertical"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:padding="16dp">
  <TextView
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="Woop, custom fonts!"
    auto:typeface_asset="my-font.ttf" />
</LinearLayout>

Pretty straightforward, right? Now all we need to do is to attach pretty to an activity so it can take care of all the LayoutInflater magic. We also need to register our new shiny decorator.

public class SampleActivity extends Activity {
 @Override
 protected void onCreate(Bundle savedInstanceState) {
  Pretty.wrap(this).with(new FontDecor());
  super.onCreate(savedInstanceState);
  setContentView(R.layout.main);
 }
}

You can try out this code by checking out the repository and looking at the samples project.

How pretty works

Pretty does its magic by replacing the view Factory in the LayoutInflater. This idea was borrowed from the android support library - thats how the support library enables one to use the <fragment> tags in OS versions that do not know about the tag.

Limitations

Currently using attributes with the auto prefix for system widgets raises a lint error Unexpected namespace prefix "auto" found for tag T. Fear not, this is a red herring. The solution is to suppress that check in your gradle buildfile:

android {
  lintOptions {
    disable 'MissingPrefix'
  }
}

License

Standard MIT. See the LICENSE file.