Hijckr

Misappropriating Your Android XML Tags since 2018

What does it do?

Hijckr interfers with Android's layout inflation and redirects named elements to other classes. For example for layout file contains Android would normally load android.widget.TextView, instead we can hijack xml tags to load com.myapp.TextView.

Getting Started

Add gradle dependency to your build.gradle file:

dependencies {
    implementation 'com.justinangel:hijckr:1+'
}

Activity-Specific Hijacking

Let's say we want all EditText's in a given activity to replace all ":)" apperances with the 😀emjoi. For that, we can subclass EditText, listen to text changes and whenever we find ":)" replace with the appropriate emoji. With Hijckr it's possible to implicitly route all usages of within an activity to our SmileyEditText.

Here's our SmileyEditText class.

public class SmileyEditText extends EditText {
    // ... c'tors 

    @Override
    protected void onTextChanged(CharSequence text, int start, int lengthBefore, int lengthAfter) {
        super.onTextChanged(text, start, lengthBefore, lengthAfter);

        if (text.toString().contains(":)")) {
            setText(text.toString().replace(":)", "\uD83D\uDE0A"));
        }
    }
}

Here's our MainActivity laoyut XML:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Type a smiley face"/>
</LinearLayout>

We'll now Hijack the EditText tag by subclassing getClassLoader and using a class loader that routes from EditText to SmileyEditText.

import com.justinangel.hijckr.HijckrClassLoader;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    @Override
    public ClassLoader getClassLoader() {
        return new HijckrClassLoader(super.getClassLoader())
                .withClassRouting(EditText.class, SmileyEditText.class);
    }
}

And here's the result:

Global Hijacking

Setting XML tag routing for each activity feels cumbersome. So any time prior to inflating XML it's possible to change the global routing of XML Tags. A best practice would be to do that during Application.onCreate. For example we can route all of our tags to load an inherting class AppTextView.

import com.justinangel.hijckr.HijckrClassLoader;

public class App extends Application {
    @Override
    public void onCreate() {
        super.onCreate();

        HijckrClassLoader.addGlobalClassRouting(TextView.class, AppTextView.class);
    }
}

In our MainActivity we'll change the base class to HijckrActivity.

import com.justinangel.hijckr.HijckrActivity;

public class MainActivity extends HijckrActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

We'll also include a tag in our layout XML.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!" />
</LinearLayout>

And for the class we want to use to replace our TextView we'll make the text Red and Font size 18.

package com.justinangel.hijckrsampleapp;

import android.content.Context;
import android.graphics.Color;
import android.util.AttributeSet;
import android.widget.TextView;

public class AppTextView extends TextView {
    public AppTextView(Context context) {
        super(context);
        init();
    }

    public AppTextView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    public AppTextView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init();
    }

    private void init() {
        this.setTextSize(28);
        this.setTextColor(Color.RED);
    }
}

Don't forget to wire up our custom Android Application class. '

   <manifest xmlns:android="http://schemas.android.com/apk/res/android"
        package="com.justinangel.hijckrsampleapp">
    
        <application
            android:name=".App" ../>
    </manifest>

When we run this sample we can see our text is colored red and font size 18.

Limitations

XML routing can't be changed on instintiated views.
For named XML tags, XML routing can only be done for classes that inherit from the originally routed class.

JustinAngel/Hijckr

JustinAngel

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