Android compiled resources - resources.arsc

TL;DR: With the help of android asset packagin tool(aapt), xml nodes get translated to Java classes and the corresponding xml attributes get translated to numerical Ids. Android run-time works with these numeric ids to instantiate classes and create the views

TL;R

Run this command to dump the binary xml
aapt d xmltree apk_file_name res/layout/activity_main.xml(aapt can be found in android-sdk-dir/build-tools/23.0.2/aapt.exe)

This will show the xml nodes (e.g. LinearLayout, RelativeLayout, etc) with their attributes(e.g. android:layout_width, android:layout_height) and their values. Note that, the constants match_parent(numeric value 0xffffffff or -1) or wrap_content(numeric value 0xfffffffe or -2) can be seen there.

As a matter of fact, you can use this command on any other xml files in the apk e.g. AndroidManifest.xml or other layout files

The apk file is just a zip archive containing all the java class files(classes.dex), all the compiled resource files and a file named resources.arsc.This resource.arsc file contains all the meta-information about the resources. Some of those are...

• the xml nodes(e.g. LinearLayout, RelativeLayout, etc),
• the attributes(e.g. android:layout_width),
• the resource id's.

The resource id's refer to the real resources in the apk-file. The attributes are resolved to a value at runtime. The resolution process is smart about any re-direction (@dimen/... as opposed to 4dp or @color/... as opposed to "#FFaabbcc") and returns a usable value(a dimen value is resolved differently than a color value).

Whats a compiled XML file:A compiled XML file is just the same XML file with the resource references changed to their corresponding ids. For example, a reference @string/ok will be replaced by 0x7f000001. Moreover, the attributes from android namespace is changed to their respective integer values(e.g. wrap_contentis changed to 0xfffffffe or -2)

How Android resolves resources at runtime:The methodinflater.inflate() parses a compiled xml file and creates a view hierarchy by instantiating the xml nodes. Each of the xml nodes is instantiated by a java class(e.g. LinearLayout.java, RelativeLayout.java). To instantiate, the inflater parses the compiled xml file, collects all the attributes of a node and creates a packed structure of type AttributeSet. This AttributeSet is passed to the class constructor. The class constructor has the responsibility of walking the AttributeSet and resolving each of the attribute values.

For example, for a layout containing RelativeLayout, the inflater will pack layout_width and layout_height into a AttributeSet and pass it to the constructor

RelativeLayout(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes).

In this case, some of the attributes are resolved by RelativeLayout.initFromAttributes(). The rest of the attributes are resolved by the parent ViewGroup.initFromAttributes(). The attribute android:id of a view is just another attribute. After inflating, the inflater stores the id of each view by calling setId(id) on that view after instantiation

Now to answer your questionR.id is a java array and R.id.my_textview is an integer in that array. The id of the view my_textview is this integer(starts with 0x7f). The method findViewById() does a depth-first search on that view hierarchy to find the respective view.

Hope this helps. The link you provided in your question already answers how the ids are generated by aapt.

Its a wonderful system of managing resources for devices with multiple dimensions of variations. Moreover, the implementation is really fast !! With this as the foundation, it allows to implement higher level functionality(e.g. Runtime Resource Overlay)

LayoutInflater inflate view by using XML strings. XML strings compiled into resource file as you mentioned in your question.

Please check these code snippets of AOSP:

public View inflate(int resource, ViewGroup root, boolean attachToRoot) {    final Resources res = getContext().getResources();    if (DEBUG) {        Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("                + Integer.toHexString(resource) + ")");    }    final XmlResourceParser parser = res.getLayout(resource);    try {        return inflate(parser, root, attachToRoot);    } finally {        parser.close();    }}

Resources.getLayout loads XML resource parser

public XmlResourceParser getLayout(int id) throws NotFoundException {    return loadXmlResourceParser(id, "layout");}XmlResourceParser loadXmlResourceParser(int id, String type)        throws NotFoundException {    synchronized (mAccessLock) {        TypedValue value = mTmpValue;        if (value == null) {            mTmpValue = value = new TypedValue();        }        getValue(id, value, true);        if (value.type == TypedValue.TYPE_STRING) {            return loadXmlResourceParser(value.string.toString(), id,                    value.assetCookie, type);        }        throw new NotFoundException(                "Resource ID #0x" + Integer.toHexString(id) + " type #0x"                + Integer.toHexString(value.type) + " is not valid");    }}

getValue uses AssetManager's getResourceValue and it calls loadResourceValue native method. This native method calls ResTable's getResource method to get XML strings stored in resource file.

Use appt for android-sdk (ex:- /build-tools/27.0.3/aapt ) run given script and get resources.arsc file content./aapt dump resources ./debug.apkPackage Groups (1)Package Group 0 id=0x7f packageCount=1 name=com.dianping.example.activity  Package 0 id=0x7f name=com.dianping.example.activity    type 1 configCount=3 entryCount=1      spec resource 0x7f020000 com.example.activity:drawable/ic_launcher: flags=0x00000100      config mdpi-v4:        resource 0x7f020000 com.example.activity:drawable/ic_launcher: t=0x03 d=0x00000000 (s=0x0008 r=0x00)      config hdpi-v4:        resource 0x7f020000 com.example.activity:drawable/ic_launcher: t=0x03 d=0x00000001 (s=0x0008 r=0x00)      config xhdpi-v4:        resource 0x7f020000 com.example.activity:drawable/ic_launcher: t=0x03 d=0x00000002 (s=0x0008 r=0x00)    type 2 configCount=1 entryCount=1      spec resource 0x7f030000 com.dianping.example.activity:string/app_name: flags=0x00000000      config (default):        resource 0x7f030000 com.dianping.example.activity:string/app_name: t=0x03 d=0x00000003 (s=0x0008 r=0x00)

This link might help http://elinux.org/Android_aapt