Activity layouts: XML vs Java
For a long while, I couldn’t really understand the difference between the XML files and the Java files. It seemed at first like the XML files determine what objects are available on the display for a particular activity, and then the actions and interactivity of those objects are determined by an accompanying Java file. However, when working later with another android developer example, I saw that the buttons were actually being created in Java and not included in the XML at all:
@Override
protected void onCreate(Bundle icicle) {
super.onCreate(icicle);
LinearLayout ll = new LinearLayout(this);
mRecordButton = new RecordButton(this);
ll.addView(mRecordButton,
new LinearLayout.LayoutParams(
ViewGroup.LayoutParams.WRAP_CONTENT,
ViewGroup.LayoutParams.WRAP_CONTENT,
0));
mPlayButton = new PlayButton(this);
ll.addView(mPlayButton,
new LinearLayout.LayoutParams(
ViewGroup.LayoutParams.WRAP_CONTENT,
ViewGroup.LayoutParams.WRAP_CONTENT,
0));
setContentView(ll);
//super.onCreate(icicle);
//setContentView(R.layout.activity_audio_record_test);
}
The commented out block at the bottom is what appears by default when creating the activity using the GUI in Android Developer Tools. I realized that the R.layout.activity_audio_record_test corresponsed to the filepath res/layout/activity_audio_record_test.xml, which was the auto-generated layout XML file. But the reason none of that was making its way to the actual app was because the setContentView function that would call it is commented out, and instead there is a setContentView(ll) above that uses an Android LinearLayout object created entirley in Java.
To compare, in XML, this would probably look something like the following:
<LinearLayout xlmns:android=“http://schemas.android.com/apk/res/android”
xlmns:tools=“http://schemas.android.com/tools”
android:layout_width=“match_parent”
android:layout_height=“match_parent”
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=“.AudioRecordTest” >
<Button
android:layout_width=“wrap_content”
android:layout_height=“wrap_content”
android:text=“Start Recording”
android:Id=“@+id/record_button”
android:onClick=“onRecord” />
<Button
android:layout_width=“wrap_content”
android:layout_height=“wrap_content”
android:text=“Start Playing”
android:Id=“@+id/play_button”
android:onClick=“onPlay” />
</LinearLayout>
Much uglier, in my opinion.