Reading the Android SDK docs and the various UI tutorials floating around you'd be forgiven for thinking that creating an Android GUI is all about nesting and wrangling the standard ViewGroup-derived layout classes in XML. In fact, for performance and to achieve non-standard arrangements, Android framework developers like Romain Guy recommend writing your own custom layout class.

Some of the best information on this subject is hiding inside a 2009 Google IO conference video with the utterly deceptive title of Supporting Multiple Devices (http://www.google.com/events/io/2009/sessions/SupportingMultipleDevicesBinary.html). Here is a bit of a table of contents for that video:

Custom layout coverage starts around the 15 minute mark: http://www.youtube.com/watch?v=PAMtKVO2ch8#t=14m55s.
Getting values from the XML file via the ViewGroup constructor taking an AttributeSet parameter: http://www.youtube.com/watch?v=PAMtKVO2ch8#t=16m01s
The onMeasure() method: http://www.youtube.com/watch?v=PAMtKVO2ch8#t=17m03s
The use of the resolveSize() method: http://www.youtube.com/watch?v=PAMtKVO2ch8#t=19m20s.
onLayout(), where your layout tells its children exactly where and how large they will be drawn: http://www.youtube.com/watch?v=PAMtKVO2ch8#t=21m20s.
Using your ViewGroup-derived Java class from your XML layout file: http://www.youtube.com/watch?v=PAMtKVO2ch8#t=23m02s.
Starting to get into pix (pixels) versus dips and similar density/resolution issues after a run of the initial version of the layout: http://www.youtube.com/watch?v=PAMtKVO2ch8#t=25m15s.
Honouring the requested (via onMeasure()) dimensions of the children: http://www.youtube.com/watch?v=PAMtKVO2ch8#t=25m35s.
getWidth() versus getMeasuredWidth(). The latter is what the View thinks it wants to be, the former is what it actually is on-screen. getMeasuredWidth() is only really useful for layouts.
Padding and margins: http://www.youtube.com/watch?v=PAMtKVO2ch8#t=27m47s. A layout accounts for its children's margins because they are outside it but not their padding which is inside them. As a shortcut for a custom layout, ignore padding and layout if you like.
They avoid going further into pix (pixels) versus dips and similar density/resolution issues: http://www.youtube.com/watch?v=PAMtKVO2ch8#t=28m53s. The example has been using pixel units up until now so child views will be different physical sizes on different screens. Example doing density suport is 54 lines of code but they skip it: bah. Skip ahead to http://www.youtube.com/watch?v=PAMtKVO2ch8#t=34m58s where they return to this subject a little.
Getting attributes from xml file: http://www.youtube.com/watch?v=PAMtKVO2ch8#t=30m17s. Use of ProjectName/res/values/attrs.xml.
Use of declare-styleable xml attribute.

Declaring attributes that you want to be visible in the layout XML file within the styleable declaration.
Units used for dimensions.
Getting custom attributes in constructor of Java layout class: http://www.youtube.com/watch?v=PAMtKVO2ch8#t=34m15s.
Use of Context.obtainStyledAttributes
Use of TypedArray.getDimensionPixelSize() to convert the attributes specified in XML in any unit to the pixel units that layout works in.

Talk veers off into commiting patches to the Google open source project: http://www.youtube.com/watch?v=PAMtKVO2ch8#t=37m26s.

There's more coverage of layouts by google guy, Romain Guy, in the first half of this 2010 talk, which has slides and example code available.



This Android 3.0 animation example also covers custom Layouts and custom Views and has full source available.