Square grid layout for Android

SquareGridLayout.java

package com.tomgibara.android.util;

import android.content.Context;
import android.content.res.TypedArray;
import android.util.AttributeSet;
import android.view.View;
import android.view.ViewGroup;

/**
 * A layout that arranges views into a grid of same-sized squares.
 * 
 * This source code contained in this file is in the Public Domain.
 * 
 * @author Tom Gibara
 *
 */

public class SquareGridLayout extends ViewGroup {

	// fields
	
	/**
	 * Records the number of views on each side of the square (ie. the number of rows and columns)
	 */
	
	private int mSize = 1;
	
	/**
	 * Records the size of the square in pixels (excluding padding).
	 * This is set during {@link #onMeasure(int, int)} 
	 */
	
	private int mSquareDimensions;
	
	// constructors

	/**
	 * Constructor used to create layout programatically.
	 */
	
	public SquareGridLayout(Context context) {
		super(context);
	}

	/**
	 * Constructor used to inflate layout from XML. It extracts the size from
	 * the attributes and sets it.
	 */
	
	/* This requires a resource to be defined like this:
	 * 
	 * <resources>
	 *   <declare-styleable name="SquareGridLayout">
	 *     <attr name="size" format="integer"/>
	 *   </declare-styleable>
	 * </resources>
	 * 
	 * So that the attribute can be set like this:
	 * 
	 * <com.tomgibara.android.util.SquareGridLayout
	 *   xmlns:android="http://schemas.android.com/apk/res/android"
	 *   xmlns:util="http://schemas.android.com/apk/res/com.tomgibara.android.background"
	 *   util:size="3"
	 *   />
	 */
	
	public SquareGridLayout(Context context, AttributeSet attrs) {
		super(context, attrs);
		TypedArray a = getContext().obtainStyledAttributes(attrs, R.styleable.SquareGridLayout);
		setSize(a.getInt(R.styleable.SquareGridLayout_size, 1));
		a.recycle();
	}

	// accessors
	
	/**
	 * Sets the number of views on each side of the square.
	 * 
	 * @param size the size of grid (at least 1)
	 */
	
	public void setSize(int size) {
		if (size < 1) throw new IllegalArgumentException("size must be positive");
		if (mSize != size) {
			mSize = size;
			requestLayout();
		}
	}
	
	// View methods
	
	@Override
	protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {

		// breakdown specs
		final int mw = MeasureSpec.getMode(widthMeasureSpec);
		final int mh = MeasureSpec.getMode(heightMeasureSpec);
		final int sw = MeasureSpec.getSize(widthMeasureSpec);
		final int sh = MeasureSpec.getSize(heightMeasureSpec);
		
		// compute padding
		final int pw = getPaddingLeft() + getPaddingRight();
		final int ph = getPaddingTop() + getPaddingBottom();
		
		// compute largest size of square (both with and without padding)
		final int s;
		final int sp;
		if (mw == MeasureSpec.UNSPECIFIED && mh == MeasureSpec.UNSPECIFIED) {
			throw new IllegalArgumentException("Layout must be constrained on at least one axis");
		} else if (mw == MeasureSpec.UNSPECIFIED) {
			s = sh;
			sp = s - ph;
		} else if (mh == MeasureSpec.UNSPECIFIED) {
			s = sw;
			sp = s - pw;
		} else {
			if (sw - pw < sh - ph) {
				s = sw;
				sp = s - pw;
			} else {
				s = sh;
				sp = s - ph;
			}
		}

		// guard against giving the children a -ve measure spec due to excessive padding
		final int spp = Math.max(sp, 0);
		
		// pass on our rigid dimensions to our children
		final int size = mSize;
		for (int y = 0; y < size; y++) {
			for (int x = 0; x < size; x++) {
				final View child = getChildAt(y * size + x);
				if (child == null) continue;
				// measure each child
				// we could try to accommodate oversized children, but we don't
				measureChildWithMargins(child,
					MeasureSpec.makeMeasureSpec((spp + x) / size, MeasureSpec.EXACTLY), 0,
					MeasureSpec.makeMeasureSpec((spp + y) / size, MeasureSpec.EXACTLY), 0
				);
			}
		}
		
		//record our dimensions
		setMeasuredDimension(
			mw == MeasureSpec.EXACTLY ? sw : sp + pw,
			mh == MeasureSpec.EXACTLY ? sh : sp + ph
		);
		mSquareDimensions = sp;
	}
	
	// ViewGroup methods
	
	@Override
	protected LayoutParams generateDefaultLayoutParams() {
		return new MarginLayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
	}
	
	@Override
	protected LayoutParams generateLayoutParams(LayoutParams p) {
		return new MarginLayoutParams(p);
	}

	@Override
	public LayoutParams generateLayoutParams(AttributeSet attrs) {
		return new MarginLayoutParams(getContext(), attrs);
	}
	
	@Override
	protected void onLayout(boolean changed, int l, int t, int r, int b) {
		
		// recover the previously computed square dimensions for efficiency
		final int s = mSquareDimensions;
		
		{
			// adjust for our padding
			final int pl = getPaddingLeft();
			final int pt = getPaddingTop();
			final int pr = getPaddingRight();
			final int pb = getPaddingBottom();
			
			// allocate any extra spare space evenly
			l = pl + (r - pr - l - pl - s) / 2;
			t = pt + (b - pb - t - pb - s) / 2;
		}
		
		final int size = mSize;
		for (int y = 0; y < size; y++) {
			for (int x = 0; x < size; x++) {
				View child = getChildAt(y * mSize + x);
				// optimization: we are moving through the children in order
				// when we hit null, there are no more children to layout so return
				if (child == null) return;
				// get the child's layout parameters so that we can honour their margins
				MarginLayoutParams lps = (MarginLayoutParams) child.getLayoutParams();
				// we don't support gravity, so the arithmetic is simplified
				child.layout(
					l + (s *  x   ) / size + lps.leftMargin,
					t + (s *  y   ) / size + lps.topMargin,
					l + (s * (x+1)) / size - lps.rightMargin,
					t + (s * (y+1)) / size - lps.bottomMargin
				);
			}
		}
	}

}

Thanks a lot for this useful piece of code !
The only problem is that generateDefaultLayoutParams generates an error because the constructor is not correct:
@OverRide
protected LayoutParams generateDefaultLayoutParams() {
return new MarginLayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
}

Instead it can be changed to:
@OverRide
protected LayoutParams generateDefaultLayoutParams() {
return new MarginLayoutParams(new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT));
}

Thank you a lot this was very helpful 👍
Thank you @yanniszark for the quick fix as well.