A custom Android layout class that arranges children in a grid-like manner, optimizing for even horizontal and vertical whitespace.

DashboardLayout.java

/*
 * Copyright 2011 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.android.apps.iosched.ui.widget;

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

/**
 * Custom layout that arranges children in a grid-like manner, optimizing for even horizontal and
 * vertical whitespace.
 */
public class DashboardLayout extends ViewGroup {

    private static final int UNEVEN_GRID_PENALTY_MULTIPLIER = 10;

    private int mMaxChildWidth = 0;
    private int mMaxChildHeight = 0;

    public DashboardLayout(Context context) {
        super(context, null);
    }

    public DashboardLayout(Context context, AttributeSet attrs) {
        super(context, attrs, 0);
    }

    public DashboardLayout(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        mMaxChildWidth = 0;
        mMaxChildHeight = 0;

        final int childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
                MeasureSpec.getSize(widthMeasureSpec), MeasureSpec.AT_MOST);
        final int childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                MeasureSpec.getSize(widthMeasureSpec), MeasureSpec.AT_MOST);

        final int count = getChildCount();
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() == GONE) {
                continue;
            }

            child.measure(childWidthMeasureSpec, childHeightMeasureSpec);

            mMaxChildWidth = Math.max(mMaxChildWidth, child.getMeasuredWidth());
            mMaxChildHeight = Math.max(mMaxChildHeight, child.getMeasuredHeight());
        }

        setMeasuredDimension(
                resolveSize(mMaxChildWidth, widthMeasureSpec),
                resolveSize(mMaxChildHeight, heightMeasureSpec));
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        int width = r - l;
        int height = b - t;

        final int count = getChildCount();

        // Calculate the number of visible children.
        int visibleCount = 0;
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() == GONE) {
                continue;
            }
            ++visibleCount;
        }

        if (visibleCount == 0) {
            return;
        }

        // Calculate what number of rows and columns will optimize for even horizontal and
        // vertical whitespace between items. Start with a 1 x N grid, then try 2 x N, and so on.
        int bestSpaceDifference = Integer.MAX_VALUE;
        int spaceDifference;

        // Horizontal and vertical space between items
        int hSpace = 0;
        int vSpace = 0;

        int cols = 1;
        int rows;

        while (true) {
            rows = (visibleCount - 1) / cols + 1;

            hSpace = ((width - mMaxChildWidth * cols) / (cols + 1));
            vSpace = ((height - mMaxChildHeight * rows) / (rows + 1));

            spaceDifference = Math.abs(vSpace - hSpace);
            if (rows * cols != visibleCount) {
                spaceDifference *= UNEVEN_GRID_PENALTY_MULTIPLIER;
            }

            if (spaceDifference < bestSpaceDifference) {
                // Found a better whitespace squareness/ratio
                bestSpaceDifference = spaceDifference;

                // If we found a better whitespace squareness and there's only 1 row, this is
                // the best we can do.
                if (rows == 1) {
                    break;
                }
            } else {
                // This is a worse whitespace ratio, use the previous value of cols and exit.
                --cols;
                rows = (visibleCount - 1) / cols + 1;
                hSpace = ((width - mMaxChildWidth * cols) / (cols + 1));
                vSpace = ((height - mMaxChildHeight * rows) / (rows + 1));
                break;
            }

            ++cols;
        }

        // Lay out children based on calculated best-fit number of rows and cols.

        // If we chose a layout that has negative horizontal or vertical space, force it to zero.
        hSpace = Math.max(0, hSpace);
        vSpace = Math.max(0, vSpace);

        // Re-use width/height variables to be child width/height.
        width = (width - hSpace * (cols + 1)) / cols;
        height = (height - vSpace * (rows + 1)) / rows;

        int left, top;
        int col, row;
        int visibleIndex = 0;
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() == GONE) {
                continue;
            }

            row = visibleIndex / cols;
            col = visibleIndex % cols;

            left = l + hSpace * (col + 1) + width * col;
            top = t + vSpace * (row + 1) + height * row;

            child.layout(left, top,
                    (hSpace == 0 && col == cols - 1) ? r : (left + width),
                    (vSpace == 0 && row == rows - 1) ? b : (top + height));
            ++visibleIndex;
        }
    }
}