Android: How does GridView auto_fit find the number of columns?

Question

I would like to understand better how Gridview works, in particular auto_fit. Here is the XML layout:

<?xml version="1.0" encoding="utf-8"?>
<GridView xmlns:android="http://schemas.android.com/apk/res/android" 
    android:id="@+id/gridview"
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content"
    android:columnWidth="60dp"
    android:numColumns="auto_fit"
/>

And it works fine with a series of six thumbnails (48*48 pixels). In portrait mode, it displays one row, six columns.

What I don't understand is why the line android:columnWidth="60dp" is necessary, because auto_fit is expected to find the right number of columns.
Without the line android:columnWidth="60dp", it displays a grid 3 rows and 2 columns.

Here is the ImageAdapter class:

package com.examples.HelloGridView;

import android.content.Context;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ImageView;

public class ImageAdapter extends BaseAdapter {
    private Context mContext;

    public ImageAdapter(Context c) {
        mContext = c;
    }

    public int getCount() {
        return mThumbIds.length;
    }

    public Object getItem(int position) {
        return null;
    }

    public long getItemId(int position) {
        return 0;
    }

    // create a new ImageView for each item referenced by the Adapter
    public View getView(int position, View convertView, ViewGroup parent) {
        ImageView imageView;
        if (convertView == null) {  // if it's not recycled, initialize some attributes
            imageView = new ImageView(mContext);
            imageView.setPadding(0, 0, 0, 0);
        } else {
            imageView = (ImageView) convertView;
        }

        imageView.setImageResource(mThumbIds[position]);
        return imageView;
    }

    // references to our images
    private Integer[] mThumbIds = {
            R.drawable.ic_1, R.drawable.ic_2,
            R.drawable.ic_3, R.drawable.ic_4,
            R.drawable.ic_5, R.drawable.ic_6
    };
}

Thank you for your help.

Answer 1

Looking at the GridView source, it is clear that setting the padding and the height on your ImageView will not help you at all. When a column width is not specified, it just chooses a preset number of columns (2):

    private void determineColumns(int availableSpace) {

    ...

    if (mRequestedNumColumns == AUTO_FIT) {
        if (requestedColumnWidth > 0) {
            // Client told us to pick the number of columns
            mNumColumns = (availableSpace + requestedHorizontalSpacing) /
                    (requestedColumnWidth + requestedHorizontalSpacing);
        } else {
            // Just make up a number if we don't have enough info
            mNumColumns = 2;
        }
    } else {
        // We picked the columns
        mNumColumns = mRequestedNumColumns;
    }

    if (mNumColumns <= 0) {
        mNumColumns = 1;
    }

    ...

The solution is to measure your column size before setting the GridView's column width. Here is a quick way to measure Views offscreen:

public int measureCellWidth( Context context, View cell )
{

    // We need a fake parent
    FrameLayout buffer = new FrameLayout( context );
    android.widget.AbsListView.LayoutParams layoutParams = new  android.widget.AbsListView.LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
    buffer.addView( cell, layoutParams);

    cell.forceLayout();
    cell.measure(1000, 1000);

    int width = cell.getMeasuredWidth();

    buffer.removeAllViews();

    return width;
}

And then you just set the GridView's column width:

gridView.setColumnWidth( width );