M7350/base/docs/html/resources/tutorials/views/hello-gridview.jd

page.title=Grid View
parent.title=Hello, Views
parent.link=index.html
@jd:body

<p>{@link android.widget.GridView} is a {@link android.view.ViewGroup} that displays items in a
two-dimensional,
scrollable grid. The grid items are automatically inserted to the layout using a {@link
android.widget.ListAdapter}.</p>

<p>In this tutorial, you'll create a grid of image thumbnails. When an item is selected, a
toast message will display the position of the image.</p>


<ol>
  <li>Start a new project named <em>HelloGridView</em>.</li>
  <li>Find some photos you'd like to use, or <a
href="{@docRoot}shareables/sample_images.zip">download these sample images</a>. Save the image files
into the project's
<code>res/drawable/</code> directory.</li>
  <li>Open the <code>res/layout/main.xml</code> file and insert the following:
<pre>
&lt;?xml version="1.0" encoding="utf-8"?>
&lt;GridView xmlns:android="http://schemas.android.com/apk/res/android" 
    android:id="@+id/gridview"
    android:layout_width="fill_parent" 
    android:layout_height="fill_parent"
    android:columnWidth="90dp"
    android:numColumns="auto_fit"
    android:verticalSpacing="10dp"
    android:horizontalSpacing="10dp"
    android:stretchMode="columnWidth"
    android:gravity="center"
/>
</pre>
  <p>This {@link android.widget.GridView} will fill the entire screen. The attributes are rather
self explanatory. For more information about valid attributes, see the {@link
android.widget.GridView} reference.</p>
</li>
  <li>Open <code>HelloGridView.java</code> and insert the following code for the
{@link android.app.Activity#onCreate(Bundle) onCreate()} method:
<pre>
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    GridView gridview = (GridView) findViewById(R.id.gridview);
    gridview.setAdapter(new ImageAdapter(this));

    gridview.setOnItemClickListener(new OnItemClickListener() {
        public void onItemClick(AdapterView&lt;?> parent, View v, int position, long id) {
            Toast.makeText(HelloGridView.this, "" + position, Toast.LENGTH_SHORT).show();
        }
    });
}
</pre>
  <p>After the {@code main.xml} layout is set for the content view, the
{@link android.widget.GridView} is captured from the layout with {@link
android.app.Activity#findViewById(int)}. The {@link
android.widget.GridView#setAdapter(T) setAdapter()} method then sets a custom adapter ({@code
ImageAdapter}) as the source for all items to be displayed in the grid. The {@code ImageAdapter} is
created in the next step.</p>
<p>To do something when an item in the grid is clicked, the {@link
android.widget.AdapterView#setOnItemClickListener(OnItemClickListener) setOnItemClickListener()}
method is passed a new {@link android.widget.AdapterView.OnItemClickListener}. This anonymous
instance defines the {@link
android.widget.AdapterView.OnItemClickListener#onItemClick(AdapterView,View,int,long)
onItemClick()} callback method to show a {@link android.widget.Toast} that displays the index
position (zero-based) of the selected item (in a real world scenario, the position could be used to
get the full sized
image for some other task).</p>

</li>
<li>Create a new class called <code>ImageAdapter</code> that extends {@link
android.widget.BaseAdapter}:
<pre>
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.setLayoutParams(new GridView.LayoutParams(85, 85));
            imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
            imageView.setPadding(8, 8, 8, 8);
        } else {
            imageView = (ImageView) convertView;
        }

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

    // references to our images
    private Integer[] mThumbIds = {
            R.drawable.sample_2, R.drawable.sample_3,
            R.drawable.sample_4, R.drawable.sample_5,
            R.drawable.sample_6, R.drawable.sample_7,
            R.drawable.sample_0, R.drawable.sample_1,
            R.drawable.sample_2, R.drawable.sample_3,
            R.drawable.sample_4, R.drawable.sample_5,
            R.drawable.sample_6, R.drawable.sample_7,
            R.drawable.sample_0, R.drawable.sample_1,
            R.drawable.sample_2, R.drawable.sample_3,
            R.drawable.sample_4, R.drawable.sample_5,
            R.drawable.sample_6, R.drawable.sample_7
    };
}
</pre>
<p>First, this implements some required methods inherited from {@link
android.widget.BaseAdapter}. The constructor and {@link
android.widget.Adapter#getCount()} are self-explanatory. Normally, {@link
android.widget.Adapter#getItem(int)} should return the actual object at the specified position in
the adapter, but it's ignored for this example. Likewise, {@link
android.widget.Adapter#getItemId(int)} should return the row id of the item, but it's not
needed here.</p>

<p>The first method necessary is {@link android.widget.Adapter#getView(int,View,ViewGroup)
getView()}. This method creates a new {@link android.view.View} for each image added to the {@code
ImageAdapter}. When this is called, a {@link android.view.View} is passed in, which is normally a
recycled object (at least after this has been called once), so there's a check to see if the
object is null. If it <em>is</em> null, an {@link android.widget.ImageView} is instantiated and
configured with desired properties for the image presentation:</p>
<ul>
  <li>{@link android.view.View#setLayoutParams(ViewGroup.LayoutParams)} sets
the height and width for the View&mdash;this ensures that, no matter the size of the drawable, each
image is resized and cropped to fit in these dimensions, as appropriate.</li>
  <li>{@link android.widget.ImageView#setScaleType(ImageView.ScaleType)} declares that images should
be cropped toward the center (if necessary).</li>
  <li>{@link android.widget.ImageView#setPadding(int,int,int,int)} defines the padding for all
sides. (Note that, if the images have different aspect-ratios, then less
padding will cause for more cropping of the image if it does not match
the dimensions given to the ImageView.)</li>
</ul>

<p>If the {@link android.view.View} passed to {@link
android.widget.Adapter#getView(int,View,ViewGroup) getView()} is <em>not</em> null, then the local
{@link android.widget.ImageView} is initialized with the recycled {@link android.view.View}
object.</p>

<p>At the end of the {@link android.widget.Adapter#getView(int,View,ViewGroup) getView()} method,
the {@code
position} integer passed into the method is used to select an image from the {@code mThumbIds}
array, which is set as the image resource for the {@link android.widget.ImageView}.</p>
<p>All that's left is to define the {@code mThumbIds} array of drawable resources.</p>
</li>
<li>Run the application.</li>
</ol>
<p>Your grid layout should look something like this:</p>
<img src="images/hello-gridview.png" width="150px" />

<p>Try experimenting with the behaviors of the {@link android.widget.GridView} and {@link
android.widget.ImageView} elements by adjusting their properties. For example, instead of using
{@link android.view.View#setLayoutParams(ViewGroup.LayoutParams)}, try using
{@link android.widget.ImageView#setAdjustViewBounds(boolean)}. </p>

<h3>References</h3>
<ul>
	<li>{@link android.widget.GridView}</li>
	<li>{@link android.widget.ImageView}</li>
	<li>{@link android.widget.BaseAdapter}</li>
  <li>{@link android.widget.AdapterView.OnItemClickListener}</li>
</ul>