DownloadWithMessages.java

package vandy.mooc.view;

import java.lang.ref.WeakReference;

import android.graphics.Bitmap;
import android.os.Handler;
import android.os.Message;

/**
 * This class downloads a bitmap image in the background using
 * Handlers and Messages.
 */
public class DownloadWithMessages 
       implements Runnable {
    /**
     * The URL to download. 
     */ 
    final String mUrl;

    /**
     * WeakReference enable garbage collection of Activity.
     */
    final WeakReference<ImageDownloadsActivity> mActivity;
    
    /*
     * Types of Messages that can be passed from a background
     * Thread to the UI Thread to specify which processing to
     * perform.
     */
    static final int SHOW_DIALOG = 1;
    static final int DISMISS_DIALOG = 2;
    static final int DISPLAY_IMAGE = 3;

    /**
     * Extends Handler and uses its handleMessage() hook method to
     * process Messages sent to it from a background Thread.
     */
    final Handler mMessageHandler =
        new Handler() {
            /**
             * Process the specified Messages passed to MessageHandler in
             * the UI Thread. These Messages instruct the Handler to start
             * showing the progress dialog, dismiss it, or display the
             * designated bitmap image via the ImageView.
             */
            public void handleMessage(Message msg) {
                // Check to see if the activity still exists and return if
                // not.
                switch (msg.what) {
                case SHOW_DIALOG:
                    // Inform the user that the download is starting.
                    mActivity.get().showDialog("downloading via Handlers and Messages");
                    break;

                case DISMISS_DIALOG:
                    // Dismiss the progress dialog.
                    mActivity.get().dismissDialog();
                    break;

                case DISPLAY_IMAGE:
                    // Display the downloaded image to the user.
                    mActivity.get().displayBitmap((Bitmap) msg.obj);
                    break;
                }
            }
        };

    /**
     * Class initializes the fields.
     * 
     * @param activity
     *            The enclosing Activity
     * @param url
     *            The bitmap image url
     */
    DownloadWithMessages(ImageDownloadsActivity activity,
                         String url) {
        mUrl = url;
        mActivity = 
            new WeakReference<>(activity);
    }

    /**
     * Download a bitmap image in a background Thread. It sends
     * various messages to the mHandler running in the UI Thread.
     */
    public void run() {
        // Factory creates a Message that instructs the
        // MessageHandler to begin showing the progress dialog to
        // the user.
        Message msg =
            mMessageHandler.obtainMessage(SHOW_DIALOG);

        // Send the Message to initiate the ProgressDialog.
        mMessageHandler.sendMessage(msg);

        // Download the image.
        final Bitmap image =
            mActivity.get().downloadBitmap(mUrl);

        // Factory creates a Message that instructs the
        // MessageHandler to dismiss the progress dialog.
        msg = mMessageHandler.obtainMessage(DISMISS_DIALOG);

        // Send the Message to dismiss the ProgressDialog.
        mMessageHandler.sendMessage(msg);

        // Factory creates a Message that instructs the
        // MessageHandler to display the image to the user.
        msg = mMessageHandler.obtainMessage(DISPLAY_IMAGE,
                                            image);

        // Send the Message to instruct the UI Thread to display the
        // image.
        mMessageHandler.sendMessage(msg);
    }
}