避免在ViewPager中切换页面，直到下一页上的内容准备就绪

Question

I am using a ViewPager so the user can browse through an infinite collection of images that need to be fetched from the Internet.

I would not like to change to the next/previous page, unless its image has already been loaded. So if the user swiped right/left, the current image would still be on display with a progressbar spinning on top, and the change to the next/previous page would not be performed until the image was ready. At that very instant pages would swap and the progressbar would disappear.

At the moment I was trying to implement this using the following layout for the children views of the ViewPager and a FragmentStatePageAdapter.

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent" >

    <ImageView 
        android:id="@+id/imageview"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:contentDescription="@string/label_painting"
        android:visibility="gone" />

        <ProgressBar
            android:id="@+id/progressbar"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center" android:visibility="gone" />

</FrameLayout>

In the Fragment I follow the steps listed below:

• In its onCreateView I make the progressbar visible.
• In that same method I start a thread that fetches the image and decodes the byte stream into a bitmap.
• Once the bitmap is available, the thread sends a message to a handler that sets the bitmap in the ImageView, makes it visible and hides the progressbar.

The problem with this approach I am following is that in case the image has not been fetched yet when the user swipes, the previous image disappears and he sees a spinning progressbar until the new image is available. It would be much nicer to keep seeing an image.

Has anyone been able to implement this with a ViewPager or knows what could I change in my code?

Thanks in advance.

Answer 1

I did nearly the same.

You can enable/disable view paging in the adapter via:

//disable viewpager

public class CustomViewPager extends ViewPager {
private boolean enabled;
public CustomViewPager(Context context, AttributeSet attrs) {
    super(context, attrs);
    this.enabled = true;
}

@Override
public boolean onTouchEvent(MotionEvent event) {
    if (this.enabled) {
        return super.onTouchEvent(event);
    } 
    return false;
}

@Override
public boolean onInterceptTouchEvent(MotionEvent event) {
    if (this.enabled) {
        return super.onInterceptTouchEvent(event);
    } 
    return false;
}

public void setPagingEnabled(boolean enabled) {
    this.enabled = enabled;
}
}

If you Load your Image use an AsyncTask

    private class DownloadImageTask extends AsyncTask<Void, Void, Void> {

    @Override
    protected void onPreExecute() {
        // TODO Auto-generated method stub
        super.onPreExecute();
    }

    @Override
    protected Void doInBackground(Void... arg0) {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    protected void onPostExecute(Void result) {
        // TODO Auto-generated method stub
        super.onPostExecute(result);
    }   

 }

Use the onPreExecute() Method to disable viewpager, and in onPostExecute() set the image and re-Enable the ViewPager again. This works perfectly for me