Add a method to pass an image thumbnail URL as placeholder

There’re several use cases where apps shows a thumbnail image and onClick() it expands to a full sized image.

It would be a great feature to be able to easily smooth transition between those two states by calling:

Picasso.with(this).load(full_img_url).placeholderUrl(thumb_url).placeHolder(drawable).into(target);

In that case, Picasso would

if( thumbnail image available from local cache ){
     // that would be expected case
     // the thumbnail is already on screen, so it should be cached too
     use the image thumbnail as placeholder
} else{
    // and fall back in case anything happens
    use the supplied drawable as placeholder
}```

[Abdelhady]

It is a worthy feature for a better UX to show the thumbnail as a placeholder until the real image loads properly, & I’m sure that @JakeWharton & others who contributed to this great library can do this extra boost 😃 just using a url for the placeholder will be super great!

but using a nested calls like the following, I think it is not the best solution from performance point of view:

Picasso.with(context)
       .load(thumb) // thumbnail url goes here
       .into(imageView, new Callback() {
            @Override
            public void onSuccess() {
                Picasso.with(context)
                        .load(url) // image url goes here
                        .placeholder(imageView.getDrawable())
                        .into(imageView);
            }
            @Override
            public void onError() {

            }
        });

[rgr-myrg]

@abdelhady Your solution works perfectly. I’m able to load a thumbnail and subsequently a full sized image in sequence using your solution. Many thanks.