You left out something very important: the code hidden under "//
reload the image" must not assume that it is not itself interrupted by
yet another call to the garbage collector. That is, instead of simply
continuing to use the soft/weak reference, it should make a strong
reference to the same object, allowing this latter reference to either
go out of scope or be set to null when it is done.
But if we are making this strong reference, what was the point of
using the weak/soft reference in the first place? Ah, that is the
tricky thing about using them. Depending on when you can make and
release the strong reference, they might not buy you much; they might
not buy you anything at all. That is why they are not recommended for
much outside of caches and normalized mappings.
On Jul 22, 11:07 am, Joseph Earl <joseph.w.e...@gmail.com> wrote:
> Suppose you had a long list of images. As the user scrolled down you
> load the images from the net, and then display them.
> To avoid having to reload the images again if the user scrolls back
> up, you put the images in a cache (probably something like a
> Map<String, Drawable>)
>
> However because it is a long list you don't want to run into an out of
> memory situation if the user scrolls very far down and lots of images
> are put in the cache.
> So instead of storing the Drawables directly in the map, you create a
> Map<String, WeakReference<Type>> (although I would use SoftReference
> for the purpose described here).
> This means that if Android is going to encounter an out of memory
> situation it will clear all of the Soft/Weak references (and thus
> hopefully avoid running out of memory). You will have to load the
> images again since your cache has been cleared, but this is far better
> than your application running out of memory and crashing.
>
> So you do something like:
>
> // caching an image
> Map<String, SoftReference> cache = new HashMap<String,
> SoftReference<Drawable>>();
> cache.put("http://mysite.com/images/1.jpg";, new
> SoftReference<Drawable>.put(myDrawable));
>
> // retrieve an image
> if (cache.containsKey(url)) {
> // looks like we have this image cached
> Drawable drawable = cache.get(url).get();
> if (drawable == null) {
> // the softreference has been cleared by the GC, reload the
> image
> } else {
> // softreference is still valid, got our image
> }
>
> }
>
> Essentially a weak reference is a weaker reference than a soft
> reference - the GC should free weak references to regain memory before
> soft references.
>
> I think that's (mostly) correct, hope it helps.
>
> On Jul 22, 6:48 pm, GodsMoon <godsm...@gmail.com> wrote:
>
> > Google just posted a new blog post
> > onhttp://android-developers.blogspot.com/2010/07/multithreading-for-per....
> > I understand the AsyncTask and I'm even using one in a list with
> > images already.
>
> > But I don't understand what a WeakReference is. I gather is is a
> > garbage collector directive, but I thought I didn't need to manage
> > garbage collection on Android.
>
> >http://developer.android.com/reference/java/lang/ref/WeakReference.html
> > isn't as helpful as I was hoping it would be.
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![http://mysite.com/images/1.jpg"](http://mysite.com/images/1.jpg"){kind=link}