Look: I know it is tedious reading, but the reference given earlier in
this thread, to Sun's own documentation of the class, answers your
question concerning when to use a WeakReference -- as well as when to
use the other kinds. Then there are IBM Java articles/tutorials on the
same topic (e.g. http://www.ibm.com/developerworks/library/j-refs/).
Since it is answered there, it should be clear that this is a Java
question, not an Android one. So why are you belaboring the point
here?
On Jul 22, 12:06 pm, Joseph Earl <joseph.w.e...@gmail.com> wrote:
> No. I'm unsure as to what to use a WeakReference for exactly - as
> Romain Guy said above it is too weak for this purpose, but I think
> (hopefully Romain will correct me if I'm wrong) that a SoftReference
> could be suitable for this purpose.
>
> A ListView already does efficient management of your Views by
> recycling. This means that you must ensure the correct details are set
> in the view each time getView is called, even if you do not inflate a
> view or call findViewById that time. Recycling does not mean that the
> ListView caches all your items or their content.
>
> Suppose you had a list of 10 items, all of the same type but only 5
> will fit on the screen at a time. The ListView only really needs 5
> views to show the rows since the other 5 won't be visible.
> Thus at the top of the list the ListView might use 'View 1' for the
> first item, but scroll down to the bottom and 'View 1' would now
> contain item 6. As far as I understand it this is recycling.
>
> Recycling does not take care of the amount time of it takes to get
> content and set it to the view - thus if it takes a long time to get a
> piece of information and display it in a list item (such as
> downloading an image from the web), you will want to cache the result
> in a way that does not adversely affect memory usage (as much as
> possible). In this case you will also want to use a Thread or Async
> task to download/get the info off the UI thread.
>
> On Jul 22, 7:36 pm, GodsMoon <godsm...@gmail.com> wrote:> So you'd only want
> to use WeakReference when you think your activity
> > might run out of memory?
> > But a list view already does efficient memory management for you
> > right?
>
> > You'd saying if I were create a large array or something like that
> > then it would be good to use WeakReference. right?
>
> > Thanks for the help guys,
> > David Shellabargerwww.nightshadelabs.com
>
> > On Jul 22, 2:26 pm, Romain Guy <romain...@android.com> wrote:
>
> > > You definitely do NOT want to use a WeakReference to cache object. If
> > > you do so, as soon as your data is put in the cache and not used
> > > outside of the cache, it gets garbage collected.
>
> > > On Thu, Jul 22, 2010 at 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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>
> > > --
> > > Romain Guy
> > > Android framework engineer
> > > romain...@android.com
>
> > > Note: please don't send private questions to me, as I don't have time
> > > to provide private support. All such questions should be posted on
> > > public forums, where I and others can see and answer them
>
>
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