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.
>
> > > --
> > > You received this message because you are subscribed to the Google
> > > Groups "Android Developers" group.
> > > To post to this group, send email to android-developers@googlegroups.com
> > > To unsubscribe from this group, send email to
> > > android-developers+unsubscr...@googlegroups.com
> > > For more options, visit this group at
> > >http://groups.google.com/group/android-developers?hl=en
>
> > --
> > 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
--
You received this message because you are subscribed to the Google
Groups "Android Developers" group.
To post to this group, send email to android-developers@googlegroups.com
To unsubscribe from this group, send email to
android-developers+unsubscr...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/android-developers?hl=en
![http://mysite.com/images/1.jpg"](http://mysite.com/images/1.jpg"){kind=link}