Perhaps the difference was because I was declaring my view in the
onCreate method instead of in the onPostExecute method of the
AsyncTask?
If I declare the view in my activity, maybe it gets GCed when my
activity is killed, but in the blog post example he tries to declare
it in the AsyncTask even though the activity is long gone.
Am I on the right track here?
David Shellabarger
www.nightshadelabs.com
On Jul 22, 4:09 pm, Joseph Earl <joseph.w.e...@gmail.com> wrote:
> I am confused somewhat about the issue myself now. Hopefully someone
> else it will clear it up once and for all.
>
> On Jul 22, 9:01 pm, GodsMoon <godsm...@gmail.com> wrote:
>
>
>
> > I have done something like that and got a null pointer reference.
> > I was starting a AsyncTask and if you backed out of the activity
> > before the AsyncTask was finished I would get a null pointer when I
> > tried to refer to the activities elements (force close).
> > So I just check to see if the views are null first. I thought that if
> > it returned null then it must have been GCed.
> > But that's not the case?
>
> > David Shellabargerwww.nightshadelabs.com
>
> > On Jul 22, 3:49 pm, Joseph Earl <joseph.w.e...@gmail.com> wrote:
>
> > > When your UI activity is killed as far as I am aware the GC will
> > > collect your Views. The problem is (I think) if the GC closes your UI
> > > thread while the downloader is still running - in this case your
> > > downloader thread maintains a reference to the ImageView so the GC
> > > cannot collect it - hence memory leak - the downloader thread cannot
> > > actually do anything to the ImageView since the UI thread no longer
> > > exists.
> > > Thus the downloader thread should only store a weak reference to the
> > > ImageView so that if the UI thread is killed the GC may reclaim the
> > > memory associated with the ImageView.
>
> > > On Jul 22, 8:34 pm, GodsMoon <godsm...@gmail.com> wrote:
>
> > > > The blog post is confusing.
> > > > "Note that this ImageView is stored as a WeakReference, so that a
> > > > download in progress does not prevent a killed activity's ImageView
> > > > from being garbage collected."
> > > > I didn't know that would cause a memory leak. I thought the garbage
> > > > collector would clean up ImageView if its activity gets killed.
> > > > Am I wrong?
>
> > > > David Shellabargerwww.nightshadelabs.com
>
> > > > On Jul 22, 3:06 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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