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 <[email protected]> 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 <[email protected]> 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 <[email protected]> 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 <[email protected]> > > > 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 <[email protected]> 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 [email protected] > > > > To unsubscribe from this group, send email to > > > > [email protected] > > > > For more options, visit this group at > > > >http://groups.google.com/group/android-developers?hl=en > > > > -- > > > Romain Guy > > > Android framework engineer > > > [email protected] > > > > 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 [email protected] To unsubscribe from this group, send email to [email protected] 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}