mvds mvds - 1 year ago 71
Android Question

Runnable is posted successfully but not run

In an existing Android project I've encountered the following piece of code (where I inserted the debugging litter)

ImageView img = null;

public void onCreate(...) {

img = (ImageView)findViewById(;

new Thread() {
public void run() {
final Bitmap bmp = BitmapFactory.decodeFile("/sdcard/someImage.jpg");
System.out.println("bitmap: "+bmp.toString()+" img: "+img.toString());
if ( ! Runnable() {
public void run() {
System.out.println("setting bitmap...");
System.out.println("bitmap set.");
}) ) System.out.println("Runnable won't run!");
System.out.println("runnable posted");

New to Android development, and having googled around, I understand that this is the way to do stuff without blocking the main (UI) thread, while still setting the image on the UI thread after decoding. (at least according to android-developers) (which I have verified by logging
at various places)

Now sometimes the image just doesn't show up, and stdout only says

I/System.out( 8066): bitmap: img: android.widget.ImageView@4339d698
I/System.out( 8066): runnable posted

with not a trace of the messages from the Runnable. So appearantly the Runnable doesn't
, although
. Pulling the ImageView in
and declaring it
doesn't help.

I'm clueless. Simply setting the bitmap directly, while blocking the UI thread, does fix things, but I want to get things right. Does anybody understand what's going on here?

(ps. this was all observed on an Android 1.6 phone and android-3 sdk)

Answer Source

If you look at the docs for there's some relevant info:

This method can be invoked from outside of the UI thread only when this View is attached to a window.

Since you're doing this in onCreate, it is likely that sometimes your View will not be attached to the window yet. You can verify this by overriding onAttachedToWindow and putting something in the logs and also logging when you post. You'll see that when the post fails, the post call happens before onAttachedToWindow.

As the others have mentioned, you can use Activity.runOnUiThread or provide your own handler. However, if you want to do it directly from the View itself, you can simply get the View's handler:


This is especially useful if you have a custom view that includes some sort of background loading. There's also the added bonus of not having to create a new separate handler.

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