Question

In my android app, I download an image from the cloud. The download is performed in a thread and in that same thread I also set an image view with the newly downloaded image. After setting the image, I then call postinvalidate().

However the image does not immediately show up. Is there a way to make the redraw happen immediately? I need a way to trigger a drawing cycle.

Answer 1

Each class which is derived from the View class has the invalidate() and the postInvalidate() method. If invalidate() gets called it tells the system that the current view has changed and it should be redrawn as soon as possible. As this method can only be called from your UIThread another method is needed for when you are not in the UIThread and still want to notify the system that your View has been changed. The postInvalidate() method notifies the system from a non-UIThread and the View gets redrawn in the next eventloop on the UIThread as soon as possible.

In your case you can achieve what you want with the help of AsyncTask (an intelligent backround thread).AsyncTask enables proper and easy use of the UI thread. This class allows to perform background operations (download image in background in your case) and publish results (set your bitmap to your ImageView) on the UI thread without having to manipulate threads and/or handlers.

An asynchronous task is defined by a computation that runs on a background thread and whose result is published on the UI thread. An asynchronous task is defined by 3 generic types, called Params, Progress and Result, and 4 steps, called begin, doInBackground, processProgress and end, as explained in the docs:

The 4 steps

When an asynchronous task is executed, the task goes through 4 steps:

onPreExecute(), invoked on the UI thread immediately after the task
is executed. This step is normally used to setup the task, for
instance by showing a progress bar in the user interface before
downloading the image from cloud and that’s used to provide a good
user experience .

doInBackground(Params...), invoked on the background thread
immediately after onPreExecute() finishes executing. This step is used
to perform background computation that can take a long time. The
parameters of the asynchronous task are passed to this step. The
result of the computation must be returned by this step and will be
passed back to the last step. This step can also use
publishProgress(Progress…) to publish one or more units of progress.
These values are published on the UI thread, in the
onProgressUpdate(Progress…) step.

onProgressUpdate(Progress...), invoked on the UI thread after a call
to publishProgress(Progress...). The timing of the execution is
undefined. This method is used to display any form of progress in the
user interface while the background computation is still executing.

onPostExecute(Result), invoked on the UI thread after the background
computation finishes. The result of the background computation is
passed to this step as a parameter in that method you can set the
bitmap to your imageView and invalidate your views