Writing Responsive Applications

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One of the keys to maintaining a responsive GUI is to do long-running tasks on a background thread so the GUI doesn't get blocked. Let's say we want to calculate a value to display to the user, but that value takes 5 seconds to calculate:

public class ThreadDemo : Activity
{
    TextView textview;

    protected override void OnCreate (Bundle bundle)
    {
        base.OnCreate (bundle);

        // Create a new TextView and set it as our view
        textview = new TextView (this);
        textview.Text = "Working..";

        SetContentView (textview);

        SlowMethod ();
    }

    private void SlowMethod ()
    {
        Thread.Sleep (5000);
        textview.Text = "Method Complete";
    }
}

This will work, but the application will "hang" for 5 seconds while the value is calculated. During this time, the app will not respond to any user interaction. To get around this, we want to do our calculations on a background thread:

public class ThreadDemo : Activity
{
    TextView textview;

    protected override void OnCreate (Bundle bundle)
    {
        base.OnCreate (bundle);

        // Create a new TextView and set it as our view
        textview = new TextView (this);
        textview.Text = "Working..";

        SetContentView (textview);

        ThreadPool.QueueUserWorkItem (o => SlowMethod ());
    }

    private void SlowMethod ()
    {
        Thread.Sleep (5000);
        textview.Text = "Method Complete";
    }
}

Now we calculate the value on a background thread so our GUI stays responsive during the calculation. However, when the calculation is done, our app crashes, leaving this in the log:

E/mono    (11207): EXCEPTION handling: Android.Util.AndroidRuntimeException: Exception of type 'Android.Util.AndroidRuntimeException' was thrown.
E/mono    (11207):
E/mono    (11207): Unhandled Exception: Android.Util.AndroidRuntimeException: Exception of type 'Android.Util.AndroidRuntimeException' was thrown.
E/mono    (11207):   at Android.Runtime.JNIEnv.CallVoidMethod (IntPtr jobject, IntPtr jmethod, Android.Runtime.JValue[] parms)
E/mono    (11207):   at Android.Widget.TextView.set_Text (IEnumerable`1 value)
E/mono    (11207):   at MonoDroidDebugging.Activity1.SlowMethod ()

This is because you must update the GUI from the GUI thread. Our code updates the GUI from the ThreadPool thread, causing the app to crash. We need to calculate our value on the background thread, but then do our update on the GUI thread, which is handled with Activity.RunOnUIThread:

public class ThreadDemo : Activity
{
    TextView textview;

    protected override void OnCreate (Bundle bundle)
    {
        base.OnCreate (bundle);

        // Create a new TextView and set it as our view
        textview = new TextView (this);
        textview.Text = "Working..";

        SetContentView (textview);

        ThreadPool.QueueUserWorkItem (o => SlowMethod ());
    }

    private void SlowMethod ()
    {
        Thread.Sleep (5000);
        RunOnUiThread (() => textview.Text = "Method Complete");
    }
}

This code works as expected. This GUI stays responsive and gets properly updated once the calculation is comple.

Note this technique isn't just used for calculating an expensive value. It can be used for any long-running task that can be done in the background, like a web service call or downloading internet data.

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