flashing background

Question

I have a LinearLayout with a few Buttons and TextViews. I want my background to flash at timed intervals, say from red to white to red and so on. Right now, I am trying this code, but it gives me a null pointer exception.

LinearLayout ll = (LinearLayout) findViewById(R.layout.activity_main);
Animation anim = new AlphaAnimation(0.0f, 1.0f);
anim.setDuration(50); 
anim.setStartOffset(20);
anim.setRepeatMode(Animation.REVERSE);
anim.setRepeatCount(Animation.INFINITE);
ll.startAnimation(anim); // shows null pointer exception at this line

Please help me where am I going wrong?

Please attach the logcat. – Dinesh Prajapati Mar 11 '13 at 04:52 — Dinesh Prajapati, Mar 11 '13 at 04:52

Vladimir Mironov · Accepted Answer · 2013-03-11T05:39:05.467

24

You have specified the wrong View id here findViewById(R.layout.activity_main). It should be something like:

findViewById(R.id.your_view_id);

Also, make sure to call setContentView(R.layout.activity_main) right after super.onCreate

EDIT:

Here is the code that allows you to change only the background color with any colors you want. It looks like AnimationDrawable.start() doesn't work if called from Activity.onCreate, so we have to use Handler.postDelayed here.

final LinearLayout layout = (LinearLayout) findViewById(R.id.layout);
final AnimationDrawable drawable = new AnimationDrawable();
final Handler handler = new Handler();

drawable.addFrame(new ColorDrawable(Color.RED), 400);
drawable.addFrame(new ColorDrawable(Color.GREEN), 400);
drawable.setOneShot(false);

layout.setBackgroundDrawable(drawable);
handler.postDelayed(new Runnable() {
    @Override
    public void run() {
        drawable.start();
    }
}, 100);

edited Mar 11 '13 at 05:39

answered Mar 11 '13 at 04:53

Vladimir Mironov

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thanks :) works now... can u help me with setting colors to the animation? also, is it possible that my buttons n textviews do not blink and just the background part does? – newbee Mar 11 '13 at 04:59
when i use this, it just sets the background color to red. no animation. also, `layout.setBackground` is undefined for type linear layout. eclipse gives 3 fixes- `setBackgroundColor()`, `setBackgroundDrawable()` and `setBackgroundResource()` – newbee Mar 11 '13 at 05:23
ooops, it should be `setBackgroundDrawable`. if you have no any animation make sure to call `drawable.start()`. Pleast note, that it **must** be called **after** calling `setBackgroundDrawable` – Vladimir Mironov Mar 11 '13 at 05:26
i called drawable.start() still it shows just the red background screen – newbee Mar 11 '13 at 05:29
`setBackgroundDrawable()` is deprecated, use `ViewCompat.setBackground(layout,drawable);` instead. – Sam Jun 28 '19 at 18:41

score 6 · Answer 2 · answered Mar 11 '13 at 04:57

Try this

LinearLayout ll = (LinearLayout) findViewById(R.id.activity_main);
Animation anim = new AlphaAnimation(0.0f, 1.0f);
anim.setDuration(50); 
anim.setStartOffset(20);
anim.setRepeatMode(Animation.REVERSE);
anim.setRepeatCount(Animation.INFINITE);
ll.startAnimation(anim);

and If activity_main is your XML file name then

setContentView(R.layout.activity_main);

and use your layout id here

LinearLayout ll = (LinearLayout) findViewById(R.id.linear_layout_id);

flashing background

2 Answers2

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