Android - Why does onKeyUp trigger a KeyEvent, but onKeyDown does not?

Question

In my current View, a keyboard is open with numbers 0 to 9, a delete key and an enter key. I added both an onKeyDown and an onKeyUp event. Both should log the keycode to the debug console when pressed.

@Override
public boolean onKeyDown(int pKeyCode, KeyEvent pKeyEvent) {
    Log.d("KEYDOWN", pKeyCode+"");
    return true; // also tried false
}
@Override
public boolean onKeyUp(int pKeyCode, KeyEvent pKeyEvent) {
    Log.d("KEYUP", pKeyCode+"");
    return true;
}

I pressed every single key on the keyboard with the following result:

D/KEYUP: 8
D/KEYUP: 9
D/KEYUP: 10
D/KEYUP: 11
D/KEYUP: 12
D/KEYUP: 13
D/KEYUP: 14
D/KEYUP: 15
D/KEYUP: 16
D/KEYDOWN: 67
D/KEYUP: 67
D/KEYUP: 7
D/KEYUP: 66

Can someone explain why the keydown event is never triggered for 0-9 and enter? I really need it to trigger when 0-9 are pressed.

EDIT: I added a solution that fixed the issue by replacing both functions with dispatchKeyEvent. This doesn't explain why onKeyDowndidn't recognize those keys, though.

Someone got an answer for this?

If you perform the same test but with ONLY the keydown event, what do you get? — Peter, Oct 17 '16 at 16:13

score 4 · Accepted Answer · answered Oct 17 '16 at 16:27

4

I only found a solution to work around this issue, but I did not find the reason WHY this happens in the first place.

This code did the job:

@Override
public boolean dispatchKeyEvent(KeyEvent event) {
    if (event.getAction()==KeyEvent.ACTION_DOWN) {
        Log.d("KEYDOWN", event.getKeyCode()+"");
    }
    else if (event.getAction()==KeyEvent.ACTION_UP) {
        Log.d("KEYUP", event.getKeyCode()+"");
    }
    Log.d("KEY", event.getKeyCode()+"");
    return false;
}

answered Oct 17 '16 at 16:27

keinabel

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What exact View you are using? – Yurii Tsap Oct 17 '16 at 17:27
I am quite new to Android, so I actually do not really know how to answer your question @YuriiTsap. I just created a standard project in Android Studio and this is the MainActivity. Don't know what View is used there. – keinabel Oct 18 '16 at 15:04
Please describe your exact problem or what you are trying to solve. – Yurii Tsap Oct 19 '16 at 05:21
I am logging sensor data during runtime. And in this data log I need to capture what keys are being (if any) pressed for each of the logged data entries. Since, theoretically, more than one data entry could be logged during a key-press, I need both the `onKeyDown` and `onKeyUp` events, so I can enable/disable a flag that will then put the corresponding keys into my data entry. – keinabel Oct 19 '16 at 12:00
Have you tried to override EditText and handle this inside of it? – Yurii Tsap Oct 19 '16 at 12:20

score 1 · Answer 2 · answered Oct 17 '16 at 20:06

To understand the workflow you need to dive inside the code a little bit, it's not so obvious but nevertheless. Your particular case depends on which view you are extending and the IME. From docs :

In general, the framework cannot guarantee that the key events it delivers to a view always constitute complete key sequences since some events may be dropped or modified by containing views before they are delivered. The view implementation should be prepared to handle FLAG_CANCELED and should tolerate anomalous situations such as receiving a new ACTION_DOWN without first having received an ACTION_UP for the prior key press.

You can use TextWatcher - this is a good solution, but I'm not aware of your needs. In any case if you are overriding onKeyDown() - it will be triggered, and you need to handle it by yourself or call parent implementation. In the case with EditText - onKeyDown will be handled by TextView, despite few cases(Back Button i.e.).

Android - Why does onKeyUp trigger a KeyEvent, but onKeyDown does not?

2 Answers2