Vladimir Gazbarov Vladimir Gazbarov - 1 year ago 133
Android Question

Android - cannot capture backspace/delete press in soft. keyboard

I am overriding the onKeyDown method of the view (openGL surface view) to capture
all of the key presses. The problem is that on several devices the KEYCODE_DEL is not
captured. I have tried adding an onKeyListener to the view, and that captured everything except backspace key.

There has to be a way to listen to this key press event, but how?

Answer Source

[11/12/2014 UPDATE: Changed scope of fix to not limit to < API level 19, since at a third party keyboard still has the bug beyond 19.] [1/9/2014 UPDATE: I've devised an approach, with code, to resolve all Google Keyboard (LatinIME) KEYCODE_DEL problems, specifically issues 42904 and 62306.] The enhancement in Turix's answer has been incorporated, with permission, into my own code here. Turix's improvements removed need to inject garbage characters into the Editable buffer by instead finding an incremental way to ensure that exactly one character was always in that buffer.

I've used (similar) code to this in a deployed app that you're welcome to test:


The workaround presented below is intended to work for all versions of the Google Keyboard, both past and future, so far as these two bugs are concerned. This workaround does not require that an app remain stuck targeting API level 15 or below, which some apps have restricted themselves to in order to take advantage of compatibility code that gets around issue 42904.

These problems are only present as bugs for a view that has implemented the override for onCreateInputConnection(), and which returns TYPE_NULL to the invoking IME (in the inputType member of the EditorInfo argument passed to that method by the IME). It is only by doing this that a view can reasonably expect that key events (including KEYCODE_DEL) will be returned to it from a soft keyboard. Consequently, the workaround presented here requires the TYPE_NULL InputType.

For apps not using TYPE_NULL, there are various overrides in the BaseInputConnection-derived object returned by a view from its onCreateInputConnection() override, that are invoked by the IME when the user performs edits, instead of the IME generating key events. This (non TYPE_NULL) approach is usually superior, because the soft keyboard's capabilities now extend far beyond the mere tapping of keys, to things like voice input, completion, etc. Key events are an older method, and those implementing LatinIME at Google have said that they would like to see the use of TYPE_NULL (and key events) go away.

If discontinuing the use of TYPE_NULL is an option, they I would urge you to proceed with the recommended approach of using the InputConnection override methods instead of key events (or, more simply, by using a class derived from EditText, which does that for you).

Nonetheless, TYPE_NULL behavior is not being officially discontinued, and thus the failure of LatinIME to generate KEYCODE_DEL events under certain circumstances is indeed a bug. I offer the following workaround to address this problem.


The problems that apps have had in receiving KEYCODE_DEL from LatinIME are due to TWO known bugs, as reported here:

https://code.google.com/p/android/issues/detail?id=42904 (listed as WorkingAsIntended, but the problem is, I maintain, a bug inasmuch as it causes a failure to support KEYCODE_DEL event generation for apps targeting API level 16 and above that have specifically listed an InputType of TYPE_NULL. The problem is fixed in the latest releases of LatinIME, but there are past releases in the wild that still exhibit this bug, and so apps using TYPE_NULL and targeting API Level 16 or above will still need a workaround that can be performed from within the app.

and here:

http://code.google.com/p/android/issues/detail?id=62306 (presently listed as fixed but not yet released - FutureRelease - but even once it is released, we will still need a workaround that can be performed from within the app to deal with the past releases that will persist "in the wild").

Consistent with this thesis (that the problems experienced with KEYCODE_DEL events are due to bugs in LatinIME), I have found that when using an external hardware keyboard, and also when using the third party SwiftKey soft keyboard, these problems do not occur, while they do occur for specific versions of LatinIME.

One or the other (but not both at once) of these problems is present in some LatinIME releases. Consequently, it is difficult for developers to know during testing whether they have worked around all KEYCODE_DEL problems, and sometimes when an Android (or Google Keyboard) update is performed, a problem will no longer be reproducible in testing. Nonetheless, the LatinIME versions that cause the problem will be present on quite a number of devices in use. This has forced me to dig into the AOSP LatinIME git repo to determine the exact scope of each of the two problems (i.e., the specific LatinIME, and Android, versions for which each of the two issues may be present). The workaround code below has been restricted to those specific versions.

The workaround code presented below includes extensive comments which should help you to understand what it is attempting to accomplish. Following the presentation of the code, I will provide some additional discussion, which will include the specific Android Open Source Project (AOSP) commits at which each of the two bugs was introduced, and at which it disappeared, and also the Android versions that might include the affected Google Keyboard releases.

I would warn anyone thinking of using this approach to perform their own testing to verify that it works for their particular app. I think that it will work in general, and have tested it on a number of devices and LatinIME versions, but the reasoning is complicated, so proceed with caution. If you find any problems, please post a comment below.


Here, then, is my workaround for both of the two problems, with an explanation included in the comments to the code:

First, include the following class (edited to taste) in your app, in its own source file InputConnectionAccomodatingLatinIMETypeNullIssues.java:

import android.view.KeyEvent;
import android.view.View;
import android.view.inputmethod.BaseInputConnection;

 * @author Carl Gunther
 * There are bugs with the LatinIME keyboard's generation of KEYCODE_DEL events 
 * that this class addresses in various ways.  These bugs appear when the app 
 * specifies TYPE_NULL, which is the only circumstance under which the app 
 * can reasonably expect to receive key events for KEYCODE_DEL.
 * This class is intended for use by a view that overrides 
 * onCreateInputConnection() and specifies to the invoking IME that it wishes 
 * to use the TYPE_NULL InputType.  This should cause key events to be returned 
 * to the view.
public class InputConnectionAccomodatingLatinIMETypeNullIssues extends BaseInputConnection {

    //This holds the Editable text buffer that the LatinIME mistakenly *thinks* 
    // that it is editing, even though the views that employ this class are 
    // completely driven by key events.
    Editable myEditable = null;

    //Basic constructor
    public InputConnectionAccomodatingLatinIMETypeNullIssues(View targetView, boolean fullEditor) {
        super(targetView, fullEditor);

    //This method is called by the IME whenever the view that returned an 
    // instance of this class to the IME from its onCreateInputConnection() 
    // gains focus.
    public Editable getEditable() {
      //Some versions of the Google Keyboard (LatinIME) were delivered with a 
      // bug that causes KEYCODE_DEL to no longer be generated once the number 
      // of KEYCODE_DEL taps equals the number of other characters that have 
      // been typed.  This bug was reported here as issue 62306.
      // As of this writing (1/7/2014), it is fixed in the AOSP code, but that 
      // fix has not yet been released.  Even when it is released, there will 
      // be many devices having versions of the Google Keyboard that include the bug
      // in the wild for the indefinite future.  Therefore, a workaround is required.
      //This is a workaround for that bug which just jams a single garbage character 
      // into the internal buffer that the keyboard THINKS it is editing even 
      // though we have specified TYPE_NULL which *should* cause LatinIME to 
      // generate key events regardless of what is in that buffer.  We have other
      // code that attempts to ensure as the user edites that there is always 
      // one character remaining.
      // The problem arises because when this unseen buffer becomes empty, the IME
      // thinks that there is nothing left to delete, and therefore stops 
      // generating KEYCODE_DEL events, even though the app may still be very 
      // interested in receiving them.
      //So, for example, if the user taps in ABCDE and then positions the 
      // (app-based) cursor to the left of A and taps the backspace key three 
      // times without any evident effect on the letters (because the app's own 
      // UI code knows that there are no letters to the left of the 
      // app-implemented cursor), and then moves the cursor to the right of the 
      // E and hits backspace five times, then, after E and D have been deleted, 
      // no more KEYCODE_DEL events will be generated by the IME because the 
      // unseen buffer will have become empty from five letter key taps followed 
      // by five backspace key taps (as the IME is unaware of the app-based cursor 
      // movements performed by the user).  
      // In other words, if your app is processing KEYDOWN events itself, and 
      // maintaining its own cursor and so on, and not telling the IME anything 
      // about the user's cursor position, this buggy processing of the hidden 
      // buffer will stop KEYCODE_DEL events when your app actually needs them - 
      // in whatever Android releases incorporate this LatinIME bug.
      // By creating this garbage characters in the Editable that is initially
      // returned to the IME here, we make the IME think that it still has 
      // something to delete, which causes it to keep generating KEYCODE_DEL 
      // events in response to backspace key presses.
      // A specific keyboard version that I tested this on which HAS this 
      // problem but does NOT have the "KEYCODE_DEL completely gone" (issue 42904)
      // problem that is addressed by the deleteSurroundingText() override below 
      // (the two problems are not both present in a single version) is 
      // 2.0.19123.914326a, tested running on a Nexus7 2012 tablet.  
      // There may be other versions that have issue 62306.
      // A specific keyboard version that I tested this on which does NOT have 
      // this problem but DOES have the "KEYCODE_DEL completely gone" (issue 
      // 42904) problem that is addressed by the deleteSurroundingText() 
      // override below is 1.0.1800.776638, tested running on the Nexus10 
      // tablet.  There may be other versions that also have issue 42904.
      // The bug that this addresses was first introduced as of AOSP commit tag 
      // 4.4_r0.9, and the next RELEASED Android version after that was 
      // android-4.4_r1, which is the first release of Android 4.4.  So, 4.4 will 
      // be the first Android version that would have included, in the original 
      // RELEASED version, a Google Keyboard for which this bug was present.
      // Note that this bug was introduced exactly at the point that the OTHER bug
      // (the one that is addressed in deleteSurroundingText(), below) was first 
      // FIXED.
      // Despite the fact that the above are the RELEASES associated with the bug, 
      // the fact is that any 4.x Android release could have been upgraded by the 
      // user to a later version of Google Keyboard than was present when the 
      // release was originally installed to the device.  I have checked the 
      // www.archive.org snapshots of the Google Keyboard listing page on the Google 
      // Play store, and all released updates listed there (which go back to early 
      // June of 2013) required Android 4.0 and up, so we can be pretty sure that 
      // this bug is not present in any version earlier than 4.0 (ICS), which means
      // that we can limit this fix to API level 14 and up.  And once the LatinIME 
      // problem is fixed, we can limit the scope of this workaround to end as of 
      // the last release that included the problem, since we can assume that 
      // users will not upgrade Google Keyboard to an EARLIER version than was 
      // originally included in their Android release.
      // The bug that this addresses was FIXED but NOT RELEASED as of this AOSP 
      // commit:
      // /b41bea65502ce7339665859d3c2c81b4a29194e4/java/src/com/android
      // /inputmethod/latin/LatinIME.java
      // so it can be assumed to affect all of KitKat released thus far 
      // (up to 4.4.2), and could even affect beyond KitKat, although I fully 
      // expect it to be incorporated into the next release *after* API level 19.
      // When it IS released, this method should be changed to limit it to no 
      // higher than API level 19 (assuming that the fix is released before API 
      // level 20), just in order to limit the scope of this fix, since poking 
      // 1024 characters into the Editable object returned here is of course a 
      // kluge.  But right now the safest thing is just to not have an upper limit 
      // on the application of this kluge, since the fix for the problem it 
      // addresses has not yet been released (as of 1/7/2014).
      if(Build.VERSION.SDK_INT >= 14) {
        if(myEditable == null) {
      myEditable = new EditableAccomodatingLatinIMETypeNullIssues(
          Selection.setSelection(myEditable, 1);
    else {
          int myEditableLength = myEditable.length(); 
          if(myEditableLength == 0) {
          //I actually HAVE seen this be zero on the Nexus 10 with the keyboard 
          // that came with Android 4.4.2
          // On the Nexus 10 4.4.2 if I tapped away from the view and then back to it, the 
          // myEditable would come back as null and I would create a new one.  This is also 
          // what happens on other devices (e.g., the Nexus 6 with 4.4.2,
          // which has a slightly later version of the Google Keyboard).  But for the 
          // Nexus 10 4.4.2, the keyboard had a strange behavior
          // when I tapped on the rack, and then tapped Done on the keyboard to close it, 
          // and then tapped on the rack AGAIN.  In THAT situation,
          // the myEditable would NOT be set to NULL but its LENGTH would be ZERO.  So, I 
          // just append to it in that situation.
          Selection.setSelection(myEditable, 1);
      return myEditable;
    else {
      //Default behavior for keyboards that do not require any fix
      return super.getEditable();

  //This method is called INSTEAD of generating a KEYCODE_DEL event, by 
  // versions of Latin IME that have the bug described in Issue 42904.
  public boolean deleteSurroundingText(int beforeLength, int afterLength) {
    //If targetSdkVersion is set to anything AT or ABOVE API level 16 
    // then for the GOOGLE KEYBOARD versions DELIVERED 
    // with Android 4.1.x, 4.2.x or 4.3.x, NO KEYCODE_DEL EVENTS WILL BE 
    // is being returned as the InputType by your view from its 
    // onCreateInputMethod() override, due to a BUG in THOSE VERSIONS.  
    // When TYPE_NULL is specified (as this entire class assumes is being done 
    // by the views that use it, what WILL be generated INSTEAD of a KEYCODE_DEL 
    // is a deleteSurroundingText(1,0) call.  So, by overriding this 
    // deleteSurroundingText() method, we can fire the KEYDOWN/KEYUP events 
    // ourselves for KEYCODE_DEL.  This provides a workaround for the bug.
    // The specific AOSP RELEASES involved are 4.1.1_r1 (the very first 4.1 
    // release) through 4.4_r0.8 (the release just prior to Android 4.4).  
    // This means that all of KitKat should not have the bug and will not 
    // need this workaround.
    // Although 4.0.x (ICS) did not have this bug, it was possible to install 
    // later versions of the keyboard as an app on anything running 4.0 and up, 
    // so those versions are also potentially affected.
    // The first version of separately-installable Google Keyboard shown on the 
    // Google Play store site by www.archive.org is Version 1.0.1869.683049, 
    // on June 6, 2013, and that version (and probably other, later ones) 
    // already had this bug.
    //Since this required at least 4.0 to install, I believe that the bug will 
    // not be present on devices running versions of Android earlier than 4.0.  
    //AND, it should not be present on versions of Android at 4.4 and higher,
    // since users will not "upgrade" to a version of Google Keyboard that 
    // is LOWER than the one they got installed with their version of Android 
    // in the first place, and the bug will have been fixed as of the 4.4 release.
    // The above scope of the bug is reflected in the test below, which limits 
    // the application of the workaround to Android versions between 4.0.x and 4.3.x.
    //UPDATE:  A popular third party keyboard was found that exhibits this same issue.  It
    // was not fixed at the same time as the Google Play keyboard, and so the bug in that case
    // is still in place beyond API LEVEL 19.  So, even though the Google Keyboard fixed this
    // as of level 19, we cannot take out the fix based on that version number.  And so I've
    // removed the test for an upper limit on the version; the fix will remain in place ad
    // infinitum - but only when TYPE_NULL is used, so it *should* be harmless even when
    // the keyboard does not have the problem...
    if((Build.VERSION.SDK_INT >= 14) // && (Build.VERSION.SDK_INT < 19) 
      && (beforeLength == 1 && afterLength == 0)) {
      //Send Backspace key down and up events to replace the ones omitted 
      // by the LatinIME keyboard.
      return super.sendKeyEvent(new KeyEvent(KeyEvent.ACTION_DOWN, KeyEvent.KEYCODE_DEL))
        && super.sendKeyEvent(new KeyEvent(KeyEvent.ACTION_UP, KeyEvent.KEYCODE_DEL));
    else {
      //Really, I can't see how this would be invoked, given that we're using 
      // TYPE_NULL, for non-buggy versions, but in order to limit the impact 
      // of this change as much as possible (i.e., to versions at and above 4.0) 
      // I am using the original behavior here for non-affected versions.
      return super.deleteSurroundingText(beforeLength, afterLength);


Next, take each View-derived class that needs to receive key events from the LatinIME soft keyboard, and edit it as follows:

First, create an override to onCreateInputConnection() in the view that is to receive key events as follows:

 public InputConnection onCreateInputConnection(EditorInfo outAttrs) {
  //Passing FALSE as the SECOND ARGUMENT (fullEditor) to the constructor 
  // will result in the key events continuing to be passed in to this 
  // view.  Use our special BaseInputConnection-derived view
  InputConnectionAccomodatingLatinIMETypeNullIssues baseInputConnection = 
    new InputConnectionAccomodatingLatinIMETypeNullIssues(this, false);

   //In some cases an IME may be able to display an arbitrary label for a 
   // command the user can perform, which you can specify here.  A null value
   // here asks for the default for this key, which is usually something 
   // like Done.
   outAttrs.actionLabel = null;

   //Special content type for when no explicit type has been specified. 
   // This should be interpreted (by the IME that invoked 
   // onCreateInputConnection())to mean that the target InputConnection 
   // is not rich, it can not process and show things like candidate text 
   // nor retrieve the current text, so the input method will need to run 
   // in a limited "generate key events" mode.  This disables the more 
   // sophisticated kinds of editing that use a text buffer.
   outAttrs.inputType = InputType.TYPE_NULL;

   //This creates a Done key on the IME keyboard if you need one
   outAttrs.imeOptions = EditorInfo.IME_ACTION_DONE;

   return baseInputConnection;

Second, make the following changes to your onKey() handler for the view:

 this.setOnKeyListener(new OnKeyListener() {   
   @Override   public
   boolean onKey(View v, int keyCode, KeyEvent event) {
     if(event.getAction() != KeyEvent.ACTION_DOWN) {
       //We only look at ACTION_DOWN in this code, assuming that ACTION_UP is redundant.  
       // If not, adjust accordingly.
       return false;
     else if(event.getUnicodeChar() == 
       //We are ignoring this character, and we want everyone else to ignore it, too, so 
       // we return true indicating that we have handled it (by ignoring it).   
       return true; 

     //Now, just do your event handling as usual...
     if(keyCode == KeyEvent.KEYCODE_ENTER) {
       //Trap the Done key and close the keyboard if it is pressed (if that's what you want to do)
       InputMethodManager imm = (InputMethodManager)
       imm.hideSoftInputFromWindow(LetterRack.this.getWindowToken(), 0);
       return true;
     else if(keyCode == KeyEvent.KEYCODE_DEL) {
       //Backspace key processing goes here...                      
       return true;
     else if((keyCode >= KeyEvent.KEYCODE_A) && (keyCode <= KeyEvent.KEYCODE_Z)) {
       //(Or, use event.getUnicodeChar() if preferable to key codes).
       //Letter processing goes here...
       return true;
     //Etc.   } };

Finally, we need to define a class for our editable that ensures that there is always at least one character in our editable buffer:

import android.text.SpannableStringBuilder;

public class EditableAccomodatingLatinIMETypeNullIssues extends SpannableStringBuilder {
  EditableAccomodatingLatinIMETypeNullIssues(CharSequence source) {

  //This character must be ignored by your onKey() code.    
  public static CharSequence ONE_UNPROCESSED_CHARACTER = "/";

  public SpannableStringBuilder replace(final int 
    spannableStringStart, final int spannableStringEnd, CharSequence replacementSequence, 
    int replacementStart, int replacementEnd) {
    if (replacementEnd > replacementStart) {
      //In this case, there is something in the replacementSequence that the IME 
      // is attempting to replace part of the editable with.
      //We don't really care about whatever might already be in the editable; 
      // we only care about making sure that SOMETHING ends up in it,
      // so that the backspace key will continue to work.
      // So, start by zeroing out whatever is there to begin with.
      super.replace(0, length(), "", 0, 0);

      //We DO care about preserving the new stuff that is replacing the stuff in the 
      // editable, because this stuff might be sent to us as a keydown event.  So, we 
      // insert the new stuff (typically, a single character) into the now-empty editable, 
      // and return the result to the caller.
      return super.replace(0, 0, replacementSequence, replacementStart, replacementEnd);
    else if (spannableStringEnd > spannableStringStart) {
      //In this case, there is NOTHING in the replacementSequence, and something is 
      // being replaced in the editable.
      // This is characteristic of a DELETION.
      // So, start by zeroing out whatever is being replaced in the editable.
      super.replace(0, length(), "", 0, 0);

      //And now, we will place our ONE_UNPROCESSED_CHARACTER into the editable buffer, and return it. 
      return super.replace(0, 0, ONE_UNPROCESSED_CHARACTER, 0, 1);

    // In this case, NOTHING is being replaced in the editable.  This code assumes that there 
    // is already something there.  This assumption is probably OK because in our 
    // InputConnectionAccomodatingLatinIMETypeNullIssues.getEditable() method 
    // we PLACE a ONE_UNPROCESSED_CHARACTER into the newly-created buffer.  So if there 
    // is nothing replacing the identified part
    // of the editable, and no part of the editable that is being replaced, then we just 
    // leave whatever is in the editable ALONE,
    // and we can be confident that there will be SOMETHING there.  This call to super.replace() 
    // in that case will be a no-op, except
    // for the value it returns.
    return super.replace(spannableStringStart, spannableStringEnd, 
      replacementSequence, replacementStart, replacementEnd);

That completes the source changes that I've found seem to handle both problems.


The problem described by Issue 42904 was introduced in the LatinIME version delivered with API level 16. Prior to that, KEYCODE_DEL events were generated regardless of whether TYPE_NULL was used. In the LatinIME released with Jelly Bean, this generation was discontinued, but no exception was made for TYPE_NULL, and so TYPE_NULL behavior was effectively disabled for apps targeted above API level 16. There was, however, compatibility code added that allowed apps that had a targetSdkVersion < 16 to continue to receive KEYCODE_DEL events, even without TYPE_NULL. See this AOSP commit at line 1493:


Therefore, you could work around this problem by setting targetSdkVersion in your app to 15 or lower.

As of commit 4.4_r0.9 (just prior to the 4.4 release), this problem was fixed by adding a test for isTypeNull() to the conditions guarding KEYCODE_DEL generation. Unfortunately, a new bug (62306) was introduced at exactly that point which caused the entire clause wrapping KEYCODE_DEL generation to be skipped if the user had typed backspace as many times as she had typed other characters. This led to a failure to generate KEYCODE_DEL under those circumstances, even with TYPE_NULL, and even with targetSdkVersion <= 15. This caused apps that had previously been able to get correct KEYCODE_DEL behavior via compatibility code (targetSdkVersion <= 15) to suddenly experience this problem when users upgraded their copies of Google Keyboard (or performed an OTA that contained a new version of Google Keyboard). See this AOSP git file at line 2146 (the clause including "NOT_A_CODE"):


This problem has persisted in released versions of Google Keyboard to the present time (1/7/2014). It has been fixed in the repo, but as of this writing has not been released.

That unreleased commit can be found here (the git commit containing this merges a commit titled "Send backspace as an event when TYPE_NULL"), at line 2110 (you can see that the "NOT_A_CODE" clause that used to prevent our reaching the clause that generates KEYCODE_DEL has been removed):


When this fix is released, that version of the Google Keyboard will no longer have either of these two problems affecting TYPE_NULL. However, there will still be older versions installed on particular devices for the indefinite future. Hence, the problem will still need a workaround. Eventually, as more people upgrade to a higher level than the last one not including the fix, this workaround will be needed less and less. But it is already scoped to phase itself out (once you make the indicated changes to put the final limit on the scope, when the final fix has actually been released so that you know what it actually is).

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