IssueTracker

The problem is reproducible using support lib 26.1.0, this is the updated activity:

class LiveDataTestActivity : AppCompatActivity() {

    private val myLiveData = MutableLiveData<Int>().apply { value = 0 }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val textView = TextView(this).apply {
            text = "AAAA"
            setOnClickListener {
                val sendIntent = Intent().apply {
                    action = Intent.ACTION_SEND
                    putExtra(Intent.EXTRA_TEXT, "This is my text to send.")
                    type = "text/plain"
                }
                startActivityForResult(sendIntent, 123)
            }
        }
        setContentView(textView)

        myLiveData.observe(this, Observer<Int> {
            textView.text = it.toString()
        })
    }

    override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        super.onActivityResult(requestCode, resultCode, data)
        myLiveData.value = (myLiveData.value ?: 0) + 1
        println("Value ${myLiveData.value}")
    }

    override fun onResume() {
        super.onResume()
        println("resume ${lifecycle.currentState}")
    }
}

thanks for the reports.

After further investigation:
good news, this is not a very serious bug, it recovers right after onResume. (also the bug does not happen in latest versions of the OS)
bad news, properly fixing it requires changes in the support library so it may not catch 1.0.0 release.

details:
Lifecycles moves state to CREATED when onSavedInstanceState is called. This is done because FragmentManager disables fragment transactions when onSavedInstanceState is called. Lifecycles API guaratees that you can run a fragment transaction inside an observer. This behavior is usually fine because onSavedInstanceState is always followed by onStop.

Turns out, this is not the case for API21..

things to do:
* make sure we have tests that fragments are handled properly and observers don't receive an unexpected STARTED event.
* in the next support lib version, make sure the state is set to at least STARTED when FragmentController#noteStateNotSaved is called.

Fabio, thanks for the bug report and example code.  We have a fix in the works that will update the TextView on 5.0 (the problem actually existed on 6.0 and below) just like it should on 6.0.

However, note that on 6.0 and below your onResume method will continue to print "CREATED" as Yigit mentioned above.

Thanks again!

reopened. fix for live data is in but  the greater fix to make lifecycles consistent is still not merged.

Norm

Fragment ransaction bug:

UncaughtException
                                                                                 java.lang.IllegalStateException: The specified child already has a parent. You must call removeView() on the child's parent first.
                                                                                     at android.view.ViewGroup.addViewInner(ViewGroup.java:4937)
                                                                                     at android.view.ViewGroup.addView(ViewGroup.java:4768)
                                                                                     at android.view.ViewGroup.addView(ViewGroup.java:4708)
                                                                                     at android.view.ViewGroup.addView(ViewGroup.java:4681)
                                                                                     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1425)
                                                                                     at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1750)
                                                                                     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1819)
                                                                                     at android.support.v4.app.BackStackRecord.executePopOps(BackStackRecord.java:855)
                                                                                     at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2587)
                                                                                     at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2377)
                                                                                     at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2332)
                                                                                     at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2239)
                                                                                     at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:700)
                                                                                     at android.os.Handler.handleCallback(Handler.java:790)
                                                                                     at android.os.Handler.dispatchMessage(Handler.java:99)
                                                                                     at android.os.Looper.loop(Looper.java:164)
                                                                                     at android.app.ActivityThread.main(ActivityThread.java:6494)
                                                                                     at java.lang.reflect.Method.invoke(Native Method)
                                                                                     at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
                                                                                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)