为什么在进行数据更改之前调用 LiveData onChanged？

Question

语境使用MutableLiveData保存的值。Button单击应该增加值。问题MutableLiveData'sObserver.onChanged()似乎在MutableLiveData.setValue()用于更改值之前被调用。例如，在Button单击之前，onChanged日志发生在初始值设置之后（onChanged纳秒大于新值纳秒）。但是，Button单击后，onChanged日志发生在设置新值之前（onChanged纳秒小于新值纳秒）。为什么会发生这种情况？：Button单击之前（onChanged日志预期在设置初始值后发生）：09-06 20:30:47.877 com.example.android.test D/TAG: initial set val ns 102107899222617    initial get val ns 10210789936709609-06 20:30:47.882 com.example.android.test D/TAG: onChanged integer 0 ns 102107903996992单击按钮后（在设置新 val 之前意外发生onChanged日志）：09-06 20:30:55.372 com.example.android.test D/TAG: onChanged integer 1 ns 10211539417823809-06 20:30:55.373 com.example.android.test D/TAG: onClick set new val 1 ns 102115394446415代码final MutableLiveData<Integer> val = new MutableLiveData<>();val.setValue(0); // triggers onChangedLog.d(TAG, "initial set val ns " + System.nanoTime());tv.setText("" + val.getValue());Log.d(TAG, "initial get val ns " + System.nanoTime());val.observe(this, new Observer<Integer>() {    @Override    public void onChanged(@Nullable Integer integer) {        // onChanged happens after initial setValue but before setValue with new value        Log.d(TAG, "onChanged integer " + integer + " ns " + System.nanoTime());        tv.setText("" + integer);    }});btn.setOnClickListener(new View.OnClickListener() {    @Override    public void onClick(View v) {        // setting new value (triggers onChanged but         // onChanged happens before setting new value via setValue)        int newVal = val.getValue() + 1;        val.setValue(newVal);        Log.d(TAG, "onClick set new val " + newVal + " ns " + System.nanoTime());    }});