jetpack-LiveData实现双向绑定原理

重要的行代码

mDbActivityBinding.lifecycleOwner = this

 override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        mDbActivityBinding = DataBindingUtil.setContentView(this, R.layout.db_activity)
        mDbActivityBinding.mdbViewModel =
                ViewModelProviders.of(this).get(DBViewModel::class.java)
        mDbActivityBinding.lifecycleOwner = this

  @OnLifecycleEvent(Lifecycle.Event.ON_START)
        public void onStart() {
            ViewDataBinding dataBinding = mBinding.get();
            if (dataBinding != null) {
                dataBinding.executePendingBindings();
            }
        }

ViewDataBinding.executeBindingsInternal

 private void executeBindingsInternal() {
        if (mIsExecutingPendingBindings) {
            requestRebind();
            return;
        }
        if (!hasPendingBindings()) {
            return;
        }
        mIsExecutingPendingBindings = true;
        mRebindHalted = false;
        if (mRebindCallbacks != null) {
            mRebindCallbacks.notifyCallbacks(this, REBIND, null);

            // The onRebindListeners will change mPendingHalted
            if (mRebindHalted) {
                mRebindCallbacks.notifyCallbacks(this, HALTED, null);
            }
        }
        if (!mRebindHalted) {
            executeBindings();
            if (mRebindCallbacks != null) {
                mRebindCallbacks.notifyCallbacks(this, REBOUND, null);
            }
        }
        mIsExecutingPendingBindings = false;
    }

executeBindings(),这里有注册监听（如果没注册）和把数据绑定到TextView

 @Override
    protected void executeBindings() {
        long dirtyFlags = 0;
        synchronized(this) {
            dirtyFlags = mDirtyFlags;
            mDirtyFlags = 0;
        }
        java.lang.String mDBViewModelUserLastName = null;
        com.seekting.demo2019.jetpack.DBViewModel mDBViewModel = mMDBViewModel;
        com.seekting.demo2019.jetpack.User mDBViewModelUserGetValue = null;
        java.lang.String mDBViewModelUserFirstName = null;
        java.lang.String mDBViewModelPwd = null;
        androidx.lifecycle.MutableLiveData<com.seekting.demo2019.jetpack.User> mDBViewModelUser = null;
        androidx.databinding.ObservableField<java.lang.String> mDBViewModelUserFullName = null;
        java.lang.String mDBViewModelUserFullNameGet = null;

        if ((dirtyFlags & 0x17L) != 0) {


            if ((dirtyFlags & 0x14L) != 0) {

                    if (mDBViewModel != null) {
                        // read mDBViewModel.pwd
                        mDBViewModelPwd = mDBViewModel.getPwd();
                    }
            }

                if (mDBViewModel != null) {
                    // read mDBViewModel.user
                    mDBViewModelUser = mDBViewModel.getUser();
                }
                updateLiveDataRegistration(0, mDBViewModelUser);


                if (mDBViewModelUser != null) {
                    // read mDBViewModel.user.getValue()
                    mDBViewModelUserGetValue = mDBViewModelUser.getValue();
                }

            if ((dirtyFlags & 0x15L) != 0) {

                    if (mDBViewModelUserGetValue != null) {
                        // read mDBViewModel.user.getValue().lastName
                        mDBViewModelUserLastName = mDBViewModelUserGetValue.getLastName();
                        // read mDBViewModel.user.getValue().firstName
                        mDBViewModelUserFirstName = mDBViewModelUserGetValue.getFirstName();
                    }
            }

                if (mDBViewModelUserGetValue != null) {
                    // read mDBViewModel.user.getValue().fullName
                    mDBViewModelUserFullName = mDBViewModelUserGetValue.getFullName();
                }
                updateRegistration(1, mDBViewModelUserFullName);


                if (mDBViewModelUserFullName != null) {
                    // read mDBViewModel.user.getValue().fullName.get()
                    mDBViewModelUserFullNameGet = mDBViewModelUserFullName.get();
                }
        }
        // batch finished
        if ((dirtyFlags & 0x15L) != 0) {
            // api target 1

            androidx.databinding.adapters.TextViewBindingAdapter.setText(this.mboundView1, mDBViewModelUserFirstName);
            androidx.databinding.adapters.TextViewBindingAdapter.setText(this.mboundView2, mDBViewModelUserLastName);
        }
        if ((dirtyFlags & 0x10L) != 0) {
            // api target 1

            androidx.databinding.adapters.TextViewBindingAdapter.setTextWatcher(this.mboundView1, (androidx.databinding.adapters.TextViewBindingAdapter.BeforeTextChanged)null, (androidx.databinding.adapters.TextViewBindingAdapter.OnTextChanged)null, (androidx.databinding.adapters.TextViewBindingAdapter.AfterTextChanged)null, mboundView1androidTextAttrChanged);
            androidx.databinding.adapters.TextViewBindingAdapter.setTextWatcher(this.mboundView2, (androidx.databinding.adapters.TextViewBindingAdapter.BeforeTextChanged)null, (androidx.databinding.adapters.TextViewBindingAdapter.OnTextChanged)null, (androidx.databinding.adapters.TextViewBindingAdapter.AfterTextChanged)null, mboundView2androidTextAttrChanged);
            androidx.databinding.adapters.TextViewBindingAdapter.setTextWatcher(this.mboundView3, (androidx.databinding.adapters.TextViewBindingAdapter.BeforeTextChanged)null, (androidx.databinding.adapters.TextViewBindingAdapter.OnTextChanged)null, (androidx.databinding.adapters.TextViewBindingAdapter.AfterTextChanged)null, mboundView3androidTextAttrChanged);
            androidx.databinding.adapters.TextViewBindingAdapter.setTextWatcher(this.mboundView4, (androidx.databinding.adapters.TextViewBindingAdapter.BeforeTextChanged)null, (androidx.databinding.adapters.TextViewBindingAdapter.OnTextChanged)null, (androidx.databinding.adapters.TextViewBindingAdapter.AfterTextChanged)null, mboundView4androidTextAttrChanged);
            this.mboundView5.setOnClickListener(mCallback1);
        }
        if ((dirtyFlags & 0x17L) != 0) {
            // api target 1

            androidx.databinding.adapters.TextViewBindingAdapter.setText(this.mboundView3, mDBViewModelUserFullNameGet);
        }
        if ((dirtyFlags & 0x14L) != 0) {
            // api target 1

            androidx.databinding.adapters.TextViewBindingAdapter.setText(this.mboundView4, mDBViewModelPwd);
        }
    }

updateLiveDataRegistration会调registerTo

 protected void registerTo(int localFieldId, Object observable,
            CreateWeakListener listenerCreator) {
        if (observable == null) {
            return;
        }
        WeakListener listener = mLocalFieldObservers[localFieldId];
        if (listener == null) {
            listener = listenerCreator.create(this, localFieldId);
            mLocalFieldObservers[localFieldId] = listener;
            if (mLifecycleOwner != null) {
                listener.setLifecycleOwner(mLifecycleOwner);
            }
        }
        listener.setTarget(observable);
    }

listener.setTarget(observable) 第i个成员变量监听了livedata的变化

public void setTarget(T object) {
    unregister();
    mTarget = object;
    if (mTarget != null) {
        "mObservable就是LiveData"
        mObservable.addListener(mTarget);
    }
}

当setValue被调用

@MainThread
protected void setValue(T value) {
    assertMainThread("setValue");
    mVersion++;
    mData = value;
    dispatchingValue(null);
}
void dispatchingValue(@Nullable ObserverWrapper initiator) {
    if (mDispatchingValue) {
        mDispatchInvalidated = true;
        return;
    }
    mDispatchingValue = true;
    do {
        mDispatchInvalidated = false;
        if (initiator != null) {
            considerNotify(initiator);
            initiator = null;
        } else {
            for (Iterator<Map.Entry<Observer<? super T>, ObserverWrapper>> iterator =
                    mObservers.iteratorWithAdditions(); iterator.hasNext(); ) {
                considerNotify(iterator.next().getValue());
                if (mDispatchInvalidated) {
                    break;
                }
            }
        }
    } while (mDispatchInvalidated);
    mDispatchingValue = false;
}

considerNotify

@SuppressWarnings("unchecked")
private void considerNotify(ObserverWrapper observer) {
    if (!observer.mActive) {
        return;
    }
    // Check latest state b4 dispatch. Maybe it changed state but we didn't get the event yet.
    //
    // we still first check observer.active to keep it as the entrance for events. So even if
    // the observer moved to an active state, if we've not received that event, we better not
    // notify for a more predictable notification order.
    if (!observer.shouldBeActive()) {
        observer.activeStateChanged(false);
        return;
    }
    if (observer.mLastVersion >= mVersion) {
        return;
    }
    observer.mLastVersion = mVersion;
    observer.mObserver.onChanged((T) mData);
}

@Override
public void onChanged(@Nullable Object o) {
    ViewDataBinding binder = mListener.getBinder();
    if (binder != null) {
        binder.handleFieldChange(mListener.mLocalFieldId, mListener.getTarget(), 0);
    }
}

private void handleFieldChange(int mLocalFieldId, Object object, int fieldId) {
    if (mInLiveDataRegisterObserver) {
        // We're in LiveData registration, which always results in a field change
        // that we can ignore. The value will be read immediately after anyway, so
        // there is no need to be dirty.
        return;
    }
    boolean result = onFieldChange(mLocalFieldId, object, fieldId);
    if (result) {
        requestRebind();
    }
}

请求重新绑定

protected void requestRebind() {
    if (mContainingBinding != null) {
        mContainingBinding.requestRebind();
    } else {
        final LifecycleOwner owner = this.mLifecycleOwner;
        if (owner != null) {
            Lifecycle.State state = owner.getLifecycle().getCurrentState();
            if (!state.isAtLeast(Lifecycle.State.STARTED)) {
                return; // wait until lifecycle owner is started
            }
        }
        synchronized (this) {
            if (mPendingRebind) {
                return;
            }
            mPendingRebind = true;
        }
        if (USE_CHOREOGRAPHER) {
            mChoreographer.postFrameCallback(mFrameCallback);
        } else {
            mUIThreadHandler.post(mRebindRunnable);
        }
    }
}

@Override
public void run() {
    synchronized (this) {
        mPendingRebind = false;
    }
    processReferenceQueue();

    if (VERSION.SDK_INT >= VERSION_CODES.KITKAT) {
        // Nested so that we don't get a lint warning in IntelliJ
        if (!mRoot.isAttachedToWindow()) {
            // Don't execute the pending bindings until the View
            // is attached again.
            mRoot.removeOnAttachStateChangeListener(ROOT_REATTACHED_LISTENER);
            mRoot.addOnAttachStateChangeListener(ROOT_REATTACHED_LISTENER);
            return;
        }
    }
    executePendingBindings();
}

又回到了executePendingBindings,它负责注册监听(如果没监听 )和数据绑定到TextView