DataBinding 原理解析记录

1.预解析
所有DataBinding的xml会先被预处理成xml文件名 n后面加上-layout的xml预处理文件
比如abc.xml被预处理后的名称是abc-layout.xml，同时也会生成新的同名layout文件并给没有id的view会打上tag。
在/app/build/intermediates/data_binding_layout_info_type_merge/release/mergeReleaseResources/out路径下

databinding库会根据@Bindable注解生成BR文件，同时BR文件还会包括xml里的data变量

BindingAdatpar
每个BindingAdatpar的注解会被预处理生成setter_store 文件，打包到aar里，并且在主项目的该目录下合并：app/build/intermediates/data_binding_dependency_artifacts/release/dataBindingMergeDependencyArtifactsRelease
3.Binding文件创建与初始化
根据-layout.xml创建，同时创建一个DataBinderMapperImpl文件，他继承于MergedDataBinderMapper，并且会根据所有的databinding文件创建一个从layoutid到打bindingimpl类的映射。

  @Override
    public ViewDataBinding getDataBinder(DataBindingComponent bindingComponent, View view,
            int layoutId) {
       //从所有的mapper里找到一个有对应映射的databinding对象
        for(DataBinderMapper mapper : mMappers) {
            ViewDataBinding result = mapper.getDataBinder(bindingComponent, view, layoutId);
            if (result != null) {
                return result;
            }
        }
        if (loadFeatures()) {
            return getDataBinder(bindingComponent, view, layoutId);
        }
        return null;
    }

先根据xml构建viewgroup对象root，之后调用DataBindingUtil方法传入getDefaultComponent跟root，前者负责处理bindingAdapter跟BindingComponent。
之后遍历之前生成的-layout文件给每个控件初始化根据id跟tag找到对应view，之后移出root里的无用tag

绑定是在setvm的时候创建的：

 public void setVm(@Nullable fm.qingting.framework.navi.viewmodel.ToolbarViewModel Vm) {
        updateRegistration(0, Vm);
        this.mVm = Vm;
        synchronized(this) {
            mDirtyFlags |= 0x1L;
        }
        notifyPropertyChanged(BR.vm);
        super.requestRebind();
    }
protected void registerTo(int localFieldId, Object observable,
        CreateWeakListener listenerCreator) {
    if (observable == null) {
        return;
    }

    // 创建对象监听并存到mLocalFieldObservers中
    WeakListener listener = mLocalFieldObservers[localFieldId];
    if (listener == null) {
        //这里会使用WeakPropertyListener去创建listener
        // CREATE_PROPERTY_LISTENER -> create(...)
        listener = listenerCreator.create(this, localFieldId);
        mLocalFieldObservers[localFieldId] = listener;
    }

    // 将监听绑定到Observable对象上
    listener.setTarget(observable);
}
   private static class WeakPropertyListener extends Observable.OnPropertyChangedCallback
            implements ObservableReference<Observable> {
        final WeakListener<Observable> mListener;

        public WeakPropertyListener(ViewDataBinding binder, int localFieldId) {
            mListener = new WeakListener<Observable>(binder, localFieldId, this);
        }

        @Override
        public WeakListener<Observable> getListener() {
            return mListener;
        }

        @Override
        public void addListener(Observable target) {
           //这里会给Observable添加回调，所以当Observable里调用对应的nofity时会触发这个
            target.addOnPropertyChangedCallback(this);
        }

        @Override
        public void removeListener(Observable target) {
            target.removeOnPropertyChangedCallback(this);
        }

        @Override
        public void setLifecycleOwner(LifecycleOwner lifecycleOwner) {
        }

        @Override
        public void onPropertyChanged(Observable sender, int propertyId) {
            ViewDataBinding binder = mListener.getBinder();
            if (binder == null) {
                return;
            }
            Observable obj = mListener.getTarget();
            if (obj != sender) {
                return; // notification from the wrong object?
            }
           //这里会触发binding对象的handleFieldChange方法从而触发onFieldChange方法
            binder.handleFieldChange(mListener.mLocalFieldId, sender, propertyId);
        }
    }
  @Override
    protected boolean onFieldChange(int localFieldId, Object object, int fieldId) {
        switch (localFieldId) {
            case 0 :
                return onChangeVm((fm.qingting.framework.navi.viewmodel.ToolbarViewModel) object, fieldId);
            case 1 :
                return onChangeNavi((fm.qingting.framework.databinding.NavigationViewBinding) object, fieldId);
        }
        return false;
    }

  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) {
         //如果onFieldChange返回true则更新
            requestRebind();
        }
    }


    private boolean onChangeVm(fm.qingting.framework.navi.viewmodel.ToolbarViewModel Vm, int fieldId) {
       //如果vm被更新则刷新所有的property
        if (fieldId == BR._all) {
            synchronized(this) {
                    mDirtyFlags |= 0x1L;
            }
            return true;
        }
        return false;
    }

双向绑定只需要在生成binding文件的时候给view加上对应的监听然后去对应的改变viewmodel的对应值就行，这里不展开了
以上是对databinding实现的一些简单分析，关于lifecycle的相关内容后续再补充