DataBinding 原理解析記錄

1.預解析
所有DataBinding的xml會先被預處理成xml文件名 n后面加上-layout的xml預處理文件
比如abc.xml被預處理后的名稱是abc-layout.xml建椰，同時也會生成新的同名layout文件并給沒有id的view會打上tag。
在/app/build/intermediates/data_binding_layout_info_type_merge/release/mergeReleaseResources/out路徑下

databinding庫會根據(jù)@Bindable注解生成BR文件局雄，同時BR文件還會包括xml里的data變量

BindingAdatpar
每個BindingAdatpar的注解會被預處理生成setter_store 文件婿牍，打包到aar里，并且在主項目的該目錄下合并：app/build/intermediates/data_binding_dependency_artifacts/release/dataBindingMergeDependencyArtifactsRelease
3.Binding文件創(chuàng)建與初始化
根據(jù)-layout.xml創(chuàng)建啊胶，同時創(chuàng)建一個DataBinderMapperImpl文件领追，他繼承于MergedDataBinderMapper庶诡，并且會根據(jù)所有的databinding文件創(chuàng)建一個從layoutid到打bindingimpl類的映射笤闯。

  @Override
    public ViewDataBinding getDataBinder(DataBindingComponent bindingComponent, View view,
            int layoutId) {
       //從所有的mapper里找到一個有對應映射的databinding對象
        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;
    }

先根據(jù)xml構建viewgroup對象root堕阔，之后調用DataBindingUtil方法傳入getDefaultComponent跟root，前者負責處理bindingAdapter跟BindingComponent颗味。
之后遍歷之前生成的-layout文件給每個控件初始化根據(jù)id跟tag找到對應view超陆，之后移出root里的無用tag

綁定是在setvm的時候創(chuàng)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;
    }

    // 創(chuàng)建對象監(jiān)聽并存到mLocalFieldObservers中
    WeakListener listener = mLocalFieldObservers[localFieldId];
    if (listener == null) {
        //這里會使用WeakPropertyListener去創(chuàng)建listener
        // CREATE_PROPERTY_LISTENER -> create(...)
        listener = listenerCreator.create(this, localFieldId);
        mLocalFieldObservers[localFieldId] = listener;
    }

    // 將監(jiān)聽綁定到Observable對象上
    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) {
           //這里會給Observable添加回調，所以當Observable里調用對應的nofity時會觸發(fā)這個
            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?
            }
           //這里會觸發(fā)binding對象的handleFieldChange方法從而觸發(fā)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則更新
            requestRebind();
        }
    }


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

雙向綁定只需要在生成binding文件的時候給view加上對應的監(jiān)聽然后去對應的改變viewmodel的對應值就行浦马，這里不展開了
以上是對databinding實現(xiàn)的一些簡單分析时呀，關于lifecycle的相關內容后續(xù)再補充