Android-DataBinding-Library分析

Android-DataBinding-GradlePlugin分析

回顾上节内容，GradlePlugin主要产出了以下文件，以便后续API功能调用

  • xx.xml：正常的资源编译文件，后续apk中保留为这份文件。主要是将原始的xml文件中的 标签移除，并未每个view设置tag
  • xx-layout.xml：记录组件的绑定信息，如标签内容，以及xml使用的表达式
  • XXBinding.java：记录View的id以及<data>定义的<variable>参数，后文会有详细介绍
  • XXBindingImpl.java：基于XXBinding的实现类，双向绑定参数的赋值逻辑均在内部实现，后文会有详细介绍
  • BR.java：记录@Bindable以及<variable>相关参数
  • DataBinderMapperImpl.java
    • 包名为 androidx.databinding.library：记录项目中ViewDataBinding的映射表，内部主要为 其他module里的DataBinderMapperImpl.java
    • 包名为module 或 appname：记录module或app中哪些布局文件使用了DataBinding，即使用<layout>包裹

相关模块

baseLibrary、extensions/library、extensions/baseAdapters

核心类

  • DataBindingUtil
  • XXBindingImpl
  • DataBinderMapperImpl

执行流程

布局与Binding绑定

在代码中通常用以下代码获取Binding对象

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//在Activity中
var binding: ActivityMainBinding = ActivityMainBinding.inflate(layoutInflater)

var binding: ActivityMainBinding = DataBindingUtil.setContentView<ActivityMainBinding>(this,R.layout.activity_main)

//在Fragment或View中
var fBinding : FragmentTestDBBinding = FragmentTestDbBinding.inflate(inflater)

var fBinding : FragmentTestDBBinding = FragmentTestDbBinding.inflate(inflater, container, false)

var fBinding : FragmentTestDBBinding = DataBindingUtil.inflate(inflater, R.layout.fragment_test_db, container, false)

主要由两个类来操作：

  • XXBinding
  • DataBindingUtil

XXBinding

以FragmentTestDBBinding为例分析

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public abstract class FragmentTestDbBinding extends ViewDataBinding {
  //xml中定义view的id，方便通过binding直接调用对应的View
  @NonNull
  public final TextView tvTxt;

  //xml中定义的<variable>，方便直接调用赋值
  @Bindable
  protected Book mBook;
  ...
    
  @NonNull
  public static FragmentTestDbBinding inflate(@NonNull LayoutInflater inflater) {
    return inflate(inflater, DataBindingUtil.getDefaultComponent());
  }
  
  @NonNull
  public static FragmentTestDbBinding inflate(@NonNull LayoutInflater inflater,
      @Nullable ViewGroup root, boolean attachToRoot) {
    return inflate(inflater, root, attachToRoot, DataBindingUtil.getDefaultComponent());
  }
  
  public static FragmentTestDbBinding inflate(@NonNull LayoutInflater inflater,
      @Nullable ViewGroup root, boolean attachToRoot, @Nullable Object component) {
    return ViewDataBinding.<FragmentTestDbBinding>inflateInternal(inflater, R.layout.fragment_test_db, root, attachToRoot, component);
  }
  
}

XXBinding.inflate()最后都指向ViewDataBinding.inflateInternal()

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//ViewDataBinding.java

    @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
    protected static <T extends ViewDataBinding> T inflateInternal(
            @NonNull LayoutInflater inflater, int layoutId, @Nullable ViewGroup parent,
            boolean attachToParent, @Nullable Object bindingComponent) {
        return DataBindingUtil.inflate(
                inflater,
                layoutId,
                parent,
                attachToParent,
                checkAndCastToBindingComponent(bindingComponent)
        );
    }

inflateInternal()调用到DataBindingUtil.inflate()

DataBindingUtil

inflate/setContentView
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public static <T extends ViewDataBinding> T inflate(
        @NonNull LayoutInflater inflater, int layoutId, @Nullable ViewGroup parent,
        boolean attachToParent, @Nullable DataBindingComponent bindingComponent) {
    final boolean useChildren = parent != null && attachToParent;
    final int startChildren = useChildren ? parent.getChildCount() : 0;
    final View view = inflater.inflate(layoutId, parent, attachToParent);
    if (useChildren) {
        return bindToAddedViews(bindingComponent, parent, startChildren, layoutId);
    } else {
      //单组件直接调用 bind
        return bind(bindingComponent, view, layoutId);
    }
}

public static <T extends ViewDataBinding> T setContentView(@NonNull Activity activity,
        int layoutId, @Nullable DataBindingComponent bindingComponent) {
    activity.setContentView(layoutId);
    View decorView = activity.getWindow().getDecorView();
    ViewGroup contentView = (ViewGroup) decorView.findViewById(android.R.id.content);
    return bindToAddedViews(bindingComponent, contentView, 0, layoutId);
}

最后都调用到bindToAddedViews

bindToAddedViews
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private static <T extends ViewDataBinding> T bindToAddedViews(DataBindingComponent component,
        ViewGroup parent, int startChildren, int layoutId) {
    final int endChildren = parent.getChildCount();
    final int childrenAdded = endChildren - startChildren;
    if (childrenAdded == 1) {
        final View childView = parent.getChildAt(endChildren - 1);
        return bind(component, childView, layoutId);
    } else {
        final View[] children = new View[childrenAdded];
        for (int i = 0; i < childrenAdded; i++) {
            children[i] = parent.getChildAt(i + startChildren);
        }
        return bind(component, children, layoutId);
    }
}
bind
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//DataBinderMapperImpl 包名为 androidx.databinding

private static DataBinderMapper sMapper = new DataBinderMapperImpl();    

    static <T extends ViewDataBinding> T bind(DataBindingComponent bindingComponent, View[] roots,
            int layoutId) {
        return (T) sMapper.getDataBinder(bindingComponent, roots, layoutId);
    }

    static <T extends ViewDataBinding> T bind(DataBindingComponent bindingComponent, View root,
            int layoutId) {
        return (T) sMapper.getDataBinder(bindingComponent, root, layoutId);
    }

最后调用到DataBinderMapperImpl.getDataBinder()

DataBinderMapperImpl

androidx.databinding.DataBinderMapperImpl
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//记录使用的 DataBinding信息
public class DataBinderMapperImpl extends MergedDataBinderMapper {
  DataBinderMapperImpl() {
    addMapper(new com.example.behaviordemo.DataBinderMapperImpl());
  }
}
MergedDataBinderMapper
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//添加映射    
public void addMapper(DataBinderMapper mapper) {
        Class<? extends DataBinderMapper> mapperClass = mapper.getClass();
        if (mExistingMappers.add(mapperClass)) {
            mMappers.add(mapper);
            final List<DataBinderMapper> dependencies = mapper.collectDependencies();
            for(DataBinderMapper dependency : dependencies) {
                addMapper(dependency);
            }
        }
    }

    @Override
    public ViewDataBinding getDataBinder(DataBindingComponent bindingComponent, View view,
            int layoutId) {
        for(DataBinderMapper mapper : mMappers) {
            ViewDataBinding result = mapper.getDataBinder(bindingComponent, view, layoutId);
            if (result != null) {
                return result;
            }
        }
        return null;
    }

其中mMappers之一对应就为com.example.behaviordemo.DataBinderMapperImpl()

${moduleName}.DataBinderMapperImpl
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@Override
public ViewDataBinding getDataBinder(DataBindingComponent component, View view, int layoutId) {
  int localizedLayoutId = INTERNAL_LAYOUT_ID_LOOKUP.get(layoutId);
  if(localizedLayoutId > 0) {
    final Object tag = view.getTag();
    if(tag == null) {
      throw new RuntimeException("view must have a tag");
    }
    switch(localizedLayoutId) {
      case  LAYOUT_FRAGMENTTESTDB: {
        if ("layout/fragment_test_db_0".equals(tag)) {
          return new FragmentTestDbBindingImpl(component, view);
        }
        throw new IllegalArgumentException("The tag for fragment_test_db is invalid. Received: " + tag);
      }
    }
  }
  return null;
}

最终返回的就是FragmentTestDbBindingImpl

graph TD

DataBinderMapper

总结如下：

  1. DataBindingUtil内部持有sMapper变量，对应的就是androidx.databinding.DataBinderMapperImpl(编译期生成)
  2. androidx.databinding.DataBinderMapperImpl内部记录了${moduleName}.DataBinderMapperImpl(实现类)

给View设置数据

初始化FragmentTestDbBindingImpl时，执行如下代码

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public FragmentTestDbBindingImpl(@Nullable androidx.databinding.DataBindingComponent bindingComponent, @NonNull View root) {
    this(bindingComponent, root, mapBindings(bindingComponent, root, 9, sIncludes, sViewsWithIds));
}    

private FragmentTestDbBindingImpl(androidx.databinding.DataBindingComponent bindingComponent, View root, Object[] bindings) {
    super(bindingComponent, root, 2
        , (android.widget.TextView) bindings[1]
        );
    this.mboundView0 = (androidx.constraintlayout.widget.ConstraintLayout) bindings[0];
    this.mboundView0.setTag(null);
    this.mboundView01 = (bindings[7] != null) ? com.example.behaviordemo.databinding.SsBinding.bind((android.view.View) bindings[7]) : null;
    this.mboundView02 = (bindings[8] != null) ? com.example.behaviordemo.databinding.AaBinding.bind((android.view.View) bindings[8]) : null;
    this.mboundView2 = (android.widget.ImageView) bindings[2];
    this.mboundView2.setTag(null);
    this.mboundView3 = (android.widget.ImageView) bindings[3];
    this.mboundView3.setTag(null);
    this.mboundView4 = (android.widget.TextView) bindings[4];
    this.mboundView4.setTag(null);
    this.mboundView5 = (android.widget.TextView) bindings[5];
    this.mboundView5.setTag(null);
    this.mboundView6 = (android.widget.TextView) bindings[6];
    this.mboundView6.setTag(null);
    this.tvTxt.setTag(null);
    setRootTag(root);
    // listeners
    invalidateAll();
}

mapBindings

取出设置了tag的组件，记录在ViewDataBinding中的bindings中，可以在外部直接获取不需要执行find

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//ViewDataBinding.java
    protected static Object[] mapBindings(DataBindingComponent bindingComponent, View[] roots,
            int numBindings, IncludedLayouts includes, SparseIntArray viewsWithIds) {
        Object[] bindings = new Object[numBindings];
        for (int i = 0; i < roots.length; i++) {
            mapBindings(bindingComponent, roots[i], bindings, includes, viewsWithIds, true);
        }
        return bindings;
    }

super(XX) -> ViewDataBinding

先执行的super()向上走到FragmentTestDbBinding，最后执行到ViewDataBinding中

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//ViewDataBinding.java
public abstract class ViewDataBinding extends BaseObservable implements ViewBinding {

    static {
        if (VERSION.SDK_INT < VERSION_CODES.KITKAT) {
            ROOT_REATTACHED_LISTENER = null;
        } else {
            ROOT_REATTACHED_LISTENER = new OnAttachStateChangeListener() {
                @TargetApi(VERSION_CODES.KITKAT)
                @Override
                public void onViewAttachedToWindow(View v) {
                    // execute the pending bindings.
                    final ViewDataBinding binding = getBinding(v);
                    binding.mRebindRunnable.run();
                    v.removeOnAttachStateChangeListener(this);
                }

                @Override
                public void onViewDetachedFromWindow(View v) {
                }
            };
        }
    }
 ... 
}

当ViewattachedToWindow时，调用mRebindRunable.run

invalidateAll

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//FragmentTestDbBindingImpl.java
    @Override
    public void invalidateAll() {
        synchronized(this) {
                mDirtyFlags = 0x40L;
        }
        requestRebind();
    }

requestRebind()向上调用到ViewDataBinding中

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//ViewDataBinding.java
    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);
            }
        }
    }

最后也是调用到mRebindRunnable

mRebindRunnable

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private final Runnable mRebindRunnable = new Runnable() {
    @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();
    }
};

mRebindRunnable.run 主要执行如下：

  1. 判断mRoot是否已经绑定在Window上，已绑定则执行executePendingBindings
  2. 没有绑定，先添加监听，等绑定时，再次调用mRebindRunnable.run()，向下继续执行到executePendingBindings

executeBindings

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//ViewDataBinding.java
public void executePendingBindings() {
       
        if (mContainingBinding == null) {
            executeBindingsInternal();
        } else {
          //包含了 <include>引入的 layout ，通过引入的 layout对应的binding执行数据处理
            mContainingBinding.executePendingBindings();
        }
    }

//交由子类实现
    protected abstract void executeBindings();

    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;
    }

此处executeBindings()的实现在FragmentTestDbBindingImpl中

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@Override
protected void executeBindings() {
    long dirtyFlags = 0;
    synchronized(this) {
        dirtyFlags = mDirtyFlags;
        mDirtyFlags = 0;
    }
    androidx.databinding.ObservableField<java.lang.String> bookName = null;
    java.lang.String bookNameGet = null;
    com.example.behaviordemo.bindingadapter.JavaBook javaBook = mJavaBook;
    java.lang.String javaBookName = null;
  ...
    if ((dirtyFlags & 0x40L) != 0) {
        // api target 1

        com.example.behaviordemo.bindingadapter.ViewExpressionKt.setShape(this.mboundView2, com.example.behaviordemo.bindingadapter.ShapeBuilder.create().setCornerRadius(10).setColors(android.graphics.Color.RED).setShapeType(0));
        androidx.databinding.adapters.ViewBindingAdapter.setBackground(this.mboundView3, com.example.behaviordemo.bindingadapter.ViewExpressionKt.colorToDrawable("#00ff00"));
    }
    if ((dirtyFlags & 0x48L) != 0) {
        // api target 1

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

通过判断dirtyFlags是否有变化，根据他的变化执行对应的赋值语句。

```



### View与Model双向更新

#### Model -> View Model变化通知View修改

> 支持监听Model数据变化的方式有两种
>
> - 在Model中参数的`getXX`添加`@Bindable`注解，在`setXX`内调用`notifyPropertyChanged()`
> - 直接设置Model参数为`ObservableField<T>`

通过设置`ObservableField`，当值发生变化时，会调用到`ViewDataBinding.updateRegistration()`，后续继续调用最终也会执行到`notifyPropertyChanged`

##### BaseObservable.notifyPropertyChanged

```java
private transient PropertyChangeRegistry mCallbacks;

public void notifyPropertyChanged(int fieldId) {
        synchronized (this) {
            if (mCallbacks == null) {
                return;
            }
        }
        mCallbacks.notifyCallbacks(this, fieldId, null);
    }

此时mCallbacks为PropertyChangeRegistry

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public class PropertyChangeRegistry extends
        CallbackRegistry<Observable.OnPropertyChangedCallback, Observable, Void> {

    private static final CallbackRegistry.NotifierCallback<Observable.OnPropertyChangedCallback, Observable, Void> NOTIFIER_CALLBACK = new CallbackRegistry.NotifierCallback<Observable.OnPropertyChangedCallback, Observable, Void>() {
        @Override
        public void onNotifyCallback(Observable.OnPropertyChangedCallback callback, Observable sender,
                int arg, Void notUsed) {
            callback.onPropertyChanged(sender, arg);
        }
    };
  
}

对应的callback就为ViewDataBinding.WeakPropertyListener

WeakPropertyListener.onPropertyChanged
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//ViewBinding.java
    private static class WeakPropertyListener extends Observable.OnPropertyChangedCallback
            implements ObservableReference<Observable> {
      ...
        @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?
            }
            binder.handleFieldChange(mListener.mLocalFieldId, sender, propertyId);
        }
      
    }

    @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
    protected void handleFieldChange(int mLocalFieldId, Object object, int fieldId) {
        if (mInLiveDataRegisterObserver || mInStateFlowRegisterObserver) {
            return;
        }
        boolean result = onFieldChange(mLocalFieldId, object, fieldId);
        if (result) {
            requestRebind();
        }
    }
requestRebind
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//ViewDataBinding.java
    protected void requestRebind() {
        if (mContainingBinding != null) {
            mContainingBinding.requestRebind();
        } else {
           ...
            if (USE_CHOREOGRAPHER) {
                mChoreographer.postFrameCallback(mFrameCallback);
            } else {
                mUIThreadHandler.post(mRebindRunnable);
            }
        }
    }

此处就续上了上一节中mRebindRunnable后续的执行流程，最终走到executeBindings刷新View中的显示数据。

View -> Model View变化修改Model

相对使用较少

避免冗余UI更新

由于DataBinding是Viwe与Model是双向绑定的，所以修改了model，View也会更新，再导致Model的修改。
可能就会存在冗余更新，DataBinding内部会对此进行处理。

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//更新文字
    @Override
    protected void executeBindings() {
      ...
      androidx.databinding.adapters.TextViewBindingAdapter.setText(this.mboundView6, javaBookName);
    }

内部并非直接调用TextView.setText()，而是通过TextViewBindingAdapter.setText进行设置

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public class TextViewBindingAdapter {
      @BindingAdapter("android:text")
    public static void setText(TextView view, CharSequence text) {
        final CharSequence oldText = view.getText();
      //优先判断文字是否相同，相同则不执行后续setText
        if (text == oldText || (text == null && oldText.length() == 0)) {
            return;
        }
        if (text instanceof Spanned) {
            if (text.equals(oldText)) {
                return; // No change in the spans, so don't set anything.
            }
        } else if (!haveContentsChanged(text, oldText)) {
            return; // No content changes, so don't set anything.
        }
        view.setText(text);
    }
  
}

参考链接

DataBinding原理

DataBinding-绑定原理