Android DataBinding布局的加载深入探究

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2023-06-17
Android DataBinding单向数据绑定深入探究
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标签   Android
目录
  • 一、布局加载
  • 二、view映射

上一章说明了DataBinding生存的类之间关系，现在这里来看看布局是如何加载的以及view是如何映射的。

一、布局加载

这里把之前的代码重新贴下方便说明，代码如下：

class MainActivity : AppCompatActivity() {
    private val viewModel: SimpleViewModel by viewModels()
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val binding: ActivityMainBinding = DataBindingUtil.setContentView(this, R.layout.activity_main)
        binding.lifecycleOwner = this
        binding.viewModel = viewModel
    }
}

其中布局加载就这一行：DataBindingUtil.setContentView(this, R.layout.activity_main)，所以进入到DataBindingUtil中，代码如下：

public static <T extends ViewDataBinding> T setContentView(@NonNull Activity activity,
        int layoutId) {
    return setContentView(activity, layoutId, sDefaultComponent);
}

就是简单的调用转发而已，继续下一步，如下：

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,, layoutId);
}

activity.setContentView(layoutId)，这和我们不用DataBinding写的一样啊，所以Databinding在这里就帮我们加载了布局。

接下来，看DataBinding是如何实现view映射的。

二、view映射

然后拿到decorView 并找到contentView ，最后调用bindToAddedViews，bindToAddedViews的函数如下：

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 ==) {
        final View childView = parent.getChildAt(endChildren -);
        return bind(component, childView, layoutId);
    } else {
        final View[] children = new View[childrenAdded];
        for (int i =; i < childrenAdded; i++) {
            children[i] = parent.getChildAt(i + startChildren);
        }
        return bind(component, children, layoutId);
    }
}

在我们的场景里面，endChildren 应该为1，childrenAdded 也为1，所以走了第一个分支，继续调用bind函数，如下：

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

调用了sMapper的getDataBinder函数，这里的sMapper类型为DataBinderMapperImpl，还记得上一章说过有两个DataBinderMapperImpl吗？为了便于说明，这里再把之前的类图贴下：

额，这就尴尬了，所以这里的Mapper到底是哪个呢？之前说过左边的是android提供的，右边的是我们自己包下面的；其实这里的sMapper属于左边这个行列，也就是androidx这个包下面的。那他们有什么区别呢？你可以认为左边的提供了一个简单的代理功能，其实它就是简单对右边的Mapper类进行包装而已。

这里需要说明下sMapper对象的初始化过程，我们知道类加载会触发类变量（静态变量）的初始化，这个时候sMapper就会被初始化，这个时候会调用DataBinderMapperImpl（左边那个mapper）的构建函数，代码如下：

package androidx.databinding;//位于androidx包下面
public class DataBinderMapperImpl extends MergedDataBinderMapper {DataBinderMapperImpl() {
//这个DataBinderMapperImpl就是我们自己包下面的了
addMapper(new com.zfang.databindingstudy.DataBinderMapperImpl());
  }
}

正如前面所说，androidx下面的mapper类包装了项目中的mapper类，addMapper代码如下：

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);
        }
    }
}

这里会把项目中的mapper（即DataBinderMapperImpl）加入到mMappers这个CopyOnWriteArrayList中，后面会用到。

此时可以继续看看getDataBinder的实现了（其实现位于MergedDataBinderMapper中），代码如下：

@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;
        }
    }
    if (loadFeatures()) {
        return getDataBinder(bindingComponent, view, layoutId);
    }
    return null;
    }

这里就是从mMappers中把mapper拿出来，再根据传递进来的参数view、layoutId找到相应的ViewDataBinding对象；这里的mMappers就是刚刚提到的那个CopyOnWriteArrayList，所以会调用到我们的DataBinderMapperImpl，其中的getDataBinder实现如下：

  private static final int LAYOUT_ACTIVITYMAIN =;private static final SparseIntArray INTERNAL_LAYOUT_ID_LOOKUP = new SparseIntArray();
  static {  INTERNAL_LAYOUT_ID_LOOKUP.put(com.zfang.databindingstudy.R.layout.activity_main, LAYOUT_ACTIVITYMAIN);
  }@Override
  public ViewDataBinding getDataBinder(DataBindingComponent component, View view, int layoutId) {  int localizedLayoutId = INTERNAL_LAYOUT_ID_LOOKUP.get(layoutId);
if(localizedLayoutId >) {
  final Object tag = view.getTag();
  if(tag == null) {
    throw new RuntimeException("view must have a tag");
  }
  switch(localizedLayoutId) {
    case  LAYOUT_ACTIVITYMAIN: {
      if ("layout/activity_main_".equals(tag)) {
        return new ActivityMainBindingImpl(component, view);
      }
      throw new IllegalArgumentException("The tag for activity_main is invalid. Received: " + tag);
    }
  }
}
return null;
  }

这里有个SparseIntArray ，它定义了我们的布局与一个整数的映射关系，上面的代码首先拿到view的tag，这里返回的tag为layout/activity_main_0（回忆下：上一章说过DataBinding会生存两个xml，其中一个加了tag，那里说的tag正是和这里对应上了，其作用就体现在这里），所以会返回ActivityMainBindingImpl，这正是需要的ViewDataBinding类。

继续进入ActivityMainBindingImpl的构建函数中，代码如下：

public ActivityMainBindingImpl(@Nullable androidx.databinding.DataBindingComponent bindingComponent, @NonNull View root) {
    this(bindingComponent, root, mapBindings(bindingComponent, root,, sIncludes, sViewsWithIds));
}
private ActivityMainBindingImpl(androidx.databinding.DataBindingComponent bindingComponent, View root, Object[] bindings) {
    super(bindingComponent, root,
        , (android.widget.TextView) bindings[]
        , (android.widget.TextView) bindings[]
        );
    this.first.setTag(null);
    this.mboundView = (androidx.constraintlayout.widget.ConstraintLayout) bindings[0];
    this.mboundView.setTag(null);
    this.second.setTag(null);
    setRootTag(root);
    // listeners
    invalidateAll();
}

先调用了第一个构造函数，然后进入第二个。第二个构造函数又调用了父类的相应构造函数，代码如下：

  protected ActivityMainBinding(Object _bindingComponent, View _root, int _localFieldCount,    TextView first, TextView second) {
super(_bindingComponent, _root, _localFieldCount);
this.first = first;
this.second = second;
  }

没错，上面的bindings数组中的bindings[1]、bindings[2]正是对应到了我们这个场景中的first和second两个view。现在的问题是bindings数组中的值是怎么来的呢？

我们继续看看ActivityMainBindingImpl类中第一个构建数据中调用的函数mapBindings，看来在mapBindings中会填充bindings数组，mapBindings代码如下：

protected static Object[] mapBindings(DataBindingComponent bindingComponent, View root,
        int numBindings, IncludedLayouts includes, SparseIntArray viewsWithIds) {
    Object[] bindings = new Object[numBindings];
    mapBindings(bindingComponent, root, bindings, includes, viewsWithIds, true);
    return bindings;
}

这里 根据numBindings新建了一个数组，继续：

private static void mapBindings(DataBindingComponent bindingComponent, View view,
        Object[] bindings, IncludedLayouts includes, SparseIntArray viewsWithIds,
        boolean isRoot) {
    final int indexInIncludes;
    final ViewDataBinding existingBinding = getBinding(view);
    if (existingBinding != null) {
        return;
    }
    Object objTag = view.getTag();
    final String tag = (objTag instanceof String) ? (String) objTag : null;
    boolean isBound = false;
    //第一次进来isRoot为true，tag为根据布局所以是以layout开头，因此这进入第一个if
    if (isRoot && tag != null && tag.startsWith("layout")) {
        final int underscoreIndex = tag.lastIndexOf('_');
        if (underscoreIndex > && isNumeric(tag, underscoreIndex + 1)) {
            final int index = parseTagInt(tag, underscoreIndex +);
            if (bindings[index] == null) {
                bindings[index] = view;//放入bindings数组，这里的view代表根布局
            }
            //处理包含布局中有include标签的情况
            indexInIncludes = includes == null ? - : index;
            isBound = true;
        } else {
            indexInIncludes = -;
        }
    } else if (tag != null && tag.startsWith(BINDING_TAG_PREFIX)) {
        //如何不是根布局，对应到我们的场景则会走到这里,我们的两个TextView的
        //tag刚是以binding开头的，其实只要写了绑定表达式就会到这里。
        int tagIndex = parseTagInt(tag, BINDING_NUMBER_START);
        if (bindings[tagIndex] == null) {
            bindings[tagIndex] = view;
        }
        isBound = true;
        indexInIncludes = includes == null ? - : tagIndex;
    } else {
        // Not a bound view
        indexInIncludes = -;
    }
    if (!isBound) {
        final int id = view.getId();
        if (id >) {
            int index;
            if (viewsWithIds != null && (index = viewsWithIds.get(id, -)) >= 0 &&
                    bindings[index] == null) {
                bindings[index] = view;
            }
        }
    }
    //如果是ViewGroup则递归处理找到相应的view
    if (view instanceof  ViewGroup) {
        final ViewGroup viewGroup = (ViewGroup) view;
        final int count = viewGroup.getChildCount();
        int minInclude =;
        for (int i =; i < count; i++) {
            final View child = viewGroup.getChildAt(i);
            boolean isInclude = false;
            //处理include标签
            if (indexInIncludes >= && child.getTag() instanceof String) {
                String childTag = (String) child.getTag();
                if (childTag.endsWith("_") &&
                        childTag.startsWith("layout") && childTag.indexOf('/') >) {
                    // This *could* be an include. Test against the expected includes.
                    int includeIndex = findIncludeIndex(childTag, minInclude,
                            includes, indexInIncludes);
                    if (includeIndex >=) {
                        isInclude = true;
                        minInclude = includeIndex +;
                        final int index = includes.indexes[indexInIncludes][includeIndex];
                        final int layoutId = includes.layoutIds[indexInIncludes][includeIndex];
                        int lastMatchingIndex = findLastMatching(viewGroup, i);
                        if (lastMatchingIndex == i) {
                            bindings[index] = DataBindingUtil.bind(bindingComponent, child,
                                    layoutId);
                        } else {
                            final int includeCount =  lastMatchingIndex - i +;
                            final View[] included = new View[includeCount];
                            for (int j =; j < includeCount; j++) {
                                included[j] = viewGroup.getChildAt(i + j);
                            }
                            bindings[index] = DataBindingUtil.bind(bindingComponent, included,
                                    layoutId);
                            i += includeCount -;
                        }
                    }
                }
            }
            //非include
            if (!isInclude) {
                mapBindings(bindingComponent, child, bindings, includes, viewsWithIds, false);
            }
        }
    }
}

这里就是实现view数组映射的关键，主要功能就是填充了bindings数组，思路就是找到包含绑定表达式的控件，然后把它们记录下来放到一个数组中，方便在相应控件的数据变化的时候能够通知到控件， 这里其实就是找到如下布局中的两个TextView然后加入到bindings中。

<?xml version=".0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"  xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:tag="layout/activity_main_"
tools:context=".MainActivity">
<TextView
    android:id="@+id/first"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginVertical="dp"
    android:tag="binding_"
    android:textColor="#"
    android:textSize="sp"
    app:layout_constraintBottom_toTopOf="@id/second"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    app:layout_constraintVertical_chainStyle="packed" />
<TextView
    android:id="@+id/second"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:tag="binding_"
    android:textColor="#"
    android:textSize="sp"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toBottomOf="@id/first" />
</androidx.constraintlayout.widget.ConstraintLayout>

上面那段代码的逻辑就是找到ConstraintLayout（android:tag="layout/activity_main_0"） 以及两个TextView（tag分别为binding_1和binding_2），总共三个控件。ConstraintLayout就是根布局，两个TextView就是我们需要操作的View。

好了，布局view映射完成，简单总结下：首先就是DataBinding会帮我们调用setContentView，所以我们不用调用这个方法；其次DataBinding会帮我们找到包含有数据绑定表达式的View其后帮我们存起来，方便在数据变化的时候操作我们的View。

下一章继续分析数据是如何与控件进行绑定的。

如果你对DataBinding生存的类关系有疑问，可以返回上一章DataBinding原理----类关系进行参考。

Android DataBinding单向数据绑定深入探究
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