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Android DataBinding布局的加载深入探究_Android

作者：昉钰更新时间： 2022-12-12 编程语言

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, 0, 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 == 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);
        }
    }

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 = 1;
  private static final SparseIntArray INTERNAL_LAYOUT_ID_LOOKUP = new SparseIntArray(1);
  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 > 0) {
      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_0".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, 3, sIncludes, sViewsWithIds));
    }
    private ActivityMainBindingImpl(androidx.databinding.DataBindingComponent bindingComponent, View root, Object[] bindings) {
        super(bindingComponent, root, 2
            , (android.widget.TextView) bindings[1]
            , (android.widget.TextView) bindings[2]
            );
        this.first.setTag(null);
        this.mboundView0 = (androidx.constraintlayout.widget.ConstraintLayout) bindings[0];
        this.mboundView0.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 > 0 && isNumeric(tag, underscoreIndex + 1)) {
                final int index = parseTagInt(tag, underscoreIndex + 1);
                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 ? -1 : index;
                isBound = true;
            } else {
                indexInIncludes = -1;
            }
        } 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 ? -1 : tagIndex;
        } else {
            // Not a bound view
            indexInIncludes = -1;
        }
        if (!isBound) {
            final int id = view.getId();
            if (id > 0) {
                int index;
                if (viewsWithIds != null && (index = viewsWithIds.get(id, -1)) >= 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 = 0;
            for (int i = 0; i < count; i++) {
                final View child = viewGroup.getChildAt(i);
                boolean isInclude = false;
                //处理include标签
                if (indexInIncludes >= 0 && child.getTag() instanceof String) {
                    String childTag = (String) child.getTag();
                    if (childTag.endsWith("_0") &&
                            childTag.startsWith("layout") && childTag.indexOf('/') > 0) {
                        // This *could* be an include. Test against the expected includes.
                        int includeIndex = findIncludeIndex(childTag, minInclude,
                                includes, indexInIncludes);
                        if (includeIndex >= 0) {
                            isInclude = true;
                            minInclude = includeIndex + 1;
                            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 + 1;
                                final View[] included = new View[includeCount];
                                for (int j = 0; j < includeCount; j++) {
                                    included[j] = viewGroup.getChildAt(i + j);
                                }
                                bindings[index] = DataBindingUtil.bind(bindingComponent, included,
                                        layoutId);
                                i += includeCount - 1;
                            }
                        }
                    }
                }
                //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="1.0" 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_0"
    tools:context=".MainActivity">
    <TextView
        android:id="@+id/first"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginVertical="12dp"
        android:tag="binding_1"
        android:textColor="#333333"
        android:textSize="18sp"
        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_2"
        android:textColor="#999"
        android:textSize="14sp"
        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。

原文链接：https://blog.csdn.net/www586089/article/details/127779260

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网站首页编程语言正文

Android DataBinding布局的加载深入探究_Android

目录

一、布局加载

二、view映射

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网站首页 编程语言 正文

Android DataBinding布局的加载深入探究_Android

目录

一、布局加载

二、view映射

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