Android DataBinding布局的加载深入探究

2022-11-28 11:12 开发作者：昉钰

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, sDhttp://www.devze.comefaultComponent);
    }

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：

Android DataBinding布局的加载深入探究

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：编程客栈

  protected ActivityMainBinding(Object _bindingCom开发者_JAVAponent, 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 existin编程客栈gBinding = 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 viejavascriptw
            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:tpythonextColor="#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----类关系说明（1）进行参考。

到此这篇关于Android DataBinding布局的加载深入探究的文章就介绍到这了,更多相关Android DataBinding 内容请搜索我们以前的文章或继续浏览下面的相关文章希望大家以后多多支持我们！

继续阅读：AndroidDataBinding AndroidDataBinding布局的加载

Android DataBinding布局的加载深入探究

目录

一、布局加载

二、view映射

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