Android DataBinding布局的加载深入探究
作者:昉钰
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, 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。
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