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目录
Jetpack学习-DataBinding

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http://www.milovetingting.cn

Jetpack学习-DataBinding

简单使用

在需要使用DataBinding的模块的build.gradle中增加

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android {
//...
defaultConfig {
//...
dataBinding{
enabled true
}
}
}

然后同步

新建一个继承自BaseObservable的类

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public class User extends BaseObservable {

private String name;

private int age;

public User(String name, int age) {
this.name = name;
this.age = age;
}

@Bindable
public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
notifyPropertyChanged(com.wangyz.jetpack.BR.name);
}

@Bindable
public int getAge() {
return age;
}

public void setAge(int age) {
this.age = age;
notifyPropertyChanged(com.wangyz.jetpack.BR.age);
}
}

在需要绑定的字段的get方法上增加@Bindable注解,在set方法里增加notifyPropertyChanged(com.wangyz.jetpack.BR.name)

build工程

新建布局文件,在布局最外层的节点上按alt+enter,在弹出的选项中选择Convert to data binding layout,布局就会转换成DataBinding格式的布局。

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<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android">

<data>

<variable
name="user"
type="com.wangyz.jetpack.databinding.User" />
</data>

<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">

<Button
android:onClick="update"
android:text="更新"
android:layout_width="match_parent"
android:layout_height="50dp"/>

<TextView
android:layout_width="match_parent"
android:layout_height="50dp"
android:gravity="center_vertical"
android:text="@{user.name}" />

<TextView
android:layout_width="match_parent"
android:layout_height="50dp"
android:gravity="center_vertical"
android:text="@{String.valueOf(user.age)}" />

</LinearLayout>
</layout>

转换后的布局,会将layout作为最外层的节点,还会在里面增加一个data节点。我们需要在这个data节点中增加variable节点,并配置nametype属性。name命名随意,type输入前面定义的User类。

对需要绑定的控件属性,如text赋值为@{user.name},意思是给text属性赋值为前面绑定的User类的name。这样当User的name发生改变时,控件的text属性就会自动改变。

在Activity中绑定User和布局

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public class DataBindingActivity extends AppCompatActivity {

User user;

ActivityDatabindingBinding binding;

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
binding = DataBindingUtil.setContentView(this, R.layout.activity_databinding);
user = new User("张三", 18);
binding.setUser(user);
}

public void update(View view) {
user.setName(user.getName() + "$");
user.setAge(user.getAge() + 1);
binding.setVariable(com.wangyz.jetpack.BR.user, user);
}
}

通过DataBindingUtil.setContentView(this, R.layout.activity_databinding)来绑定,并返回Binding,然后通过Binding的setUser方法,就可以给布局设置数据。

原理

布局文件

下面来看下原理。DataBinding相比前面的LifecycleLiveData要复杂。

我们将应用运行到手机上,这个在开发者看来是很简单的一件事,但是Android Studio却为我们做了很多事。

首先,布局文件会分为两个xml文件。

databinding-layout

app/buil/intermediates/data_binding_layout_info_type_merge/debug/mergeDebugResources/out/目录下,可以看到生成了一个activity_databinding-layout.xml文件,这个文件名称是在我们原来的布局名称后加上了-layout。它的内容如上图右侧所示。在最外层的节点里记录了对应的布局文件,然后通过定义Variables节点,记录了对应的数据类。在Targets节点中,记录了原布局的tag,及使用了DataBinding的控件的tag,然后通过Expression节点,记录对应的数据。

databinding-activity

app/build/intermediates/incremental/mergeDebugResources/stripped.dir/layout/目录下,可以看到activity_databinding.xml文件,这个文件就是我们原来的文件名,不过里面稍作了一些变动,增加了一些tag,这些tag的值和前面的activity_databinding-layout.xml记录的是对应的。

DataBindingUtil.setContentView

我们从DataBindingUtil.setContentView(this, R.layout.activity_databinding)这个方法看起。

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

这个方法里又调用了setContentView方法

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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);
}

这个方法里调用了bindToAddedViews方法

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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);
}
}

然后调用bind方法

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

这里调用了sMappergetDataBinder方法,sMapper其实就是自动生成的DataBinderMapperImpl文件

databinding-databindermapperimpl

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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_ACTIVITYDATABINDING: {
if ("layout/activity_databinding_0".equals(tag)) {
return new ActivityDatabindingBindingImpl(component, view);
}
throw new IllegalArgumentException("The tag for activity_databinding is invalid. Received: " + tag);
}
}
}
return null;
}

在这里调用了ActivityDatabindingBindingImpl的构造方法

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public ActivityDatabindingBindingImpl(@Nullable androidx.databinding.DataBindingComponent bindingComponent, @NonNull View root) {
this(bindingComponent, root, mapBindings(bindingComponent, root, 3, sIncludes, sViewsWithIds));
}

这里调用了ViewDataBindingmapBindings方法

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

这里调用mapBindings方法

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private static void mapBindings(DataBindingComponent bindingComponent, View view, Object[] bindings, ViewDataBinding.IncludedLayouts includes, SparseIntArray viewsWithIds, boolean isRoot) {
ViewDataBinding existingBinding = getBinding(view);
if (existingBinding == null) {
Object objTag = view.getTag();
String tag = objTag instanceof String ? (String)objTag : null;
boolean isBound = false;
int indexInIncludes;
int id;
int count;
if (isRoot && tag != null && tag.startsWith("layout")) {
id = tag.lastIndexOf(95);
if (id > 0 && isNumeric(tag, id + 1)) {
count = parseTagInt(tag, id + 1);
if (bindings[count] == null) {
bindings[count] = view;
}

indexInIncludes = includes == null ? -1 : count;
isBound = true;
} else {
indexInIncludes = -1;
}
} else if (tag != null && tag.startsWith("binding_")) {
id = parseTagInt(tag, BINDING_NUMBER_START);
if (bindings[id] == null) {
bindings[id] = view;
}

isBound = true;
indexInIncludes = includes == null ? -1 : id;
} else {
indexInIncludes = -1;
}

if (!isBound) {
id = view.getId();
if (id > 0 && viewsWithIds != null && (count = viewsWithIds.get(id, -1)) >= 0 && bindings[count] == null) {
bindings[count] = view;
}
}

if (view instanceof ViewGroup) {
ViewGroup viewGroup = (ViewGroup)view;
count = viewGroup.getChildCount();
int minInclude = 0;

for(int i = 0; i < count; ++i) {
View child = viewGroup.getChildAt(i);
boolean isInclude = false;
if (indexInIncludes >= 0 && child.getTag() instanceof String) {
String childTag = (String)child.getTag();
if (childTag.endsWith("_0") && childTag.startsWith("layout") && childTag.indexOf(47) > 0) {
int includeIndex = findIncludeIndex(childTag, minInclude, includes, indexInIncludes);
if (includeIndex >= 0) {
isInclude = true;
minInclude = includeIndex + 1;
int index = includes.indexes[indexInIncludes][includeIndex];
int layoutId = includes.layoutIds[indexInIncludes][includeIndex];
int lastMatchingIndex = findLastMatching(viewGroup, i);
if (lastMatchingIndex == i) {
bindings[index] = DataBindingUtil.bind(bindingComponent, child, layoutId);
} else {
int includeCount = lastMatchingIndex - i + 1;
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;
}
}
}
}

if (!isInclude) {
mapBindings(bindingComponent, child, bindings, includes, viewsWithIds, false);
}
}
}

}
}

这个方法其实就是解析前面的两个xml文件,将绑定的控件保存起来。

DataBindingUtil.setContentView方法执行完成后,就可以获取到对应的DataBinding对象。

来一张简单的时序图

databinding-setcontentview

binding.setUser

binding.setUser对应的是ActivityDatabindingBindingImpl的setUser方法

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public void setUser(@Nullable com.wangyz.jetpack.databinding.User User) {
updateRegistration(0, User);
this.mUser = User;
synchronized(this) {
mDirtyFlags |= 0x1L;
}
notifyPropertyChanged(BR.user);
super.requestRebind();
}

updateRegistration

先来看updateRegistration,这里就是注册过程。调用ViewDataBinding的updateRegistration方法

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protected boolean updateRegistration(int localFieldId, Observable observable) {
return updateRegistration(localFieldId, observable, CREATE_PROPERTY_LISTENER);
}

然后调用updateRegistration方法

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private boolean updateRegistration(int localFieldId, Object observable,
CreateWeakListener listenerCreator) {
if (observable == null) {
return unregisterFrom(localFieldId);
}
WeakListener listener = mLocalFieldObservers[localFieldId];
if (listener == null) {
registerTo(localFieldId, observable, listenerCreator);
return true;
}
if (listener.getTarget() == observable) {
return false;//nothing to do, same object
}
unregisterFrom(localFieldId);
registerTo(localFieldId, observable, listenerCreator);
return true;
}

调用registerTo方法

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protected void registerTo(int localFieldId, Object observable,
CreateWeakListener listenerCreator) {
if (observable == null) {
return;
}
WeakListener listener = mLocalFieldObservers[localFieldId];
if (listener == null) {
listener = listenerCreator.create(this, localFieldId);
mLocalFieldObservers[localFieldId] = listener;
if (mLifecycleOwner != null) {
listener.setLifecycleOwner(mLifecycleOwner);
}
}
listener.setTarget(observable);
}

这个方法将传进来的数据保存到WeakListener

notifyPropertyChanged

执行完成updateRegistration方法后,需要执行notifyPropertyChanged方法,通知更新,这个方法在BaseObservable里

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

然后调用CallbackRegistrynotifyCallbacks方法

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public synchronized void notifyCallbacks(T sender, int arg, A arg2) {
mNotificationLevel++;
notifyRecurse(sender, arg, arg2);
mNotificationLevel--;
if (mNotificationLevel == 0) {
if (mRemainderRemoved != null) {
for (int i = mRemainderRemoved.length - 1; i >= 0; i--) {
final long removedBits = mRemainderRemoved[i];
if (removedBits != 0) {
removeRemovedCallbacks((i + 1) * Long.SIZE, removedBits);
mRemainderRemoved[i] = 0;
}
}
}
if (mFirst64Removed != 0) {
removeRemovedCallbacks(0, mFirst64Removed);
mFirst64Removed = 0;
}
}
}

这里调用了notifyRecurse方法去递归通知

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private void notifyRecurse(T sender, int arg, A arg2) {
final int callbackCount = mCallbacks.size();
final int remainderIndex = mRemainderRemoved == null ? -1 : mRemainderRemoved.length - 1;

// Now we've got all callbakcs that have no mRemainderRemoved value, so notify the
// others.
notifyRemainder(sender, arg, arg2, remainderIndex);

// notifyRemainder notifies all at maxIndex, so we'd normally start at maxIndex + 1
// However, we must also keep track of those in mFirst64Removed, so we add 2 instead:
final int startCallbackIndex = (remainderIndex + 2) * Long.SIZE;

// The remaining have no bit set
notifyCallbacks(sender, arg, arg2, startCallbackIndex, callbackCount, 0);
}

这个方法里调用notifyCallbacks方法

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private void notifyCallbacks(T sender, int arg, A arg2, final int startIndex,
final int endIndex, final long bits) {
long bitMask = 1;
for (int i = startIndex; i < endIndex; i++) {
if ((bits & bitMask) == 0) {
mNotifier.onNotifyCallback(mCallbacks.get(i), sender, arg, arg2);
}
bitMask <<= 1;
}
}

调用NotifierCallbackonNotifyCallback方法,它的具体回调在PropertyChangeRegistry

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

然后回调Observable的内部类OnPropertyChangedCallbackonPropertyChanged方法,而这个方法的实现是ViewDataBinding的内部类WeakPropertyListener

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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);
}

然后调用handleFieldChange方法

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private void handleFieldChange(int mLocalFieldId, Object object, int fieldId) {
if (mInLiveDataRegisterObserver) {
// We're in LiveData registration, which always results in a field change
// that we can ignore. The value will be read immediately after anyway, so
// there is no need to be dirty.
return;
}
boolean result = onFieldChange(mLocalFieldId, object, fieldId);
if (result) {
requestRebind();
}
}

调用onFieldChange方法,它的具体实现是ActivityDatabindingBindingImpl

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protected boolean onFieldChange(int localFieldId, Object object, int fieldId) {
switch (localFieldId) {
case 0 :
return onChangeUser((com.wangyz.jetpack.databinding.User) object, fieldId);
}
return false;
}

调用onChangeUser

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private boolean onChangeUser(com.wangyz.jetpack.databinding.User User, int fieldId) {
if (fieldId == BR._all) {
synchronized(this) {
mDirtyFlags |= 0x1L;
}
return true;
}
else if (fieldId == BR.name) {
synchronized(this) {
mDirtyFlags |= 0x2L;
}
return true;
}
else if (fieldId == BR.age) {
synchronized(this) {
mDirtyFlags |= 0x4L;
}
return true;
}
return false;
}

在执行完onFieldChange方法后,会再执行requestRebind方法

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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);
}
}
}

在这里post了一个mRebindRunnable到主线程中,看下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();
}
};

在这个方法里执行了executePendingBindings方法

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public void executePendingBindings() {
if (mContainingBinding == null) {
executeBindingsInternal();
} else {
mContainingBinding.executePendingBindings();
}
}

然后执行executeBindingsInternal方法

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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;
}

这个方法里执行了executeBindings方法,它在实现是ActivityDatabindingBindingImpl

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protected void executeBindings() {
long dirtyFlags = 0;
synchronized(this) {
dirtyFlags = mDirtyFlags;
mDirtyFlags = 0;
}
java.lang.String userName = null;
int userAge = 0;
com.wangyz.jetpack.databinding.User user = mUser;
java.lang.String stringValueOfUserAge = null;

if ((dirtyFlags & 0xfL) != 0) {


if ((dirtyFlags & 0xbL) != 0) {

if (user != null) {
// read user.name
userName = user.getName();
}
}
if ((dirtyFlags & 0xdL) != 0) {

if (user != null) {
// read user.age
userAge = user.getAge();
}


// read String.valueOf(user.age)
stringValueOfUserAge = java.lang.String.valueOf(userAge);
}
}
// batch finished
if ((dirtyFlags & 0xbL) != 0) {
// api target 1

androidx.databinding.adapters.TextViewBindingAdapter.setText(this.mboundView1, userName);
}
if ((dirtyFlags & 0xdL) != 0) {
// api target 1

androidx.databinding.adapters.TextViewBindingAdapter.setText(this.mboundView2, stringValueOfUserAge);
}
}

可以看到,是通过androidx.databinding.adapters.TextViewBindingAdapter.setText来实现的

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public static void setText(TextView view, CharSequence text) {
final CharSequence oldText = view.getText();
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);
}

这里我们界面就发生了变更。

来一张简单的时序图

databinding-setVariable

setVariable

java
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public boolean setVariable(int variableId, @Nullable Object variable)  {
boolean variableSet = true;
if (BR.user == variableId) {
setUser((com.wangyz.jetpack.databinding.User) variable);
}
else {
variableSet = false;
}
return variableSet;
}

最终还是通过setUser来实现的。

文章作者: milovetingting
文章链接: http://www.milovetingting.cn/2020/04/15/Android/Jetpack%E5%AD%A6%E4%B9%A0-DataBinding/
版权声明: 本博客所有文章除特别声明外,均采用 CC BY-NC-SA 4.0 许可协议。转载请注明来自 milovetingting

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