寫在前面：Jetpack的更新速度非诚延溃快尤溜，可能你一個(gè)月前看WorkManager是這樣用的，下個(gè)月這個(gè)使用方法就有可能被廢棄了（我看源碼的時(shí)候是遇到過的逃片，而且源碼也變了，但核心原理是不變的）只酥，所以我們這一系列文章偏重講原理褥实，使用就一帶而過（因?yàn)橹v了也沒用啊，會(huì)變的层皱。性锭。。叫胖。草冈。，讀者使用最好看官方文檔官方文檔
，當(dāng)然我這里講的也是截止到目前的最新用法）怎棱。
Jetpack使用（一）Lifecycles核心原理
Jetpack使用（二）LiveData核心原理
Jetpack使用（三）DataBinding核心原理
Jetpack使用（四）ViewModel核心原理
Jetpack使用（五）Navigation核心原理
Jetpack使用（六） WorkManager的4種用法
DataBinding
是 Google 在 Jetpack 中推出的一款數(shù)據(jù)綁定的支持庫哩俭，利用該庫可以實(shí)現(xiàn)在頁面組件和數(shù)據(jù)的雙向綁定，類似與MVVM拳恋。
具體使用
- 在build.gradle里配置
dataBinding{
enabled true
}
- 新建一個(gè)實(shí)體類繼承BaseObservable凡资，并且在對(duì)應(yīng)的get方法上加上注解 @Bindable，在set方法里加 notifyPropertyChanged(BR.name);
public class User extends BaseObservable {
@Bindable
private String name;
@Bindable
private String password;
public User(String name, String password) {
this.name = name;
this.password = password;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
}
- 在布局文件里導(dǎo)入要用的實(shí)體類
<?xml version="1.0" encoding="utf-8"?>
<layout 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">
<data>
<variable
name="user"
type="com.example.databinding.User" />
</data>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context=".MainActivity">
<TextView
android:id="@+id/tv1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@{user.name}"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="@+id/tv2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@{user.password}"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</LinearLayout>
</layout>
- MainActivity里具體去使用
public class MainActivity extends AppCompatActivity {
ActivityMainBinding binding;
User user;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
user = new User("小三爺", "123");
user.setName(user.getName() + "1");
binding.setVariable(BR.user, user);
}
}
在你想要更新的地方谬运，調(diào)用binding.setVariable(BR.user, user)就ok隙赁。
核心原理分析
當(dāng)我們的代碼在編譯的時(shí)候，系統(tǒng)會(huì)給我們的xml文件分離成兩個(gè)文件
1梆暖、app/build/imtermediates/data_binding_layout_info_type_merge/
debug/activity_main-layout.xml伞访，相當(dāng)于把我們的xml文件用另一種xml方式翻譯了一下
<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<Layout directory="layout" filePath="/Users/wudengyao/android-projects/DataBinding_Demo_20200329/app/src/main/res/layout/activity_main.xml"
isMerge="false"
layout="activity_main" modulePackage="com.example.databinding_demo_20200329">
<Variables name="user" declared="true" type="com.example.databinding_demo_20200329.User">
<location endLine="7" endOffset="63" startLine="5" startOffset="8" />
</Variables>
<Targets>
<Target tag="layout/activity_main_0" view="LinearLayout">
<Expressions />
<location endLine="35" endOffset="18" startLine="10" startOffset="4" />
</Target>
<Target id="@+id/tv1" tag="binding_1" view="TextView">
<Expressions>
<Expression attribute="android:text" text="user.name">
<Location endLine="20" endOffset="38" startLine="20" startOffset="12" />
<TwoWay>false</TwoWay>
<ValueLocation endLine="20" endOffset="36" startLine="20" startOffset="28" />
</Expression>
</Expressions>
<location endLine="24" endOffset="55" startLine="16" startOffset="8" />
</Target>
<Target id="@+id/tv2" tag="binding_2" view="TextView">
<Expressions>
<Expression attribute="android:text" text="user.password">
<Location endLine="29" endOffset="42" startLine="29" startOffset="12" />
<TwoWay>false</TwoWay>
<ValueLocation endLine="29" endOffset="40" startLine="29" startOffset="28" />
</Expression>
</Expressions>
<location endLine="33" endOffset="55" startLine="25" startOffset="8" />
</Target>
</Targets>
</Layout>
2、app/build/imtermediates/incremental/mergeDebugResources/stripped.dir/
layout/activity_main.xml轰驳，把我們的xml去掉了<data></data>包含的代碼塊
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context=".MainActivity" android:tag="layout/activity_main_0" 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">
<TextView
android:id="@+id/tv1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:tag="binding_1"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="@+id/tv2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:tag="binding_2"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</LinearLayout>
還有會(huì)通過apt生成三個(gè)類文件ActivityMainBindingImpl厚掷、BR、DataBinderMapperImpl
BR就相當(dāng)于R文件级解，里面存放xml布局里控件的id
public class BR {
public static final int _all = 0;
public static final int password = 1;
public static final int name = 2;
public static final int user = 3;
}
image.png
知道了這些我們現(xiàn)在真正進(jìn)入源碼分析階段
流程一冒黑、binding.setVariable（設(shè)置數(shù)據(jù)）
我們?cè)谠贛ainActicty里調(diào)用setVariable來設(shè)置數(shù)據(jù)的時(shí)候，會(huì)進(jìn)入到ViewDataBinding類
public abstract boolean setVariable(int variableId, @Nullable Object value);
這是一個(gè)抽象類勤哗，具體實(shí)現(xiàn)會(huì)在ActivityMainBingdingImpl里抡爹，
@Override
public boolean setVariable(int variableId, @Nullable Object variable) {
boolean variableSet = true;
if (BR.user == variableId) {
setUser((com.example.databinding_demo.User) variable);
}
else {
variableSet = false;
}
return variableSet;
}
setVariable里會(huì)調(diào)用到ActivityMainBingdingImpl里的另一個(gè)方法setUser，我們?cè)冱c(diǎn)進(jìn)去
public void setUser(@Nullable com.example.databinding_demo.User User) {
updateRegistration(0, User);
this.mUser = User;
synchronized(this) {
mDirtyFlags |= 0x1L;
}
notifyPropertyChanged(BR.user);
super.requestRebind();
}
setUser里先調(diào)用注冊(cè)u(píng)pdateRegistration方法俺陋，然后把User設(shè)置給成員變量mUser
初始化了mDirtyFlags標(biāo)志豁延，最后是調(diào)用notifyPropertyChanged通知更新。
我們先進(jìn)入注冊(cè)u(píng)pdateRegistration方法
protected boolean updateRegistration(int localFieldId, Observable observable) {
return updateRegistration(localFieldId, observable, CREATE_PROPERTY_LISTENER);
}
CREATE_PROPERTY_LISTENER實(shí)際是返回了WeakPropertyListener對(duì)象
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;
}
最后會(huì)走到這個(gè)方法里來腊状，我們?cè)冱c(diǎn)到registerTo方法
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);
}
這個(gè)方法實(shí)會(huì)判斷mLocalFieldObservers里是否存在WeakListener listener诱咏，如果不存在就創(chuàng)建一個(gè)存進(jìn)去，mLocalFieldObservers里面保存的就是我們控件在BR文件里的id缴挖，每個(gè)id就對(duì)應(yīng)一個(gè)監(jiān)聽器袋狞，然后通過listener.setLifecycleOwner(mLifecycleOwner);把lifecycle關(guān)聯(lián)起來，再通過listener.setTarget(observable);把User對(duì)象關(guān)聯(lián)起來映屋，所以這個(gè)注冊(cè)方法簡單的說就是：把我們的實(shí)體對(duì)象User（觀察者）苟鸯、DataBinding、Lifecycle（被觀察者這里就是activity）建立綁定關(guān)系棚点，還有把控件的BR文件里的id放到一個(gè)數(shù)組里早处。
流程二、setContentView（讀取XML信息）
我們?cè)贛ainActivty里調(diào)用DataBindingUtil.setContentView(this,R.layout.activity_main)實(shí)際會(huì)調(diào)用到
DataBinderMapperImpl的getDataBinder方法瘫析，
@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;
}
ActivityMainBindingImpl再點(diǎn)進(jìn)去砌梆，最后會(huì)調(diào)到ViewDataBinding的mapBindings的方法
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;
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;
}
indexInIncludes = includes == null ? -1 : index;
isBound = true;
} else {
indexInIncludes = -1;
}
} else if (tag != null && tag.startsWith(BINDING_TAG_PREFIX)) {
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;
}
}
}
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;
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;
}
}
}
}
if (!isInclude) {
mapBindings(bindingComponent, child, bindings, includes, viewsWithIds, false);
}
}
}
}
mapBindings里就是解析xml文件信息默责，并存入bingdings數(shù)組，
流程三咸包、notifyPropertyChanged（更新數(shù)據(jù)）
我們知道在流程一setVariable方法之后會(huì)調(diào)用notifyPropertyChanged去更新數(shù)據(jù)桃序，我們點(diǎn)進(jìn)去最終會(huì)跑到ViewDataBingding的 onPropertyChanged里
@Override
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);
}
一層層點(diǎn)進(jìn)去發(fā)現(xiàn)是一個(gè)抽象方法
protected abstract boolean onFieldChange(int localFieldId, Object object, int fieldId);
最終它的實(shí)現(xiàn)類在ActivityMainBingdingImpl的onFieldChange里
@Override
protected boolean onFieldChange(int localFieldId, Object object, int fieldId) {
switch (localFieldId) {
case 0 :
return onChangeUser((com.example.databinding_demo_20200329.User) object, fieldId);
}
return false;
}
private boolean onChangeUser(com.example.databinding_demo_20200329.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.password) {
synchronized(this) {
mDirtyFlags |= 0x4L;
}
return true;
}
return false;
}
這個(gè)方法去通過位或操作設(shè)置mDirtyFlags的初始值的，我們?cè)倏吹絍iewDataBinding里的一個(gè)靜態(tài)代碼塊
static {
if (VERSION.SDK_INT < VERSION_CODES.KITKAT) {
ROOT_REATTACHED_LISTENER = null;
} else {
ROOT_REATTACHED_LISTENER = new OnAttachStateChangeListener() {
@TargetApi(VERSION_CODES.KITKAT)
@Override
public void onViewAttachedToWindow(View v) {
// execute the pending bindings.
final ViewDataBinding binding = getBinding(v);
binding.mRebindRunnable.run();
v.removeOnAttachStateChangeListener(this);
}
@Override
public void onViewDetachedFromWindow(View v) {
}
};
}
}
這里有個(gè)監(jiān)聽器烂瘫，在監(jiān)聽executeBindings媒熊，這個(gè)方法最終的實(shí)現(xiàn)類在ActivityMainBindingImpl里
@Override
protected void executeBindings() {
long dirtyFlags = 0;
synchronized(this) {
dirtyFlags = mDirtyFlags;
mDirtyFlags = 0;
}
java.lang.String userName = null;
com.example.databinding_demo_20200329.User user = mUser;
java.lang.String userPassword = 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.password
userPassword = user.getPassword();
}
}
}
// batch finished
if ((dirtyFlags & 0xbL) != 0) {
// api target 1
androidx.databinding.adapters.TextViewBindingAdapter.setText(this.tv1, userName);
}
if ((dirtyFlags & 0xdL) != 0) {
// api target 1
androidx.databinding.adapters.TextViewBindingAdapter.setText(this.tv2, userPassword);
}
}
我們前面通過onFieldChange設(shè)置的標(biāo)志就是在這里通過位與操作來進(jìn)行控件的更新的。到此坟比，DataBingding核心原理就一目了然了
