lzyprime 博客 (github)
创建时间：2021.04.23
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kotlin & android 笔记
λ：
# ViewBinding DataBinding
# 仓库地址: https://github.com/lzyprime/android_demos
# branch: viewBinding
git clone -b viewBinding https://github.com/lzyprime/android_demos
最近几个月忙于写需求，积累了太多要总结的东西。当然也正是这几个月的大量实践，对一些知识有了新的认识和发现。
ViewBinding DataBinding 通过 xml 声明，生成对应代码，刨开生成的源码看一下，大概就能明白原理。
有用的可能就是 val binding by viewBinding<T>() 的两个拓展函数实现。其余就是如官网文档一样的备忘录内容，方便知识点查找。
ViewBinding
生成的源码
ViewBinding 库代替之前的kotlin-android-extensions, 根据布局文件 layout/example.xml 生成对应的[ExampleBinding].
以[FragmentDetailBinding]为例, 看一下生成的源码。
public final class FragmentDetailBinding implements ViewBinding {
@NonNull
private final FrameLayout rootView;
@NonNull
public final ImageView imageView;
private FragmentDetailBinding(@NonNull FrameLayout rootView, @NonNull ImageView imageView) {
this.rootView = rootView;
this.imageView = imageView;
}
@Override
@NonNull
public FrameLayout getRoot() {
return rootView;
}
@NonNull
public static FragmentDetailBinding inflate(@NonNull LayoutInflater inflater) {
return inflate(inflater, null, false);
}
@NonNull
public static FragmentDetailBinding inflate(@NonNull LayoutInflater inflater,
@Nullable ViewGroup parent, boolean attachToParent) {
View root = inflater.inflate(R.layout.fragment_detail, parent, false);
if (attachToParent) {
parent.addView(root);
}
return bind(root);
}
@NonNull
public static FragmentDetailBinding bind(@NonNull View rootView) {
// The body of this method is generated in a way you would not otherwise write.
// This is done to optimize the compiled bytecode for size and performance.
int id;
missingId: {
id = R.id.imageView;
ImageView imageView = rootView.findViewById(id);
if (imageView == null) {
break missingId;
}
return new FragmentDetailBinding((FrameLayout) rootView, imageView);
}
String missingId = rootView.getResources().getResourceName(id);
throw new NullPointerException("Missing required view with ID: ".concat(missingId));
}
}
基类[ViewBinding]是interface, 只有一个getRoot方法，返回显示的View
/** A type which binds the views in a layout XML to fields. */
public interface ViewBinding {
/**
* Returns the outermost {@link View} in the associated layout file. If this binding is for a
* {@code <merge>} layout, this will return the first view inside of the merge tag.
*/
@NonNull
View getRoot();
}
每份生成的代码:
- 根据
layout/fragment_detail.xml下划线名称生成对应驼峰类名FragmentDetailBinding - 根据布局文件中组件
id, 生成对应驼峰式成员名，类型为组件类型. 如imageView: ImageView - 根部局生成为
rootView
构造函数私有，需要的参数为上述根据id生成的成员.
private FragmentDetailBinding(@NonNull FrameLayout rootView, @NonNull ImageView imageView)
同时生成3个静态函数作为工厂构造
- 两个
inflate用传入的[inflater: LayoutInflater]获得对应的View. - 调用
bind，通过findViewById获得各个组件, 然后通过私有构造得到[FragmentDetailBinding]
也就是说, findViewById 的过程靠生成代码解决，所以在拿到一个ViewBinding实例时, 可以通过成员直接访问。
kotlin 伪代码大概写一下工厂构造的调用关系
fun inflate(inflater: LayoutInflater): FragmentDetailBinding = inflate(inflater, null, false)
fun inflate(inflater: LayoutInflater,
parent: ViewGroup,
attachToParent: Boolean,
): FragmentDetailBinding {
...
val root: View = inflater.inflate(...)
...
return bind(root)
}
fun bind(rootView: View): FragmentDetailBinding {
// findViewById
val imageView = rootView.findViewById(R.id.imageView)
return FragmentDetailBinding(rootView, imageView)
}
使用
- 当前没有View, 需要新建
// 官网例子：
// Activity
class ResultProfileActivity : AppCompatActivity(){
private lateinit var binding: ResultProfileBinding
override fun onCreate(savedInstanceState: Bundle) {
super.onCreate(savedInstanceState)
// 通过 inflate 新建
binding = ResultProfileBinding.inflate(layoutInflater)
val view = binding.root
setContentView(view)
}
}
// Fragment
class ResultProfileFragment : Fragment() {
private var _binding: ResultProfileBinding? = null
private val binding get() = _binding!!
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
_binding = ResultProfileBinding.inflate(inflater, container, false)
val view = binding.root
return view
}
override fun onDestroyView() {
super.onDestroyView()
_binding = null
}
}
- 已有视图，直接通过
bind获得
// Fragment 构造直接传 R.layout.fragment_detail
class DetailFragment : Fragment(R.layout.fragment_detail) {
private var _binding: FragmentDetailBinding? = null
private val binding get() = _binding!!
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
// 此时R.layout.fragment_detail对应View已存在，直接 bind
_binding = FragmentDetailBinding.bind(view)
...
}
override fun onDestroyView() {
super.onDestroyView()
_binding = null
}
}
同理其他地方，没有视图调用inflate构造，有视图调用bind直接获得.
Activity, Fragment 使用优化
存在的问题:
- 过程重复。 每个
Activity和Fragment中，流程相同，仅仅是具体[ViewBinding]的区别。 -
Fragment中,onDestroyView时要将_binding置空，对于binding的操作时机靠自己保证，时序自己保证。 -
lateinit var在代码扫描中视为风险行为，不建议使用(个人项目随意)。
仿照
val model: VM by viewModels<VM>()
通过拓展函数, 委托, 反射, 实现类似
val binding: FragmentDetailBinding by viewBinding<FragmentDetailBinding>()
/**
* 用于[Activity]生成对应[ViewBinding].
*
* @exception ClassCastException 当 [VB] 无法通过
* `VB.inflate(LayoutInflater.from(this#Activity))` 构造成功时抛出
* */
@MainThread
inline fun <reified VB : ViewBinding> Activity.viewBinding() = object : Lazy<VB> {
private var cached: VB? = null
override val value: VB
get() =
cached ?: VB::class.java.getMethod(
"inflate",
LayoutInflater::class.java,
).invoke(null, layoutInflater).let {
if (it is VB) {
cached = it
it
} else {
throw ClassCastException()
}
}
override fun isInitialized(): Boolean = cached != null
}
// example
class MainActivity : AppCompatActivity() {
private val binding by viewBinding<ActivityMainBinding>()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
// 确保调用该函数设置binding.root
setContentView(binding.root)
}
}
Activity内联拓展函数，通过调用inflate(inflater: LayoutInflater)版本生成binding。需要自己确保在onCreate之后使用，否则拿不到Activity.layoutInflater, 构造失败
/**
* 用于 [Fragment] 内构造对应 [ViewBinding].
*
* @exception ClassCastException 当 [VB] 无法通过 `VB.bind(view)` 构造成功时抛出
*
* 函数会自动注册[Fragment.onDestroyView]时的注销操作.
* */
@MainThread
inline fun <reified VB : ViewBinding> Fragment.viewBinding() = object : Lazy<VB> {
private var cached: VB? = null
override val value: VB
get() = cached ?: VB::class.java.getMethod(
"bind",
View::class.java,
).invoke(VB::class.java, this@viewBinding.requireView()).let {
if (it is VB) {
// 监听Destroy事件
viewLifecycleOwner.lifecycle.addObserver(object : LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
fun onDestroyView() {
cached = null
}
})
cached = it
it
} else {
throw ClassCastException()
}
}
override fun isInitialized(): Boolean = cached != null
}
// example
class ExampleFragment:Fragment(R.layout.example_fragment) {
private val binding by viewBinding<ExampleFragmentBinding>()
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
// 确保在此之后使用binding
binding.xxxTextView.text = "sssss"
}
}
Fragment内联拓展函数，通过调用bind(rootView: View)版本生成binding。
前提是调用Fragment(@LayoutRes)版本构造, 利用Fragment默认的onCreateView行为得到View。因此要在onViewCreated后使用binding。否则Fragment.requireView()拿不到view, bind失败。
通过viewLifecycleOwner.lifecycle监听Destroy行为，将cached赋为null, 当重新构建View时，binding的isInitialized() == false, 认为没有初始化，重新走value get()中的逻辑，达到重新绑定的效果。
总结：原有问题仍有一部分未解决(如: 自己保证执行时序), 但一定程度上减少了重复代码，尤其是Fragment中。
DataBinding
DataBinding相当于ViewBinding++
在xml中传递和使用数据
<?xml version="1.0" encoding="utf-8"?>
<!-- layout作为根 -->
<layout xmlns:android="http://schemas.android.com/apk/res/android">
<!-- 数据 -->
<data>
<variable name="user" type="com.example.User"/>
</data>
<!-- 布局 -->
<LinearLayout
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@{user.firstName}"/> <!-- 使用数据 -->
<TextView android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@{user.lastName}"/> <!-- 使用数据 -->
</LinearLayout>
</layout>
// data class User(val firstName: String, val lastName: String)
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
val binding: ActivityMainBinding = DataBindingUtil.setContentView(
this, R.layout.activity_main)
binding.user = User("Test", "User")
}
基类[ViewDataBinding]
public abstract class ViewDataBinding extends BaseObservable implements ViewBinding
实现了
[ViewBinding], 生成的代码中inflate, bind函数签名相同，内部实现略有不同，所以上边by viewBinding<T>()仍然适用。同时继承
[BaseObservable], 使得本身成为[Observable], 可观察者
除了像ViewBinding中构造方式, 还可以使用DataBindingUtil：
// Activity, 等价于 inflate + setContentView
val binding = DataBindingUtil.setContentView(this, R.layout.activity_main)
// or
val binding = DataBindingUtil.inflate(layoutInflater, R.layout.list_item, viewGroup, false)
绑定表达式
<data> 中
<data>
<!-- 声明 -->
<variable name="user" type="com.example.User"/>
<!-- 导入 -->
<import type="android.view.View"/>
<!-- 类型别名 -->
<import type="com.example.real.estate.View" alias="Vista"/>
<!-- 集合 -->
<import type="android.util.SparseArray"/>
<import type="java.util.Map"/>
<import type="java.util.List"/>
<variable name="list" type="List<String>"/>
<variable name="sparse" type="SparseArray<String>"/>
<variable name="map" type="Map<String, String>"/>
<variable name="index" type="int"/>
<variable name="key" type="String"/>
<!-- 在布局中使用
android:text="@{list[index]}"
android:text="@{sparse[index]}"
android:text="@{map[key]}"
-->
</data>
布局中，表达式
- 算术运算符
+ - / * % - 字符串连接运算符
+ - 逻辑运算符
&& || - 二元运算符
& | ^ - 一元运算符
+ - ! ~ - 移位运算符
>> >>> << - 比较运算符
== > < >= <= instanceof- 分组运算符
() - 字面量运算符 - 字符、字符串、数字、null
- 类型转换
- 方法调用
- 字段访问
- 数组访问
[] - 三元运算符
?:
<!-- 当链式调用中存在可空类型时, 如： -->
<TextView android:text="@{a.b.c.d.e}"/>
<!-- 相当于 -->
<TextView android:text="@{a?.b?.c?.d?.e}"/>
<!-- 其中有一环为空, 则表达式值为null -->
<TextView android:text="@{expr ?? defautValue}"/>
<!-- 相当于 -->
<TextView android:text="@{expr != null ? expr : defautValue}"/>
<!-- 资源引用 -->
android:padding="@{large ? @dimen/largePadding : @dimen/smallPadding}"
android:text="@{@string/nameFormat(firstName, lastName)}"
...
<!-- function -->
<data>
<variable name="task" type="com.android.example.Task" />
<variable name="presenter" type="com.android.example.Presenter" />
</data>
<LinearLayout android:onClick="@{() -> presenter.onSaveClick(task)}" />
...
</LinearLayout>
<!--
class Presenter {
fun onSaveClick(view: View, task: Task){}
}
-->
android:onClick="@{(theView) -> presenter.onSaveClick(theView, task)}"
<!--
class Presenter {
fun onCompletedChanged(task: Task, completed: Boolean){}
}
-->
android:onCheckedChanged="@{(cb, isChecked) -> presenter.completeChanged(task, isChecked)}"
<!-- ?: -->
android:onClick="@{(v) -> v.isVisible() ? doSomething() : void}"
适配器
现有的 资源引用表达式 满足大多数情况，但也有例外，常见为ImageView中。所以用适配器指定处理方法
@BindingMethods
// 将 android:tint 交由 setImageTintList(ColorStateList) 处理, 而非原有 setTint()
@BindingMethods(value = [
BindingMethod(
type = android.widget.ImageView::class,
attribute = "android:tint",
method = "setImageTintList")])
@BindingAdapter
@BindingAdapter(value = ["imageUrl", "placeholder"], requireAll = false)
fun setImageUrl(imageView: ImageView, url: String?, placeHolder: Drawable?) {
if (url == null) {
imageView.setImageDrawable(placeholder);
} else {
MyImageLoader.loadInto(imageView, url, placeholder);
}
}
//xml
<ImageView app:imageUrl="@{venue.imageUrl}" app:error="@{@drawable/venueError}" />
-
@BindingConversion, 自定义转换
@BindingConversion
fun convertColorToDrawable(color: Int) = ColorDrawable(color)
//xml
<View android:background="@{isError ? @drawable/error : @color/white}" .../>
-
@TargetApi, 监听器有多个方法时，需要拆分处理
// View.OnAttachStateChangeListener 为例
// 有两个方法：onViewAttachedToWindow(View) 和 onViewDetachedFromWindow(View)
// 1. 拆分
@TargetApi(Build.VERSION_CODES.HONEYCOMB_MR1)
interface OnViewDetachedFromWindow {
fun onViewDetachedFromWindow(v: View)
}
@TargetApi(Build.VERSION_CODES.HONEYCOMB_MR1)
interface OnViewAttachedToWindow {
fun onViewAttachedToWindow(v: View)
}
// 2. BindAdapter
@BindingAdapter(
"android:onViewDetachedFromWindow",
"android:onViewAttachedToWindow",
requireAll = false
)
fun setListener(view: View, detach: OnViewDetachedFromWindow?, attach:OnViewAttachedToWindow?) {
...
}
// 3. xml中使用
Observable, LiveData作为数据
数据更新时，UI自动刷新
ObservableBooleanObservableByteObservableCharObservableShortObservableIntObservableLongObservableFloatObservableDoubleObservableParcelableObservableArrayListObservableArrayMap
// 自定义
class User : BaseObservable() {
@get:Bindable // 给getter方法打标签, BR中会生成对应条目
var firstName: String = ""
set(value) {
field = value
notifyPropertyChanged(BR.firstName) // 刷新UI
}
@get:Bindable
var lastName: String = ""
set(value) {
field = value
notifyPropertyChanged(BR.lastName) // 刷新UI
}
}
或者用 LiveData, 在代码中需要调用setLifecycleOwner()
<!-- data class User(val firstName: LiveData<String>, val lastName: LiveData<String>) -->
<!-- xml中 -->
<data>
<variable name="duration" type="LiveData<String>"/>
<variable name="user" type="com.example.User"/>
</data>
<TextView android:text="@{user.firstName}"/>
<TextView android:text="@{duration}"/>
// kotlin
class ExampleFragment : Fragment(R.layout.example_fragment) {
...
binding.duration = liveData<String> { emitSource(...) }
binding.user = model.user
binding.setLifecycleOwner(viewLifecycleOwner)
...
}
结合两者使用：
open class ObservableViewModel : ViewModel(), Observable {
private val callbacks: PropertyChangeRegistry = PropertyChangeRegistry()
// 添加订阅
override fun addOnPropertyChangedCallback(
callback: Observable.OnPropertyChangedCallback) {
callbacks.add(callback)
}
// 取消订阅
override fun removeOnPropertyChangedCallback(
callback: Observable.OnPropertyChangedCallback) {
callbacks.remove(callback)
}
// 全量刷新
fun notifyChange() {
callbacks.notifyCallbacks(this, 0, null)
}
// 精确刷新
fun notifyPropertyChanged(fieldId: Int) {
callbacks.notifyCallbacks(this, fieldId, null)
}
}
数据双向绑定 @={}
<CheckBox
android:id="@+id/rememberMeCheckBox"
android:checked="@={viewmodel.rememberMe}"
/>
class LoginViewModel : BaseObservable {
// val data = ...
@Bindable
fun getRememberMe(): Boolean = data.rememberMe
fun setRememberMe(value: Boolean) {
if (data.rememberMe != value) {
data.rememberMe = value
// React to the change.
saveData()
notifyPropertyChanged(BR.remember_me)
}
}
}
使用@InverseBindingAdapter和@InverseBindingMethod, 自定义双向绑定
// 1. 数据变动时调用的方法
@BindingAdapter("time")
@JvmStatic fun setTime(view: MyView, newValue: Time) {
// Important to break potential infinite loops.
if (view.time != newValue) {
view.time = newValue
}
}
// 2. view变动时调用的方法
@InverseBindingAdapter("time")
@JvmStatic fun getTime(view: MyView) : Time {
return view.getTime()
}
// 3. 变动时机和方式, 后缀`AttrChanged`
@BindingAdapter("app:timeAttrChanged")
@JvmStatic fun setListeners(
view: MyView,
attrChange: InverseBindingListener
) {
// 使用 InverseBindingListener 告知数据绑定系统，特性已更改
// 数据绑定系统调用@InverseBindingAdapter绑定的方法
// warning: 避免陷入循环刷新.
}
