Databinding

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android.databinding

@Bindable

The Bindable annotation should be applied to any getter accessor method of an Observable class.

@Bindable用于修饰方法或变量。

Observable接口提供给开发者添加/移除监听者的机制。为了使开发更便捷我们创建了BaseObservable类，它已经实现了Observable接口中的注册监听者的机制。

继承自BaseObservable的数据类仍需手动的通知监听者们数据变更。因而，你可以在setter方法中通知变更，在getter方法中标记注解@Bindable。

注解@Bindable在编译期间生成一个BR类以此持有对应的实例，作用同R文件。

private static class User extends BaseObservable {
   private String firstName;
   @Bindable
   public String getFirstName() {
       return this.firstName;
   }
   public void setFirstName(String firstName) {
       this.firstName = firstName;
       notifyPropertyChanged(BR.firstName);
   }
}

@BindingAdapter

BindingAdapter is applied to methods that are used to manipulate how values with expressions are set to views.

@BindingAdapter用于修饰方法。

一些属性需要定制绑定逻辑，一个用@BindingAdapter修饰的静态方法可以自定义属性的setter操作。

android自身实现了大量的Adapter，你可以在项目module的android.databinding.adapters包下找到这些代码。

public class CardViewBindingAdapter {
    @BindingAdapter("contentPadding")
    public static void setContentPadding(CardView view, int padding) {
        view.setContentPadding(padding, padding, padding, padding);
    }
}

1、默认的你的自定义的命名空间，在匹配时会被忽略。

@BindingAdapter("contentPadding")

2、允许重写android的命名空间。

@BindingAdapter("android:contentPadding")

app:contentPadding与android:contentPadding处理行为可以不一样。
app:contentPadding与custom:contentPadding处理行为是一致的。（仅android是特殊的命名空间）。

需要注意，当你创建的适配器属性与系统默认的产生冲突时，你的自定义适配器将会覆盖掉系统原先定义的注解，这将会产生一些意外的问题。

假设需要对下面接口，做适配。

public interface ILogAction{
      void login();
      void logout();
}

则需要一个方法一个接口，这么做的原因是避免login()的修改影响到logout()。所以根据业务需要，可能需要排列组合适配这两个接口。

1、适配 login
2、适配 logout
3、适配 login + logout

@BindingBuildInfo

@BindingBuildInfo(
buildId="3fefc6ba-1e95-4dcf-8ffa-278fe0f449bd",
modulePackage="com.ipudong.library",
sdkRoot="/Users/robert/Library/Android/sdk",
layoutInfoDir="/Users/robert/android/develops/pudong-d-android/lib_basic/build/intermediates/data-binding-info/debug",
exportClassListTo="/Users/robert/android/develops/pudong-d-android/lib_basic/build/intermediates/data-binding-info/debug/_generated.txt",
isLibrary=true,
minSdk=14,
enableDebugLogs=false,
printEncodedError=true
)
public class DataBindingInfo {}

在SOURCE阶段会自动生成DataBindingInfo.class，并标记注解如上。

@BindingConversion

Annotate methods that are used to automatically convert from the expression type to the value used in the setter.

有时候会遇到类型不匹配的问题，比如R.color.white是int，但是通过Data Binding赋值给android:background属性后，需要把int转换为ColorDrawable。

@BindingConversion
public static Drawable convertColorToDrawable(int drawable) {
  return new ColorDrawable(drawable);
}

@BindingMethod && @BindingMethods

Used within an BindingMethods annotation to describe a renaming of an attribute to the setter used to set that attribute.
Used to enumerate attribute-to-setter renaming.

@BindingMethods用于修饰类。

一些属性虽然拥有setters但是并不与名字相匹配，这些方法的属性可以通过 @BindingMethod && @BindingMethods 注释 setters。

@BindingMethods({
       @BindingMethod(type = "android.widget.ImageView",
                      attribute = "android:tint",
                      method = "setImageTintList"),
})

开发人员不太可能需要重命名 setters ，因为android框架属性已经实现了这一部分。

@InverseBindingAdapter

InverseBindingAdapter is associated with a method used to retrieve the value for a View when setting values gathered from the View.

InverseBindingAdapter is associated with a method used to retrieve the value for a View when setting values gathered from the View. This is similar to BindingAdapters:

 @InverseBindingAdapter(attribute = "android:text", event = "android:textAttrChanged")
 public static String captureTextValue(TextView view, CharSequence originalValue) {
     CharSequence newValue = view.getText();
     CharSequence oldValue = value.get();
     if (oldValue == null) {
         value.set(newValue);
     } else if (!contentEquals(newValue, oldValue)) {
         value.set(newValue);
     }
 }

he default value for event is the attribute name suffixed with "AttrChanged". In the above example, the default value would have been android:textAttrChanged even if it wasn't provided.

The event attribute is used to notify the data binding system that the value has changed. The developer will typically create a BindingAdapter to assign the event. For example:

@BindingAdapter(value = {"android:beforeTextChanged", "android:onTextChanged",
                          "android:afterTextChanged", "android:textAttrChanged"},
                          requireAll = false)
 public static void setTextWatcher(TextView view, final BeforeTextChanged before,
                                   final OnTextChanged on, final AfterTextChanged after,
                                   final InverseBindingListener textAttrChanged) {
     TextWatcher newValue = new TextWatcher() {
         ...
         @Override
         public void onTextChanged(CharSequence s, int start, int before, int count) {
             if (on != null) {
                 on.onTextChanged(s, start, before, count);
             }
             if (textAttrChanged != null) {
                 textAttrChanged.onChange();
             }
         }
     }
     TextWatcher oldValue = ListenerUtil.trackListener(view, newValue, R.id.textWatcher);
     if (oldValue != null) {
         view.removeTextChangedListener(oldValue);
     }
     view.addTextChangedListener(newValue);
 }

Like BindingAdapters, InverseBindingAdapter methods may also take DataBindingComponent as the first parameter and may be an instance method with the instance retrieved from the DataBindingComponent.

InverseBindingListener is useful. See InverseBindingListener

@InverseBindingMethod

InverseBindingMethod is used to identify how to listen for changes to a View property and which getter method to call.

InverseBindingMethod is used to identify how to listen for changes to a View property and which getter method to call. InverseBindingMethod should be associated with any class as part of InverseBindingMethods.

@InverseBindingMethods({@InverseBindingMethod(
     type = android.widget.TextView.class,
     attribute = "android:text",
     event = "android:textAttrChanged",
     method = "getText")})
 public class MyTextViewBindingAdapters { ... }

method is optional. If it isn't provided, the attribute name is used to find the method name, either prefixing with "is" or "get". For the attribute android:text, data binding will search for a public CharSequence getText() method on TextView.

event is optional. If it isn't provided, the event name is assigned the attribute name suffixed with AttrChanged. For the android:text attribute, the default event name would be android:textAttrChanged. The event should be set using a BindingAdapter. For example:

 @BindingAdapter(value = {"android:beforeTextChanged", "android:onTextChanged",
                          "android:afterTextChanged", "android:textAttrChanged"},
                          requireAll = false)
 public static void setTextWatcher(TextView view, final BeforeTextChanged before,
                                   final OnTextChanged on, final AfterTextChanged after,
                                   final InverseBindingListener textAttrChanged) {
     TextWatcher newValue = new TextWatcher() {
         ...
         @Override
         public void onTextChanged(CharSequence s, int start, int before, int count) {
             if (on != null) {
                 on.onTextChanged(s, start, before, count);
             }
             if (textAttrChanged != null) {
                 textAttrChanged.onChange();
             }
         }
     }
     TextWatcher oldValue = ListenerUtil.trackListener(view, newValue, R.id.textWatcher);
     if (oldValue != null) {
         view.removeTextChangedListener(oldValue);
     }
     view.addTextChangedListener(newValue);
 }

@InverseBindingMethods

Used to enumerate attribute, getter, and event association.

@Untaggable

Data Binding相关的jar包由四部分组成，

1. baseLibrary-2.1.0-rc1.jar
   作为运行时类库被打进APK中；

2. DataBinderPlugin（gradle plugin）
   在编译期使用，利用gradle-api（之前叫transform-api，1.5生，2.0改名）处理xml文件，生成DataBindingInfo.java；

3. compiler-2.1.0-rc1.jar
   在编译器使用，入口类继承自AbstractProcessor，用于处理注解，并生成Binding类，DataBindingCompoent.java，DataBinderMapper.java类；

4. compilerCommon-2.1.0-rc1.jar
   被DataBinderPlugin和compiler-2.1.0-rc1.jar所依赖

相关编译流程

STEP1 资源处理

aapt或者gradle执行时，都会触发资源处理。
在资源处理过程中，DataBinding都会扫描一遍现有的资源，生成不包含的data-binding-layout-out以及DataBinding所需要的data-binding-info；

STEP2 DataBindingInfo.class生成

在完成资源处理后，aapt或者gradle-api都会去执行DataBindingInfo.class生成操作，把相关的信息写入DataBindingInfo.class的@BindingBuildInfo注解中；

STEP3 监听到注解变化

生成@BindingBuildInfo注解，或者code中发现有新的注解写入，AbstractProcessor注解处理器就开始执行注解处理。
DataBinding中有一个ProcessDataBinding.java类专门来处理DataBinding相关的注解；

STEP4 ProcessDataBinding处理注解，生成bin

ProcessDataBinding中处理注解永远会按顺执行3步，ProcessMethodAdapter，ProcessExpressions，ProcessBindable。
每次执行都会从磁盘反序列化对应的bin文件，然后往bin中写入新的，完成后再序列化到磁盘；

STEP5 生成最终产物

执行ProcessMethodAdapter生成DataBindingComponents.class；
执行ProcessExpressions生成ViewDataBinding.class子类（ActivityDetail2Binding.class），并触发DataBindingMapper.class更新；
执行ProcessBindable生成BR.class，并触发DataBindingMapper.class更新；

参考链接：
http://www.jianshu.com/p/eb29c691d370
https://developer.android.com/topic/libraries/data-binding/index.html
https://developer.android.com/reference/android/databinding/InverseBindingAdapter.html