这个适配器会在imageUrl和error都在ImageView中设置，imageUrl是字符串，error时drawable时调用

Binding adapter methods may optionally take the old values in their handlers. A method taking old and new values should have all old values for the attributes come first, followed by the new values:

@BindingAdapter("android:paddingLeft")
public static void setPaddingLeft(View view, int oldPadding, int newPadding) {
   if (oldPadding != newPadding) {
       view.setPadding(newPadding,
                       view.getPaddingTop(),
                       view.getPaddingRight(),
                       view.getPaddingBottom());
   }
}

事件适配器必须使用带有一个抽象方法的接口或抽象类，如：

@BindingAdapter("android:onLayoutChange")
public static void setOnLayoutChangeListener(View view, View.OnLayoutChangeListener oldValue,
       View.OnLayoutChangeListener newValue) {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
        if (oldValue != null) {
            view.removeOnLayoutChangeListener(oldValue);
        }
        if (newValue != null) {
            view.addOnLayoutChangeListener(newValue);
        }
    }
}

当一个listener有多个方法时，必须分成多个listener。比如View.OnAttachStateChangeListener有两个方法：onViewAttachedToWindow() 和 onViewDetachedFromWindow()，就必须创建两个接口来区分他们。

@TargetApi(VERSION_CODES.HONEYCOMB_MR1)
public interface OnViewDetachedFromWindow {
    void onViewDetachedFromWindow(View v);
}

@TargetApi(VERSION_CODES.HONEYCOMB_MR1)
public interface OnViewAttachedToWindow {
    void onViewAttachedToWindow(View v);
}

因为改变一个listener经常会影响另一个，所以我们需要有3个不同的绑定适配器。

@BindingAdapter("android:onViewAttachedToWindow")
public static void setListener(View view, OnViewAttachedToWindow attached) {
    setListener(view, null, attached);
}

@BindingAdapter("android:onViewDetachedFromWindow")
public static void setListener(View view, OnViewDetachedFromWindow detached) {
    setListener(view, detached, null);
}

@BindingAdapter({"android:onViewDetachedFromWindow", "android:onViewAttachedToWindow"})
public static void setListener(View view, final OnViewDetachedFromWindow detach,
        final OnViewAttachedToWindow attach) {
    if (VERSION.SDK_INT >= VERSION_CODES.HONEYCOMB_MR1) {
        final OnAttachStateChangeListener newListener;
        if (detach == null && attach == null) {
            newListener = null;
        } else {
            newListener = new OnAttachStateChangeListener() {
                @Override
                public void onViewAttachedToWindow(View v) {
                    if (attach != null) {
                        attach.onViewAttachedToWindow(v);
                    }
                }

                @Override
                public void onViewDetachedFromWindow(View v) {
                    if (detach != null) {
                        detach.onViewDetachedFromWindow(v);
                    }
                }
            };
        }
        final OnAttachStateChangeListener oldListener = ListenerUtil.trackListener(view,
                newListener, R.id.onAttachStateChangeListener);
        if (oldListener != null) {
            view.removeOnAttachStateChangeListener(oldListener);
        }
        if (newListener != null) {
            view.addOnAttachStateChangeListener(newListener);
        }
    }
}

The above example is slightly more complicated than normal because View uses add and remove for the listener instead of a set method for View.OnAttachStateChangeListener. The android.databinding.adapters.ListenerUtil class helps keep track of the previous listeners so that they may be removed in the Binding Adaper.

By annotating the interfaces OnViewDetachedFromWindow and OnViewAttachedToWindow with @TargetApi(VERSION_CODES.HONEYCOMB_MR1), the data binding code generator knows that the listener should only be generated when running on Honeycomb MR1 and new devices, the same version supported by addOnAttachStateChangeListener(View.OnAttachStateChangeListener).

九、转换器（Converters）

Object Conversions

当binding表达式返回一个对象时，会从automatic, renamed, 和 custom setters 中选择一个，这个对象会被转换成选择的setter的参数类型。
这方便了用ObservableMaps保存数据，例如：

userMap 返回了一个 Object ，这个 Object 将会自动转换成 setText(CharSequence)的参数类型。当参数类型不明确时，开发者需要在表达式中进行转换。

Custom Conversions

有时特殊的类型需要自动转换。比如，当设置背景时：

这里，背景是一个 Drawable，但是颜色是一个整数，需要把 int 转换成 ColorDrawable，可以使用带有 BindingConversion 注解的一个静态方法：

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

Note：不要将上面的和以下混淆