這個適配器會在imageUrl和error都在ImageView中設置，imageUrl是字符串，error時drawable時調用

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());
   }
}

事件適配器必須使用帶有一個抽象方法的接口或抽象類，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);
        }
    }
}

當一個listener有多個方法時，必須分成多個listener。比如View.OnAttachStateChangeListener有兩個方法：onViewAttachedToWindow() 和 onViewDetachedFromWindow()，就必須創建兩個接口來區分他們。

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

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

因為改變一個listener經常會影響另一個，所以我們需要有3個不同的綁定適配器。

@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、轉換器（Converters）

Object Conversions

當binding表達式返回一個對象時，會從automatic, renamed, 和 custom setters 中選擇一個，這個對象會被轉換成選擇的setter的參數類型。
這方便了用ObservableMaps保存數據，例如：

userMap 返回了一個 Object ，這個 Object 將會自動轉換成 setText(CharSequence)的參數類型。當參數類型不明確時，開發者需要在表達式中進行轉換。

Custom Conversions

有時特殊的類型需要自動轉換。比如，當設置背景時：

這裡，背景是一個 Drawable，但是顏色是一個整數，需要把 int 轉換成 ColorDrawable，可以使用帶有 BindingConversion 注解的一個靜態方法：

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

Note：不要將上面的和以下混淆