先看View中是如何定义clickable和onClick的：
case com.android.internal.R.styleable.View_clickable:
if (a.getBoolean(attr, false)) {
viewFlagValues |= CLICKABLE;
viewFlagMasks |= CLICKABLE;
}
break;
上面这段代码是View源码中对clickable属性的定义，缺省值为false。
case R.styleable.View_onClick:
if (context.isRestricted()) {
throw new IllegalStateException("The android:onClick attribute cannot "
+ "be used within a restricted context");
}
final String handlerName = a.getString(attr);
if (handlerName != null) {
setOnClickListener(new OnClickListener() {
private Method mHandler;
public void onClick(View v) {
if (mHandler == null) {
try {
mHandler = getContext().getClass().getMethod(handlerName,
View.class);
} catch (NoSuchMethodException e) {
int id = getId();
String idText = id == NO_ID ? "" : " with id ‘"
+ getContext().getResources().getResourceEntryName(
id) + "‘";
throw new IllegalStateException("Could not find a method " +
handlerName + "(View) in the activity "
+ getContext().getClass() + " for onClick handler"
+ " on view " + View.this.getClass() + idText, e);
}
}
try {
mHandler.invoke(getContext(), View.this);
} catch (IllegalAccessException e) {
throw new IllegalStateException("Could not execute non "
+ "public method of the activity", e);
} catch (InvocationTargetException e) {
throw new IllegalStateException("Could not execute "
+ "method of the activity", e);
}
}
});
}
break;
上面是onClick属性在View源码中的定义。可以看到它调用了setOnClickListener()方法对点击事件进行注册，下面看下setOnClickListener()方法的源码：
/**
* Register a callback to be invoked when this view is clicked. If this view is not
* clickable, it becomes clickable.
*
* @param l The callback that will run
*
* @see #setClickable(boolean)
*/
public void setOnClickListener(OnClickListener l) {
if (!isClickable()) {
setClickable(true);
}
getListenerInfo().mOnClickListener = l;
}
如果clickable属性是false那么在调用此方法后就会变为true。
继承自View的Button、ImageView控件在xml里使用onClick属性时，通过打印日志可以看到clickable由默认的false变为了true，可是同样继承自View的TextView依旧是false。为什么会出现这种情况呢？
那就要了解Android加载xml文件的过程。
以View为例进行说明。我们使用setOnContentView()方法加载布局文件时。布局文件中的View元素会调用下面的这个构造器来新建一个View对象
/**
* Constructor that is called when inflating a view from XML. This is called
* when a view is being constructed from an XML file, supplying attributes
* that were specified in the XML file. This version uses a default style of
* 0, so the only attribute values applied are those in the Context‘s Theme
* and the given AttributeSet.
*
* <p>
* The method onFinishInflate() will be called after all children have been
* added.
*
* @param context The Context the view is running in, through which it can
* access the current theme, resources, etc.
* @param attrs The attributes of the XML tag that is inflating the view.
* @see #View(Context, AttributeSet, int)
*/
public View(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
this(context, attrs, 0);又会调用 public View(Context context, AttributeSet attrs, int defStyleAttr){}构造器。可以看到onClick属性在定义时调用的setOnClickListener()方法是在对象被创建时调用的。
下面再来看TextView控件，他在构造器中对clickable又进行了一些处理
boolean focusable = mMovement != null || getKeyListener() != null;
boolean clickable = focusable;
boolean longClickable = focusable;
n = a.getIndexCount();
for (int i = 0; i < n; i++) {
int attr = a.getIndex(i);
switch (attr) {
case com.android.internal.R.styleable.View_focusable:
focusable = a.getBoolean(attr, focusable);
break;
case com.android.internal.R.styleable.View_clickable:
clickable = a.getBoolean(attr, clickable);
break;
case com.android.internal.R.styleable.View_longClickable:
longClickable = a.getBoolean(attr, longClickable);
break;
}
}
从代码中可以看到clickable的缺省值变成了focuseable的值，TextView在调用构造器创造新的对象的时候会默认调用父类的构造器，也就是说经过setOnClickListener()处理过的clickable值是变成了true，可是在TextView的构造器中又被赋予了新的值。这也可以解释说为什么TextView控件在xml文件中用了onClick属性后clickable的值依旧是false，而在代码中调用setOnClickListener（）方法后，clickable值就变成了true，那是因为在代码中调用的setOnClickListener（）方法是在TextView对象被建立之后。