界面库中值得注意的一点就是对象响应事件的时候自身被删除了，那么后续的访问自然就会出问题，所以需要在响应事件之后先添加引用，相关处理之后再查看自身是否已经被删除，如果已经被删除那么就直接退出。juce中通过BailOutChecker来进行这处检查，内部实现很简单也就是通过弱引用来进行，关于弱引用请看上一篇文章
//==============================================================================
/** A class to keep an eye on a component and check for it being deleted.
This is designed for use with the ListenerList::callChecked() methods, to allow
the list iterator to stop cleanly if the component is deleted by a listener callback
while the list is still being iterated.
*/
class JUCE_API BailOutChecker
{
public:
/** Creates a checker that watches one component. */
BailOutChecker (Component* component);
/** Returns true if either of the two components have been deleted since this object was created. */
bool shouldBailOut() const noexcept;
private:
const WeakReference<Component> safePointer;
JUCE_DECLARE_NON_COPYABLE (BailOutChecker)
};
红色部份标识了进行检查的部份：
void Component::internalMouseWheel (MouseInputSource source, Point<float> relativePos,
Time time, const MouseWheelDetails& wheel)
{
Desktop& desktop = Desktop::getInstance();
BailOutChecker checker (this);
const MouseEvent me (source, relativePos, source.getCurrentModifiers(), MouseInputSource::invalidPressure,
this, this, time, relativePos, time, 0, false);
if (isCurrentlyBlockedByAnotherModalComponent())
{
// allow blocked mouse-events to go to global listeners..
desktop.mouseListeners.callChecked (checker, &MouseListener::mouseWheelMove, me, wheel);
}
else
{
mouseWheelMove (me, wheel);
if (checker.shouldBailOut())
return;
//历遍的过程中同样需要检查
desktop.mouseListeners.callChecked (checker, &MouseListener::mouseWheelMove, me, wheel);
if (! checker.shouldBailOut())
MouseListenerList::sendWheelEvent (*this, checker, me, wheel);
}
}
原文：http://www.cnblogs.com/csxy/p/5472362.html