juce中的BailOutChecker

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);

    }

}