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);
    }
}