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Fix dangling child widgets from Screen's focus/drag #389

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Fix dangling child widgets from Screen's focus/drag #389

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85e405b
Remove dangling children from Screen
chpatton013 May 23, 2018
958f46a
Ensure a Widget removes itself from the Screen on destruction
Apr 3, 2019
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2 changes: 1 addition & 1 deletion include/nanogui/screen.h
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Original file line number Diff line number Diff line change
Expand Up @@ -179,7 +179,7 @@ class NANOGUI_EXPORT Screen : public Widget {

/* Internal helper functions */
void updateFocus(Widget *widget);
void disposeWindow(Window *window);
void disposeWidget(Widget *widget);
void centerWindow(Window *window);
void moveWindowToFront(Window *window);
void drawWidgets();
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2 changes: 1 addition & 1 deletion python/py_doc.h
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Expand Up @@ -2598,7 +2598,7 @@ static const char *__doc_nanogui_Screen_charCallbackEvent = R"doc()doc";

static const char *__doc_nanogui_Screen_cursorPosCallbackEvent = R"doc()doc";

static const char *__doc_nanogui_Screen_disposeWindow = R"doc()doc";
static const char *__doc_nanogui_Screen_disposeWidget = R"doc(Remove Screen's references to a widget and its children)doc";
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@svenevs svenevs Apr 5, 2019 •
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Please un-do changes to this file. If we want it documented, it must be done in include/nanogui/screen.h and then run make mkdoc (that will regenerate all python docs). Personally I think it's OK to leave it undocumented for now at least ;)

So to clarify: all you should need to do is run make mkdoc or ninja mkdoc, it's a special target added at the bottom of CMakeLists.txt. If you need help with that let me know :)


static const char *__doc_nanogui_Screen_drawAll = R"doc(Draw the Screen contents)doc";

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10 changes: 6 additions & 4 deletions src/screen.cpp
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Expand Up @@ -690,12 +690,14 @@ void Screen::updateFocus(Widget *widget) {
moveWindowToFront((Window *) window);
}

void Screen::disposeWindow(Window *window) {
if (std::find(mFocusPath.begin(), mFocusPath.end(), window) != mFocusPath.end())
void Screen::disposeWidget(Widget *widget) {
if (std::find(mFocusPath.begin(), mFocusPath.end(), widget) != mFocusPath.end())
mFocusPath.clear();
if (mDragWidget == window)

if (mDragWidget == widget) {
mDragWidget = nullptr;
removeChild(window);
mDragActive = false;
}
}

void Screen::centerWindow(Window *window) {
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5 changes: 5 additions & 0 deletions src/widget.cpp
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Expand Up @@ -30,6 +30,11 @@ Widget::Widget(Widget *parent)
}

Widget::~Widget() {
try {
screen()->disposeWidget(this);
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I think doing this kind of exception stuff in the destructor may not be a good decision. I think there are two good options:

  1. Make screen() able to return nullptr rather than throwing an std::runtime_error. That's kind of a significant change though.

  2. Less pervasive,just do it manually -- the Widget destructor is a special case, exceptions + destructors is dangerous territory. I think we could just do this:

    Widget::~Widget() {
    /* Similar to Widget::screen(), but does not throw:
     * 1. Destruction of orphan widgets should not throw.
     * 2. Special case: every Screen is a Widget.
     */
    Screen *screen = dynamic_cast<Screen *>(this);
    if (screen == nullptr) { // only dispose widgets, screen cannot dispose itself
        Widget *widget = this;
        while (true) {
            if (!widget) break;// 1: orphan widgets (no parent screen)
            screen = dynamic_cast<Screen *>(widget);
            if (screen) break;
            widget = widget->parent();
        }
        if (screen)
            screen->disposeWidget(this);
    }

    for (auto child : mChildren) {
        if (child)
            child->decRef();
    }
}

Basically, in the destructor we need to be extra careful. "Orphan" widgets (no screen in parent chain) are not exactly supported, but not explicitly forbidden. No orphan ever gets drawn until it has a screen in the parent chain, so all drawing code assumes that a screen() can be found. We just want to make sure that we don't crash on what you mentioned (when it's a Screen being destructed), but also in the rare (but possibly abusively used) orphan widget case.

}
catch (const std::runtime_error&) {}

for (auto child : mChildren) {
if (child)
child->decRef();
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2 changes: 1 addition & 1 deletion src/window.cpp
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Expand Up @@ -145,7 +145,7 @@ void Window::dispose() {
Widget *widget = this;
while (widget->parent())
widget = widget->parent();
((Screen *) widget)->disposeWindow(this);
((Screen *) widget)->removeChild(this);
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This does not work. Run Example 1 and use the Info / Warn / Ask dialog. That's the "real" test case for Window::dispose (gets called by the message diaolgs in src/messagedialog.cpp).

}

void Window::center() {
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