Preface
I have a simple app with a viewmodel, a custom UI control, and a TextView. Databinding is setup like this:
LiveData -> control.value -> control.preValue -> TextView
When LiveData is changed, databinding notifies control.value of the new value. control.value setter has a line which also gives the new value to control.preValue. Both properties have calls to their respective databinding listeners to notify databinding that the values have changed and that the UI should be updated. The text value of TextView is bound to control.preValue, so when the listener is notified, the TextView is updated.
This works well at runtime, however there is a problem at initialization.
The Problem
When the UI is first constructed, the LiveData value is not correctly propagated to the TextView. This is because the listeners have not yet been created by the android databinding library, so when control.preValue is set by control.value's setter, the listener is still null.
Diving deeper into executeBindings we can see the cause of the problem.
executeBindings is a function which is part of the *BindingImpl file automatically generated by the databinding library based on the Binding Adapters I have defined. It is responsible for initializing databinding, e.g. creating listeners, registering livedatas, and setting initial values to the UI.
executeBindings starts like this. It initializes variables for all the databound values.
#Override
protected void executeBindings() {
long dirtyFlags = 0;
synchronized(this) {
dirtyFlags = mDirtyFlags;
mDirtyFlags = 0;
}
java.lang.Integer viewmodelBpmGetValue = null;
androidx.lifecycle.MutableLiveData<java.lang.Integer> viewmodelBpm = null;
int bpmPickerPreValue = 0;
androidx.lifecycle.MutableLiveData<java.lang.Boolean> viewmodelPlaying = null;
java.lang.String integerToStringBpmPickerPreValue = null;
int androidxDatabindingViewDataBindingSafeUnboxViewmodelBpmGetValue = 0;
com.okos.metronome.MetViewModel viewmodel = mViewmodel;
Next, it gets the value of control.preValue property and stores it in the earlier created variable. This is already the core of the problem. At this point control.preValue is still at the default value that is defined in the control's definition class, not the LiveData value which will be assigned to it a bit later.
if ((dirtyFlags & 0x18L) != 0) {
// read bpmPicker.preValue
bpmPickerPreValue = bpmPicker.getPreValue();
// read Integer.toString(bpmPicker.preValue)
integerToStringBpmPickerPreValue = java.lang.Integer.toString(bpmPickerPreValue);
}
Next we get the LiveData value from the viewmodel and register it with databinding
if ((dirtyFlags & 0x15L) != 0) {
if (viewmodel != null) {
// read viewmodel.bpm
viewmodelBpm = viewmodel.getBpm();
}
updateLiveDataRegistration(0, viewmodelBpm);
if (viewmodelBpm != null) {
// read viewmodel.bpm.getValue()
viewmodelBpmGetValue = viewmodelBpm.getValue();
}
// read androidx.databinding.ViewDataBinding.safeUnbox(viewmodel.bpm.getValue())
androidxDatabindingViewDataBindingSafeUnboxViewmodelBpmGetValue = androidx.databinding.ViewDataBinding.safeUnbox(viewmodelBpmGetValue);
}
Here it sets control.value to the value of the LiveData in the first if block. This line will trigger the control.value setter, which will set control.preValue, and those setters will both try to call their respective onChange listeners but they will be null because executeBindings hasn't created them yet. They are created in the 2nd if block.
if ((dirtyFlags & 0x15L) != 0) {
// api target 1
this.bpmPicker.setValue(androidxDatabindingViewDataBindingSafeUnboxViewmodelBpmGetValue);
}
if ((dirtyFlags & 0x10L) != 0) {
// api target 1
com.okos.metronome.view.DialPickerBindingAdapter.setPreValueListener(this.bpmPicker, (com.okos.metronome.view.PrePickerBase.OnValueChangeListener)null, bpmPickerpreValueAttrChanged);
com.okos.metronome.view.DialPickerBindingAdapter.setValueListener(this.bpmPicker, (com.okos.metronome.view.PrePickerBase.OnValueChangeListener)null, bpmPickervalueAttrChanged);
}
Finally, the value of the TextView is set, but it is set to the original value of preValue which we cached in a variable in the very first if block. **Not the new value which has been updated to preValue from the LiveData since then.
if ((dirtyFlags & 0x18L) != 0) {
// api target 1
androidx.databinding.adapters.TextViewBindingAdapter.setText(this.tvBpmDisplay, integerToStringBpmPickerPreValue);
}
This seems like an oversight in the databinding library, and I wonder if anyone has any ideas of getting around this? The fix seems pretty simple to just move the first if block in executeBindings down so integerToStringBpmPickerPreValue is set after the value has been set from LiveData, but because executeBindings is automatically generated, I can't do that. There are some ways of changing the order of execution in executeBindings, like which order the bindings are defined in the xaml, but none of that affects the parts I want to change.
Related
I am building simple application that will track a certain tabletop game of 2 players.
I have a view called MatchView
class MatchView : View() {
// data
private var currentRound = SimpleIntegerProperty(0)
private var currentTurn = SimpleIntegerProperty(0)
override val root = borderpane {
center = label(currentRound.stringBinding{ "Round %d".format(it) })
// other layout-related stuff
subscribe<EndTurnEvent> {
endPlayerTurn()
}
}
private fun endPlayerTurn() {
// increment turn
currentTurn.plus(1)
currentRound.plus(currentTurn.value % 2)
}
}
that is subscribed to EndTurnEvent - event emitted by one of the fragments used by view.
The called method is supposed to increment value of currentTurn and if needed currentRound (round increments after second player ends their turn)
However neither the value of currentRound nor the one of currentTurn are getting increased when i call .plus() method.
I have tried editting values differently :
private fun endPlayerTurn() {
// increment turn
currentTurn.value = currentTurn.value + 1
currentRound.value = currentTurn.value % 2
}
But this throws me java.lang.IllegalStateException: Not on FX application thread
I am aware that putting properties into views is anti-pattern, but since I just want to keep track of 2 properties, I thought I could put them directly into View
Platform.runLater(() -> {
// Update GUI from another Thread here
});
I have a setup with a main model (QStandardModel), a proxy model which changes the output of the DisplayRole, and a separate tableview displaying each model. Inside the main model data is a user role that stores a pointer to another QObject which is used by the proxy model to get the desired display value.
I'm running into problems when the object pointed to by that variable is deleted. I am handling deletion in the main model via the destroyed(QObject*) signal. Inside the slot, I search through the model looking for any items that are pointing to the object and delete the reference.
That part works fine on its own but I also have connected to the onDataChanged(...) signal of the proxy model, where I call resizeColumnsToContents() on the proxy model. This then calls the proxy's data() function. Here I check to see if the item has a pointer and, if it does, get some information from the object for display.
The result of all this becomes:
Object about to be deleted triggers destroyed(...) signal
Main model looks for any items using the deleted object and calls setData to remove the reference
Tableview catches onDataChanged signal for the proxy model and resizes columns
Proxy model's data(...) is called. It checks if the item in the main model has the object pointer and, if so, displays a value from the object. If not, it displays something else.
The problem is, at step 4 the item from the main model apparently still hasn't been deleted; the pointer address is still stored. The object the pointer was referencing, though, has been deleted by this point resulting in a segfault.
How can I fix my setup to make sure the main model is finished deleting pointer references before the proxy model tries to update?
Also, here is pseudo-code for the relevant sections:
// elsewhere
Object *someObject = new QObject();
QModelIndex index = mainModel->index(0,0);
mainModel->setData(index, someObject, ObjectPtrRole);
// do stuff
delete someObject; // Qt is actually doing this, I'm not doing it explicitly
// MainModel
void MainModel::onObjectDestroyed(QObject *obj)
{
// iterating over all model items
// if item has pointer to obj
item->setData(QVariant::fromValue(NULL), ObjectPtrRole));
}
// receives onDataChanged signal
void onProxyModelDataChanged(...)
{
ui->tblProxyView->reseizeColumnsToContents();
}
void ProxyModel::data(const QModelIndex &index, int role) const
{
QModelIndex srcIndex = mapToSource(index);
if(role == Qt::DisplayRole)
{
QVariant v = sourceModel()->data(srcIndex, ObjectPtrRole);
Object *ptr = qvariant_cast<Object*>(v);
if(ptr != NULL)
return ptr->getDisplayData();
else
return sourceModel->data(srcIndex, role);
}
}
The problem is ptr is not NULL, but the referenced object is deleted, at the time ProxyModel::data(...) is called so I end up with a segfault.
To avoid dangling pointer dereferences with instances of QObject, you can do one of two things:
Use object->deleteLater - the object will be deleted once the control returns to the event loop. Such functionality is also known as autorelease pools.
Use a QPointer. It will set itself to null upon deletion of the object, so you can check it before use.
Within qt's item/view framework, I'm trying to save a QColorDialog as user data and then retrieve that dialog as the editor, as well as during paint, in a tableview.
In my class constructor I do
QStandardItem *item = new QStandardItem();
QColorDialog *colorDlg = new QColorDialog(QColor(0,0,255), this);
item->setData(QVariant::fromValue(colorDlg), ColorDialogRole);
mTableModel->setItem(0,2,item);
then, inside my delegate's paint function I have
void ReportFigureTableDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const
{
QVariant vColorDlg= index.data(ReportFigure::ColorDialogRole);
if(vColorDlg.isValid())
{
////////////////////////////////////////////////
// Program segfaults on the next line ... why?
////////////////////////////////////////////////
QColorDialog *colorDlg = qvariant_cast<QColorDialog*>(vColorDlg);
if(colorDlg != NULL)
{
painter->save();
QStyleOptionViewItem opt = option;
initStyleOption(&opt, index);
painter->fillRect(opt.rect, colorDlg->selectedColor());
painter->restore();
}
else
QStyledItemDelegate::paint(painter, option, index);
}
else
QStyledItemDelegate::paint(painter, option, index);
}
During runtime, the table shows up the first time (although with the wrong color ... different issue I assume). I double click to edit the cell and it brings up the dialog as expected. When I close, though, it segfaults on the indicated line. I don't understand why since I think I'm doing all the necessary checks.
You set the data on a QStandardItem object. Meanwhile you are retrieving the data on a QModelIndex object. Now why is the variant valid is a mystery. Maybe because ReportFigure::ColorDialogRole is equal to a build-in Qt role while it should be at least Qt::UserRole.
Anyway In the paint() method you can access the previously set item using
QStandardItem *item = mTableModel->itemFromIndex(index);
I am creating a plugin and i need to know the value of porosity of reservoir. If these properties exist somewhere it would be much easier if I could just access them.
So how can we take these value using "drop target button" ?
You must subscribe to the DropTarget.DragDrop event. The following callback method shows you how to get the object dropped on the DropTarget button.
void DropTarget_DragDrop(object sender, DragEventArgs e)
{
Property property = e.Data.GetData(typeof(object)) as Property;
if (property == null)
return;
// Do something with property, like show it in a
// PresentationBox or store it for use later.
}
In my title screen, i have a code saying that the first controller using A is the PlayerIndex.one.
Here is the code:
public override void HandleInput(InputState input)
{
for (int anyPlayer = 0; anyPlayer <4; anyPlayer++)
{
if (GamePad.GetState((PlayerIndex)anyPlayer).Buttons.A == ButtonState.Pressed)
{
FirstPlayer = (PlayerIndex)anyPlayer;
this.ExitScreen();
AddScreen(new Background());
}
}
}
My question is: How can i use the "FirstPlayer" in other classes? (without this, there is no interest in this code)
I tried the Get Set thing but i can't make it work. Does i need to put my code in another class? Do you use other code to make this?
Thanks.
You can make a static variable say : SelectedPlayer,
and assign first player to it!
then you can call the first player through this class,
for example
class GameManager
{
public static PlayerIndex SelectedPlayer{get;set;}
..
..
..
}
and right after the loop in your code, you can say:
GameManager.SelectedPlayer = FirstPlayer;
I hope this helps, if your code cold be clearer that would be easier to help :)
Ok, so to do this properly you're going to have to redesign a little.
First off, you should be checking for a new gamepad input (i.e. you should be exiting the screen only when 'A' has been newly pressed). To do this you should be storing previous and current gamepad states:
private GamePadState currentGamePadState;
private GamePadState lastGamePadState;
// in your constructor
currentGamePadState = new GamePadState();
lastGamePadState = new GamePadState();
// in your update
lastGamePadState = currentGamePadState;
currentGamePadState = GamePad.GetState(PlayerIndex.One);
Really what you need to do is modify your class that deals with input. The basic functionality from your HandleInput function should be moved into your input class. Input should have a collection of functions that test for new/current input. For example, for the case you posted:
public Bool IsNewButtonPress(Buttons buton)
{
return (currentGamePadState.IsButtonDown(button) && lastGamePadState.IsButtonUp(button));
}
Then you can write:
public override void HandleInput(InputState input)
{
if (input.IsNewButtonPress(Buttons.A)
{
this.ExitScreen();
AddScreen(new Background());
}
}
Note: this will only work for one controller. To extend the implementation, you'll need to do something like this:
private GamePadState[] currentGamePadStates;
private GamePadState[] lastGamePadStates;
// in your constructor
currentGamePadStates = new GamePadState[4];
currentGamePadStates[0] = new GamePadState(PlayerIndex.One);
currentGamePadStates[1] = new GamePadController(PlayerIndex.Two);
// etc.
lastGamePadStates[0] = new GamePadState(PlayerIndex.One);
// etc.
// in your update
foreach (GamePadState s in currentGamePadStates)
{
// update all of this as before...
}
// etc.
Now, you want to test every controller for input, so you'll need to generalise by writing a function that returns a Bool after checking each GamePadState in the arrays for a button press.
Check out the MSDN Game State Management Sample for a well developed implementation. I can't remember if it supports multiple controllers, but the structure is clear and can easily be adapted if not.