from ..Qt import QtCore, QtWidgets

if not hasattr(QtCore, 'Signal'):
    QtCore.Signal = QtCore.pyqtSignal
import weakref


class CanvasManager(QtCore.QObject):
    SINGLETON = None
    
    sigCanvasListChanged = QtCore.Signal()
    
    def __init__(self):
        if CanvasManager.SINGLETON is not None:
            raise Exception("Can only create one canvas manager.")
        # It is important to save SINGLETON *after* the call to QObject.__init__, see #2838.
        QtCore.QObject.__init__(self)
        CanvasManager.SINGLETON = self
        self.canvases = weakref.WeakValueDictionary()

    @classmethod
    def instance(cls):
        return CanvasManager.SINGLETON
        
    def registerCanvas(self, canvas, name):
        n2 = name
        i = 0
        while n2 in self.canvases:
            n2 = "%s_%03d" % (name, i)
            i += 1
        self.canvases[n2] = canvas
        self.sigCanvasListChanged.emit()
        return n2
        
    def unregisterCanvas(self, name):
        c = self.canvases[name]
        del self.canvases[name]
        self.sigCanvasListChanged.emit()
        
    def listCanvases(self):
        return list(self.canvases.keys())
        
    def getCanvas(self, name):
        return self.canvases[name]
        
    
manager = CanvasManager()


class CanvasCombo(QtWidgets.QComboBox):
    def __init__(self, parent=None):
        QtWidgets.QComboBox.__init__(self, parent)
        man = CanvasManager.instance()
        man.sigCanvasListChanged.connect(self.updateCanvasList)
        self.hostName = None
        self.updateCanvasList()
        
    def updateCanvasList(self):
        canvases = CanvasManager.instance().listCanvases()
        canvases.insert(0, "")
        if self.hostName in canvases:
            canvases.remove(self.hostName)
            
        sel = self.currentText()
        if sel in canvases:
            self.blockSignals(True)  ## change does not affect current selection; block signals during update
        self.clear()
        for i in canvases:
            self.addItem(i)
            if i == sel:
                self.setCurrentIndex(self.count())
            
        self.blockSignals(False)
        
    def setHostName(self, name):
        self.hostName = name
        self.updateCanvasList()