Add workaround for loading forms with custom widgets from PyQt4

	More information: qt-project.org/forums/viewthread/27098/

Author: NathanW2

python/gui/qgisinterface.sip

@@ -427,6 +427,8 @@ class QgisInterface : QObject
     virtual bool openFeatureForm( QgsVectorLayer *l, QgsFeature &f, bool updateFeatureOnly = false ) = 0;
 
     virtual QDialog* getFeatureForm( QgsVectorLayer *l, QgsFeature &f ) = 0;
+
+    virtual void preloadForm(QString uifile) = 0;
     /** Return vector layers in edit mode
      * @param modified whether to return only layers that have been modified
      * @returns list of layers in legend order, or empty list

src/app/qgisappinterface.cpp

@@ -21,6 +21,9 @@
 #include <QMenu>
 #include <QDialog>
 #include <QAbstractButton>
+#include <QSignalMapper>
+#include <QTimer>
+#include <QUiLoader>
 
 #include "qgisappinterface.h"
 #include "qgisappstylesheet.h"
@@ -541,6 +544,37 @@ bool QgisAppInterface::openFeatureForm( QgsVectorLayer *vlayer, QgsFeature &f, b
   }
 }
 
+void QgisAppInterface::preloadForm( QString uifile )
+{
+    QSignalMapper* signalMapper = new QSignalMapper ( this );
+    mTimer = new QTimer(this);
+
+    connect( mTimer ,SIGNAL( timeout() ), signalMapper,SLOT( map() ) );
+    connect( signalMapper, SIGNAL( mapped(QString) ), mTimer, SLOT( stop() ) );
+    connect( signalMapper, SIGNAL( mapped(QString) ), this, SLOT( cacheloadForm( QString ) ) );
+
+    signalMapper->setMapping( mTimer, uifile );
+
+    mTimer->start(0);
+}
+
+void QgisAppInterface::cacheloadForm( QString uifile )
+{
+    QUiLoader loader;
+
+    QFile file( uifile );
+
+    if ( file.open( QFile::ReadOnly ) )
+    {
+      QUiLoader loader;
+
+      QFileInfo fi( uifile );
+      loader.setWorkingDirectory( fi.dir() );
+      QWidget *myWidget = loader.load( &file );
+      file.close();
+    }
+}
+
 QDialog* QgisAppInterface::getFeatureForm( QgsVectorLayer *l, QgsFeature &f )
 {
   QgsDistanceArea myDa;

src/app/qgisappinterface.h

@@ -382,6 +382,19 @@ class QgisAppInterface : public QgisInterface
 
     virtual QDialog* getFeatureForm( QgsVectorLayer *l, QgsFeature &f );
 
+    /** This method is only needed when using a UI form with a custom widget plugin and calling
+     * openFeatureForm or getFeatureForm from Python (PyQt4) and you havn't used the info tool first.
+     * Python will crash bringing QGIS wtih it
+     * if the custom form is not loaded from a C++ method call.
+     *
+     * This method uses a QTimer to call QUiLoader in order to load the form via C++
+     * you only need to call this once after that you can call openFeatureForm/getFeatureForm
+     * like normal
+     *
+     * More information here: http://qt-project.org/forums/viewthread/27098/
+     */
+    virtual void preloadForm(QString uifile);
+
     /** Return vector layers in edit mode
      * @param modified whether to return only layers that have been modified
      * @returns list of layers in legend order, or empty list
@@ -395,6 +408,10 @@ class QgisAppInterface : public QgisInterface
   signals:
     void currentThemeChanged( QString );
 
+  private slots:
+
+    void cacheloadForm( QString uifile );
+
   private:
 
     /// QgisInterface aren't copied
@@ -406,6 +423,8 @@ class QgisAppInterface : public QgisInterface
     //! Pointer to the QgisApp object
     QgisApp *qgis;
 
+    QTimer *mTimer;
+
     //! Pointer to the LegendInterface object
     QgsAppLegendInterface legendIface;
 };

src/gui/qgisinterface.h

@@ -477,6 +477,8 @@ class GUI_EXPORT QgisInterface : public QObject
 
     virtual QDialog* getFeatureForm( QgsVectorLayer *l, QgsFeature &f ) = 0;
 
+    virtual void preloadForm( QString uifile ) = 0;
+
     /** Return vector layers in edit mode
      * @param modified whether to return only layers that have been modified
      * @returns list of layers in legend order, or empty list