Implement prefix based locator searching

Filters can indicate their preferred search prefix. Searches which
begin with this character will be restricted to the single matching
filter.

E.g. entering 'l buffer' will searching only layers containing 'buffer'

Other prefixes are:
- . search actions
- pl search print layouts
- a search algorithms

Plugins are restricted to a minimum 3 character prefix. We do this
to avoid plugins 'stealing' desirable prefixes, and instead we
want to reserve them for future core filters.

python/gui/locator/qgslocatorfilter.sip

@@ -101,6 +101,20 @@ class QgsLocatorFilter : QObject
  :rtype: Priority
 %End
 
+    virtual QString prefix() const;
+%Docstring
+ Returns the search prefix character(s) for this filter. Prefix a search
+ with these characters will restrict the locator search to only include
+ results from this filter.
+.. note::
+
+   Plugins are not permitted to utilise prefixes with < 3 characters,
+ as these are reserved for core QGIS functions. If a plugin registers
+ a filter with a prefix shorter than 3 characters then the prefix will
+ be ignored.
+ :rtype: str
+%End
+
     virtual void fetchResults( const QString &string, const QgsLocatorContext &context, QgsFeedback *feedback ) = 0;
 %Docstring
  Retrieves the filter results for a specified search ``string``. The ``context``

python/plugins/processing/gui/AlgorithmLocatorFilter.py

@@ -48,6 +48,9 @@ def displayName(self):
     def priority(self):
         return QgsLocatorFilter.Low
 
+    def prefix(self):
+        return 'a'
+
     def fetchResults(self,string,context,feedback):
         for a in QgsApplication.processingRegistry().algorithms():
             if feedback.isCanceled():

src/app/locator/qgsinbuiltlocatorfilters.cpp

@@ -141,7 +141,7 @@ void QgsActionLocatorFilter::searchActions( const QString &string, QWidget *pare
       continue;
     }
 
-    if ( !action->isEnabled() || !action->isVisible() )
+    if ( !action->isEnabled() || !action->isVisible() || action->text().isEmpty() )
       continue;
     if ( found.contains( action ) )
       continue;

src/app/locator/qgsinbuiltlocatorfilters.h

@@ -31,6 +31,7 @@ class QgsLayerTreeLocatorFilter : public QgsLocatorFilter
     virtual QString name() const override { return QStringLiteral( "layertree" ); }
     virtual QString displayName() const override { return tr( "Project layers" ); }
     virtual Priority priority() const override { return Highest; }
+    QString prefix() const override { return QStringLiteral( "l" ); }
 
     void fetchResults( const QString &string, const QgsLocatorContext &context, QgsFeedback *feedback ) override;
     void triggerResult( const QgsLocatorResult &result ) override;
@@ -47,6 +48,7 @@ class QgsLayoutLocatorFilter : public QgsLocatorFilter
     virtual QString name() const override { return QStringLiteral( "layouts" ); }
     virtual QString displayName() const override { return tr( "Project layouts" ); }
     virtual Priority priority() const override { return Highest; }
+    QString prefix() const override { return QStringLiteral( "pl" ); }
 
     void fetchResults( const QString &string, const QgsLocatorContext &context, QgsFeedback *feedback ) override;
     void triggerResult( const QgsLocatorResult &result ) override;
@@ -63,6 +65,7 @@ class QgsActionLocatorFilter : public QgsLocatorFilter
     virtual QString name() const override { return QStringLiteral( "actions" ); }
     virtual QString displayName() const override { return tr( "Actions" ); }
     virtual Priority priority() const override { return Lowest; }
+    QString prefix() const override { return QStringLiteral( "." ); }
 
     void fetchResults( const QString &string, const QgsLocatorContext &context, QgsFeedback *feedback ) override;
     void triggerResult( const QgsLocatorResult &result ) override;

src/gui/locator/qgslocator.cpp

@@ -37,6 +37,9 @@ void QgsLocator::deregisterFilter( QgsLocatorFilter *filter )
 {
   cancelRunningQuery();
   mFilters.removeAll( filter );
+  QString key = mPrefixedFilters.key( filter );
+  if ( !key.isEmpty() )
+    mPrefixedFilters.remove( key );
   delete filter;
 }
 
@@ -50,6 +53,20 @@ void QgsLocator::registerFilter( QgsLocatorFilter *filter )
   mFilters.append( filter );
   filter->setParent( this );
   connect( filter, &QgsLocatorFilter::resultFetched, this, &QgsLocator::filterSentResult, Qt::QueuedConnection );
+
+  if ( !filter->prefix().isEmpty() )
+  {
+    if ( filter->name() == QStringLiteral( "actions" ) || filter->name() == QStringLiteral( "processing_alg" )
+         || filter->name() == QStringLiteral( "layertree" ) || filter->name() == QStringLiteral( "layouts" ) )
+    {
+      //inbuilt filter, no prefix check
+      mPrefixedFilters.insert( filter->prefix(), filter );
+    }
+    else if ( filter->prefix().length() >= 3 )
+    {
+      mPrefixedFilters.insert( filter->prefix(), filter );
+    }
+  }
 }
 
 void QgsLocator::fetchResults( const QString &string, const QgsLocatorContext &context, QgsFeedback *feedback )
@@ -72,13 +89,26 @@ void QgsLocator::fetchResults( const QString &string, const QgsLocatorContext &c
   }
   mFeedback = feedback;
 
-  auto gatherFilterResults = [string, context, feedback]( QgsLocatorFilter * filter )
+  mActiveFilters = mFilters;
+  QString searchString = string;
+  if ( searchString.indexOf( ' ' ) > 0 )
+  {
+    QString prefix = searchString.left( searchString.indexOf( ' ' ) );
+    if ( mPrefixedFilters.contains( prefix ) )
+    {
+      mActiveFilters.clear();
+      mActiveFilters << mPrefixedFilters.value( prefix );
+      searchString = searchString.mid( prefix.length() + 1 );
+    }
+  }
+
+  auto gatherFilterResults = [searchString, context, feedback]( QgsLocatorFilter * filter )
   {
     if ( !feedback->isCanceled() )
-      filter->fetchResults( string, context, feedback );
+      filter->fetchResults( searchString, context, feedback );
   };
 
-  mFuture = QtConcurrent::map( mFilters, gatherFilterResults );
+  mFuture = QtConcurrent::map( mActiveFilters, gatherFilterResults );
   mFutureWatcher.setFuture( mFuture );
 }
 

src/gui/locator/qgslocator.h

@@ -147,6 +147,8 @@ class GUI_EXPORT QgsLocator : public QObject
     std::unique_ptr< QgsFeedback > mOwnedFeedback;
 
     QList< QgsLocatorFilter * > mFilters;
+    QList< QgsLocatorFilter * > mActiveFilters;
+    QMap< QString, QgsLocatorFilter *> mPrefixedFilters;
     QFuture< void > mFuture;
     QFutureWatcher< void > mFutureWatcher;
 

src/gui/locator/qgslocatorfilter.h

@@ -119,6 +119,17 @@ class GUI_EXPORT QgsLocatorFilter : public QObject
      */
     virtual Priority priority() const { return Medium; }
 
+    /**
+     * Returns the search prefix character(s) for this filter. Prefix a search
+     * with these characters will restrict the locator search to only include
+     * results from this filter.
+     * \note Plugins are not permitted to utilise prefixes with < 3 characters,
+     * as these are reserved for core QGIS functions. If a plugin registers
+     * a filter with a prefix shorter than 3 characters then the prefix will
+     * be ignored.
+     */
+    virtual QString prefix() const { return QString(); }
+
     /**
      * Retrieves the filter results for a specified search \a string. The \a context
      * argument encapsulates the context relating to the search (such as a map