Allow generator effects to use preview without requiring a selection to be made.

Documentation: This does not affect any currently shipped effects. This will need documenting on the wiki for writing Nyquist plug-ins.

src/effects/Effect.cpp

@@ -1409,7 +1409,8 @@ void Effect::Preview(bool dryOnly)
    if (t1 > mT1)
       t1 = mT1;
 
-   if (t1 <= t0)
+   // Generators can run without a selection.
+   if (!GeneratorPreview() && (t1 <= t0))
       return;
 
    bool success = ::MixAndRender(mTracks, mFactory, rate, floatSample, t0, t1,

src/effects/Effect.h

@@ -181,6 +181,12 @@ class AUDACITY_DLL_API Effect : public EffectHostInterface
       return (mFlags & BUILTIN_EFFECT) != 0;
    }
 
+   // Preview normally requires a selection, but INSERT_EFFECTs do not.
+   // Return true to override the need for a selection.
+   virtual bool GeneratorPreview(){
+      return false;
+   }
+
    // Called to set or retrieve parameter values.  Return true if successful.
    virtual bool TransferParameters( Shuttle & WXUNUSED(shuttle) ) {
       return true;

src/effects/nyquist/Nyquist.cpp

@@ -508,6 +508,12 @@ bool EffectNyquist::TransferParameters( Shuttle & shuttle )
    return true;
 }
 
+bool EffectNyquist::GeneratorPreview()
+{
+   // Enable Nyquist generator plug-ins to create preview without a selection
+   return (mEnablePreview && (GetEffectFlags() & INSERT_EFFECT));
+}
+
 bool EffectNyquist::PromptUser()
 {
    while (mInteractive) {

src/effects/nyquist/Nyquist.h

@@ -112,6 +112,7 @@ class AUDACITY_DLL_API EffectNyquist:public Effect
       return mAction;
    }
 
+   virtual bool GeneratorPreview();
    virtual bool PromptUser(); 
 
    virtual bool Process();