feat(Playback): Can set number of precount measures (#20) (playbackManager.PreCountMeasures)

e.g.:
osmd.renderingManager.PlaybackManager.PreCountMeasures = 1;
osmd.renderingManager.PlaybackManager.DoPreCount = true;

fixes #20

demo/index.js

@@ -937,7 +937,8 @@ import { TransposeCalculator } from '../src/Plugins/Transpose/TransposeCalculato
         var timingSource = new LinearTimingSource();
         var playbackManager = new PlaybackManager(timingSource, undefined, new BasicAudioPlayer(), undefined);
         playbackManager.DoPlayback = true;
-        playbackManager.DoPreCount = false;
+        playbackManager.DoPreCount = false; 
+        playbackManager.PreCountMeasures = 1; // note that DoPreCount has to be true for a precount to happen
         var playbackControlPanel = new ControlPanel();
         playbackControlPanel.addListener(playbackManager);
         playbackControlPanel.addListener(playbackListener);

src/Playback/PlaybackManager.ts

@@ -142,6 +142,7 @@ export class PlaybackManager implements IPlaybackParametersListener {
         }
     }
 
+    public PreCountMeasures: number;
     public PreCountBeats: number;
 
     public get Metronome(): ISettableInstrument {
@@ -372,7 +373,7 @@ export class PlaybackManager implements IPlaybackParametersListener {
     public reset(): void {
         // lock(this.playbackThreadSyncObject) {
         //this.resetRequested = true;
-        this.doReset(this.DoPreCount);
+        this.doReset(this.DoPreCount, this.PreCountMeasures);
 
         if (this.musicPartManager === undefined) {
             return;
@@ -935,7 +936,7 @@ export class PlaybackManager implements IPlaybackParametersListener {
         }
     }
 
-    protected doReset(shallPrecount: boolean): void {
+    protected doReset(shallPrecount: boolean, preCountMeasures: number = 1): void {
         this.nextIteratorTimestamp = undefined;
         this.playNextMetronomeAt = undefined;
         this.closestNextTimestamp = undefined;
@@ -967,10 +968,12 @@ export class PlaybackManager implements IPlaybackParametersListener {
         let preCountDuration: Fraction = new Fraction();
         if (shallPrecount) {
             this.isPreCounting = true;
-            const rhythmDuration: Fraction = new Fraction(this.startRhythmBeats, this.startRhythmDenominator);
+            const rhythmDuration: Fraction = new Fraction(this.startRhythmBeats * preCountMeasures, this.startRhythmDenominator);
+            const sourceMeasureDuration: Fraction = this.musicPartManager.MusicSheet.SourceMeasures[this.currentMeasureIndex].Duration;
+            const measureDuration: Fraction = new Fraction(sourceMeasureDuration.Numerator * preCountMeasures, sourceMeasureDuration.Denominator);
             const duration: Fraction =
-                Fraction.plus(  this.musicPartManager.MusicSheet.SourceMeasures[this.currentMeasureIndex].AbsoluteTimestamp,
-                                this.musicPartManager.MusicSheet.SourceMeasures[this.currentMeasureIndex].Duration).Sub(this.currentIteratorSourceTimeStamp);
+                Fraction.plus(this.musicPartManager.MusicSheet.SourceMeasures[this.currentMeasureIndex].AbsoluteTimestamp,
+                    measureDuration).Sub(this.currentIteratorSourceTimeStamp);
 
             preCountDuration = rhythmDuration;