handle bidirectional transitions differently

package.json

@@ -21,6 +21,7 @@
     "build:shared": "rollup -c rollup/rollup.config.shared.js",
     "build:ssr": "rollup -c rollup/rollup.config.ssr.js",
     "dev": "rollup -c rollup/rollup.config.main.js -w",
+    "dev:shared": "rollup -c rollup/rollup.config.shared.js -w",
     "pretest": "npm run build",
     "prepublish": "npm run lint && npm run build"
   },

src/generators/dom/index.js

@@ -159,7 +159,7 @@ export default function dom ( parsed, source, options ) {
 		` );
 	}
 
-	if ( generator.hasComponents ) {
+	if ( generator.hasComponents || generator.hasIntroTransitions ) {
 		const statement = `this._flush();`;
 
 		builders.init.addBlock( statement );

src/generators/dom/visitors/Element/Element.js

@@ -62,7 +62,7 @@ export default function visitElement ( generator, block, state, node ) {
 				visitors[ attribute.type ]( generator, block, childState, node, attribute );
 			});
 
-		addTransitions( generator, block, childState, node, intro, outro );
+		if ( intro || outro ) addTransitions( generator, block, childState, node, intro, outro );
 
 		if ( !outro && !state.parentNode ) {
 			// TODO this probably doesn't belong here. We eventually need to consider

src/generators/dom/visitors/Element/addTransitions.js

@@ -1,19 +1,42 @@
 import deindent from '../../../../utils/deindent.js';
 
 export default function addTransitions ( generator, block, state, node, intro, outro ) {
-	const introName = intro && block.getUniqueName( `${state.name}_intro` );
+	const wrapTransition = generator.helper( 'wrapTransition' );
-	const outroName = outro && block.getUniqueName( `${state.name}_outro` );
 
-	const introSnippet = intro && intro.expression ? block.contextualise( intro.expression ).snippet : '{}';
+	if ( intro === outro ) {
+		const name = block.getUniqueName( `${state.name}_transition` );
+		const snippet = intro.expression ? block.contextualise( intro.expression ).snippet : '{}';
 
-	const outroSnippet = outro === intro ?
+		block.addVariable( name );
-		introSnippet :
-		outro && outro.expression ? block.contextualise( outro.expression ).snippet : '{}';
 
-	const wrapTransition = generator.helper( 'wrapTransition' );
+		const fn = `${generator.alias( 'template' )}.transitions.${intro.name}`;
+
+		block.builders.intro.addBlock( deindent`
+			${block.component}._renderHooks.push( function () {
+				if ( !${name} ) ${name} = ${wrapTransition}( ${state.name}, ${fn}, ${snippet}, true, null );
+				${name}.run( ${name}.t, 1, function () {
+					${block.component}.fire( 'intro.end', { node: ${state.name} });
+				});
+			});
+		` );
+
+		block.builders.outro.addBlock( deindent`
+			${name}.run( ${name}.t, 0, function () {
+				detachNode( ${state.name} );
+				${block.component}.fire( 'outro.end', { node: ${state.name} });
+				if ( --${block.alias( 'outros' )} === 0 ) ${block.alias( 'outrocallback' )}();
+				${name} = null;
+			});
+		` );
+	}
+
+	else {
+		const introName = intro && block.getUniqueName( `${state.name}_intro` );
+		const outroName = outro && block.getUniqueName( `${state.name}_outro` );
 
 		if ( intro ) {
 			block.addVariable( introName );
+			const snippet = intro.expression ? block.contextualise( intro.expression ).snippet : '{}';
 
 			const fn = `${generator.alias( 'template' )}.transitions.${intro.name}`; // TODO add built-in transitions?
 
@@ -23,16 +46,17 @@ export default function addTransitions ( generator, block, state, node, intro, o
 
 			block.builders.intro.addBlock( deindent`
 				${block.component}._renderHooks.push( function () {
-				${introName} = ${wrapTransition}( ${state.name}, ${fn}, ${introSnippet}, true, null, function () {
+					${introName} = ${wrapTransition}( ${state.name}, ${fn}, ${snippet}, true, null );
+					${introName}.run( 0, 1, function () {
 						${block.component}.fire( 'intro.end', { node: ${state.name} });
 					});
-				${generator.helper( 'transitionManager' )}.add( ${introName} );
 				});
 			` );
 		}
 
 		if ( outro ) {
 			block.addVariable( outroName );
+			const snippet = outro.expression ? block.contextualise( outro.expression ).snippet : '{}';
 
 			const fn = `${generator.alias( 'template' )}.transitions.${outro.name}`;
 
@@ -41,12 +65,13 @@ export default function addTransitions ( generator, block, state, node, intro, o
 			}
 
 			block.builders.outro.addBlock( deindent`
-			${outroName} = ${wrapTransition}( ${state.name}, ${fn}, ${outroSnippet}, false, null, function () {
+				${outroName} = ${wrapTransition}( ${state.name}, ${fn}, ${snippet}, false, null );
-				detachNode( div );
+				${outroName}.run( 1, 0, function () {
+					detachNode( ${state.name} );
 					${block.component}.fire( 'outro.end', { node: ${state.name} });
 					if ( --${block.alias( 'outros' )} === 0 ) ${block.alias( 'outrocallback' )}();
 				});
-			transitionManager.add( ${outroName} );
 			` );
 		}
 	}
+}

src/shared/transitions.js

@@ -4,12 +4,10 @@ export function linear ( t ) {
 	return t;
 }
 
-export function wrapTransition ( node, fn, params, intro, outgroup, callback ) {
+export function wrapTransition ( node, fn, params, intro, outgroup ) {
 	var obj = fn( node, params, intro );
 
-	var start = window.performance.now() + ( obj.delay || 0 );
 	var duration = obj.duration || 300;
-	var end = start + duration;
 	var ease = obj.easing || linear;
 
 	if ( obj.tick ) {
@@ -17,19 +15,40 @@ export function wrapTransition ( node, fn, params, intro, outgroup, callback ) {
 		if ( intro ) obj.tick( 0 );
 
 		return {
-			start: start,
+			start: null,
-			end: end,
+			end: null,
+			a: null,
+			d: null,
+			running: false,
+			t: intro ? 0 : 1,
+			callback: null,
 			update: function ( now ) {
-				const p = intro ? now - start : end - now;
+				const p = now - this.start;
-				obj.tick( ease( p / duration ) );
+				this.t = this.a + this.d * ease( p / this.duration );
+				obj.tick( this.t );
 			},
 			done: function () {
 				obj.tick( intro ? 1 : 0 );
-				callback();
+				this.callback();
+				this.running = false;
 			},
 			abort: function () {
 				if ( !intro ) obj.tick( 1 ); // reset styles for intro
-				this.aborted = true;
+				this.running = false;
+			},
+			run: function ( a, b, callback ) {
+				this.a = a;
+				this.d = b - a;
+				this.start = window.performance.now() + ( obj.delay || 0 );
+				this.duration = duration * Math.abs( b - a );
+				this.end = this.start + this.duration;
+
+				this.callback = callback;
+
+				if ( !this.running ) {
+					this.running = true;
+					transitionManager.add( this );
+				}
 			}
 		};
 	} else {
@@ -75,6 +94,9 @@ export function wrapTransition ( node, fn, params, intro, outgroup, callback ) {
 			abort: function () {
 				node.style.cssText = getComputedStyle( node ).cssText;
 				this.aborted = true;
+			},
+			run: function ( a, b, callback ) {
+				// TODO...
 			}
 		};
 	}
@@ -110,11 +132,17 @@ export var transitionManager = {
 		while ( i-- ) {
 			var transition = transitionManager.transitions[i];
 
-			if ( now < transition.end && !transition.aborted ) {
+			if ( transition.running ) {
-				if ( now > transition.start ) transition.update( now );
+				if ( now >= transition.end ) {
+					transition.done();
+				} else if ( now > transition.start ) {
+					transition.update( now );
+				}
+			}
+
+			if ( transition.running ) {
 				transitionManager.running = true;
 			} else {
-				if ( !transition.aborted ) transition.done();
 				transitionManager.transitions.splice( i, 1 );
 			}
 		}