support dynamic simple if-blocks

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

@@ -63,14 +63,13 @@ export default function visitElement ( generator, block, state, node ) {
 			});
 
 		if ( intro || outro ) addTransitions( generator, block, childState, node, intro, outro );
+	}
 
-		if ( !outro && !state.parentNode ) {
+	if ( !state.parentNode ) {
-			// TODO this probably doesn't belong here. We eventually need to consider
+		// TODO we eventually need to consider what happens to elements
-			// what happens to elements that belong to the same outgroup as an
+		// that belong to the same outgroup as an outroing element...
-			// outroing element...
 		block.builders.detach.addLine( `${generator.helper( 'detachNode' )}( ${name} );` );
 	}
-	}
 
 	if ( node.name !== 'select' ) {
 		// <select> value attributes are an annoying special case — it must be handled

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

@@ -22,7 +22,6 @@ export default function addTransitions ( generator, block, state, node, intro, o
 
 		block.builders.outro.addBlock( deindent`
 			${name}.run( ${name}.t, 0, function () {
-				${generator.helper( 'detachNode' )}( ${state.name} );
 				${block.component}.fire( 'outro.end', { node: ${state.name} });
 				if ( --${block.alias( 'outros' )} === 0 ) ${block.alias( 'outrocallback' )}();
 				${name} = null;
@@ -60,10 +59,11 @@ export default function addTransitions ( generator, block, state, node, intro, o
 
 			const fn = `${generator.alias( 'template' )}.transitions.${outro.name}`;
 
+			// TODO hide elements that have outro'd (unless they belong to a still-outroing
+			// group) prior to their removal from the DOM
 			block.builders.outro.addBlock( deindent`
 				${outroName} = ${wrapTransition}( ${state.name}, ${fn}, ${snippet}, false, null );
 				${outroName}.run( 1, 0, function () {
-					${generator.helper( 'detachNode' )}( ${state.name} );
 					${block.component}.fire( 'outro.end', { node: ${state.name} });
 					if ( --${block.alias( 'outros' )} === 0 ) ${block.alias( 'outrocallback' )}();
 				});

src/generators/dom/visitors/IfBlock.js

@@ -86,37 +86,26 @@ function simple ( generator, block, state, node, branch, dynamic, { name, anchor
 
 	const parentNode = state.parentNode || `${anchor}.parentNode`;
 
-	const exit = branch.hasOutroTransitions ?
+	const enter = dynamic ?
-		deindent`
+		( branch.hasIntroTransitions ?
-			${name}.outro( function () {
-				${name}.destroy( true );
-				${name} = null;
-			});
-		` :
 			deindent`
-			${name}.destroy( true );
+				if ( ${name} ) {
-			${name} = null;
+					${name}.update( changed, ${params} );
-		`;
+				} else {
-
+					${name} = ${branch.block}( ${params}, ${block.component} );
-	if ( dynamic ) {
-		if ( branch.hasIntroTransitions ) {
-			throw new Error( 'TODO simple dynamic if-block with intro transitions' );
 				}
 
-		block.builders.update.addBlock( deindent`
+				${name}.intro( ${parentNode}, ${anchor} );
-			if ( ${branch.condition} ) {
+			` :
+			deindent`
 				if ( ${name} ) {
 					${name}.update( changed, ${params} );
 				} else {
 					${name} = ${branch.block}( ${params}, ${block.component} );
 					${name}.mount( ${parentNode}, ${anchor} );
 				}
-			} else if ( ${name} ) {
+			` ) :
-				${exit}
+		( branch.hasIntroTransitions ?
-			}
-		` );
-	} else {
-		const enter = branch.hasIntroTransitions ?
 			deindent`
 				if ( !${name} ) ${name} = ${branch.block}( ${params}, ${block.component} );
 				${name}.intro( ${parentNode}, ${anchor} );
@@ -126,6 +115,20 @@ function simple ( generator, block, state, node, branch, dynamic, { name, anchor
 					${name} = ${branch.block}( ${params}, ${block.component} );
 					${name}.mount( ${parentNode}, ${anchor} );
 				}
+			` );
+
+	// no `update()` here — we don't want to update outroing nodes,
+	// as that will typically result in glitching
+	const exit = branch.hasOutroTransitions ?
+		deindent`
+			${name}.outro( function () {
+				${name}.destroy( true );
+				${name} = null;
+			});
+		` :
+		deindent`
+			${name}.destroy( true );
+			${name} = null;
 		`;
 
 	block.builders.update.addBlock( deindent`
@@ -135,7 +138,6 @@ function simple ( generator, block, state, node, branch, dynamic, { name, anchor
 			${exit}
 		}
 	` );
-	}
 }
 
 function compound ( generator, block, state, node, branches, dynamic, { name, anchor, params } ) {

test/runtime/samples/transition-js-dynamic-if-block-bidi/_config.js

@@ -0,0 +1,47 @@
+export default {
+	data: {
+		name: 'world'
+	},
+
+	test ( assert, component, target, window ) {
+		global.count = 0;
+		let now = 0;
+		let callback;
+
+		window.performance = { now: () => now };
+		global.requestAnimationFrame = cb => callback = cb;
+
+		component.set({ visible: true });
+		assert.equal( global.count, 1 );
+		const div = target.querySelector( 'div' );
+		assert.equal( div.foo, 0 );
+
+		now = 300;
+		callback();
+		component.set({ name: 'everybody' });
+		assert.equal( div.foo, 0.75 );
+		assert.htmlEqual( div.innerHTML, 'hello everybody!' );
+
+		component.set({ visible: false, name: 'again' });
+		assert.htmlEqual( div.innerHTML, 'hello everybody!' );
+
+		now = 500;
+		callback();
+		assert.equal( div.foo, 0.25 );
+
+		component.set({ visible: true });
+		now = 700;
+		callback();
+		assert.equal( div.foo, 0.75 );
+		assert.htmlEqual( div.innerHTML, 'hello again!' );
+
+		now = 800;
+		callback();
+		assert.equal( div.foo, 1 );
+
+		now = 900;
+		callback();
+
+		component.destroy();
+	}
+};

test/runtime/samples/transition-js-dynamic-if-block-bidi/main.html

@@ -0,0 +1,19 @@
+{{#if visible}}
+	<div transition:foo>hello {{name}}!</div>
+{{/if}}
+
+<script>
+	export default {
+		transitions: {
+			foo: function ( node, params ) {
+				global.count += 1;
+				return {
+					duration: 400,
+					tick: t => {
+						node.foo = t;
+					}
+				};
+			}
+		}
+	};
+</script>