Fix issue with conditional expressions

For conditionals we failed to setup the observer for both branches
as required. This lead to us only observing either the true or false
branch but not both.

Fixes googlearchive#40

src/polymer-expressions.js

@@ -297,7 +297,17 @@
       consequent = getFn(consequent);
       alternate = getFn(alternate);
 
+      // We need to ensure we set up observers on both branches the first time.
+      var firstTime = true;
+
       return function(model, observer, filterRegistry) {
+        if (firstTime) {
+          var t = test(model, observer, filterRegistry);
+          var c = consequent(model, observer, filterRegistry);
+          var a = alternate(model, observer, filterRegistry);
+          firstTime = false;
+          return t ? c : a;
+        }
         return test(model, observer, filterRegistry) ?
             consequent(model, observer, filterRegistry) :
             alternate(model, observer, filterRegistry);

tests/tests.js

@@ -749,6 +749,44 @@ suite('PolymerExpressions', function() {
     });
   });
 
+  test('Expressions Conditional 2', function(done) {
+    var div = createTestHtml(
+        '<template bind>' +
+            '{{ checked ? a : b }}' +
+        '</template>');
+    var model = {
+      checked: false,
+      a: 'A',
+      b: 'B'
+    };
+
+    recursivelySetTemplateModel(div, model);
+
+    then(function() {
+      assert.strictEqual('B', div.childNodes[1].textContent);
+
+      model.checked = true;
+
+    }).then(function() {
+      assert.strictEqual('A', div.childNodes[1].textContent);
+
+      model.a = 'AAA';
+    }).then(function() {
+      assert.strictEqual('AAA', div.childNodes[1].textContent);
+
+      model.checked = false;
+
+    }).then(function() {
+      assert.strictEqual('B', div.childNodes[1].textContent);
+
+      model.b = 'BBB';
+    }).then(function() {
+      assert.strictEqual('BBB', div.childNodes[1].textContent);
+
+      done();
+    });
+  });
+
   test('Expressions Literals', function(done) {
     var div = createTestHtml(
         '<template bind>' +
@@ -1794,6 +1832,11 @@ suite('PolymerExpressions', function() {
     assert.isTrue(getExpression('a[a.b] + d[e].f').dynamicDeps);
     assert.isTrue(getExpression('a[b + c]').dynamicDeps);
     assert.isTrue(getExpression('a[b + 2].c + d').dynamicDeps);
+
+    assert.isFalse(getExpression('a ? b : c').dynamicDeps);
+    assert.isTrue(getExpression('a[a.b] ? b : c').dynamicDeps);
+    assert.isTrue(getExpression('a ? b[b.c] : c').dynamicDeps);
+    assert.isTrue(getExpression('a ? b : c[c.d]').dynamicDeps);
   });
 
   // https://github.com/Polymer/polymer-expressions/issues/24