Encapsulate state needed for re-render

This commit introduces the notion of a “result” data type, which
encapsulates all of the state needed to be passed to a compiled template
in order to successfully re-render with an existing element.

Rather than returning a document fragment, `render()` now returns this
result type. For compatibility, consumers of this API will need to
use the `fragment` property of the result rather than the result
directly.

In order to re-render, you can pass the last result as the `lastResult`
option, and it will use that element and morph to update.

One small additional change: contextual elements must now be passed in
the options hash. We decided to make this change as the list of
arguments was growing unwieldy, and we wanted to make this durable to
changes in the future. We will also investigate incorporating block
params into the options hash.

Author: Tom Dale and Yehuda Katz

packages/htmlbars-compiler/lib/hydration-javascript-compiler.js

@@ -30,13 +30,14 @@ prototype.compile = function(opcodes, options) {
 
   processOpcodes(this, opcodes);
 
-  var i, l, morphs;
+  var i, l;
 
   var indent = this.indent + '  ';
 
+  var morphs = indent+'var morphs;\n';
+
   if (this.morphs.length) {
-    morphs =
-      indent+'var morphs = env.morphs;\n' +
+    morphs +=
       indent+'if (!morphs) {\n' +
       indent+'  morphs = new Array(' + this.morphs.length + ');\n';
 
@@ -46,8 +47,6 @@ prototype.compile = function(opcodes, options) {
       }
 
       morphs += indent+'}\n';
-  } else {
-    morphs = indent+'var morphs;\n';
   }
 
   this.source.unshift(morphs);

packages/htmlbars-compiler/lib/template-compiler.js

@@ -83,7 +83,7 @@ TemplateCompiler.prototype.endProgram = function(program, programDepth) {
 
   var blockParams = program.blockParams || [];
 
-  var templateSignature = 'context, env, contextualElement';
+  var templateSignature = 'context, env, options';
   if (blockParams.length > 0) {
     templateSignature += ', blockArguments';
   }
@@ -100,8 +100,11 @@ TemplateCompiler.prototype.endProgram = function(program, programDepth) {
     indent+'    render: function render(' + templateSignature + ') {\n' +
     indent+'      var dom = env.dom;\n' +
     this.getHydrationHooks(indent + '      ', this.hydrationCompiler.hooks) +
+    indent+'      var contextualElement = options && options.contextualElement;\n' +
+    indent+'      var lastResult = options && options.lastResult;\n' +
     indent+'      dom.detectNamespace(contextualElement);\n' +
-    indent+'      var fragment = env.target;\n' +
+    indent+'      var fragment = lastResult ? lastResult.fragment : null;\n' +
+    indent+'      var morphs = lastResult ? lastResult.morphs : null;\n' +
     indent+'      if (!fragment && env.useFragmentCache && dom.canClone) {\n' +
     indent+'        if (this.cachedFragment === null) {\n' +
     indent+'          fragment = this.build(dom);\n' +
@@ -118,7 +121,7 @@ TemplateCompiler.prototype.endProgram = function(program, programDepth) {
     indent+'        fragment = this.build(dom);\n' +
     indent+'      }\n' +
     hydrationProgram +
-    indent+'      return { fragment: fragment, morphs: morphs };\n' +
+    indent+'      return lastResult || { fragment: fragment, morphs: morphs };\n' +
     indent+'    }\n' +
     indent+'  };\n' +
     indent+'}())';