support watching a function

Author: yyx990803

src/watcher.js

@@ -22,10 +22,11 @@ var uid = 0
  * @constructor
  */
 
-function Watcher (vm, expression, cb, options) {
+function Watcher (vm, expOrFn, cb, options) {
+  var isFn = typeof expOrFn === 'function'
   this.vm = vm
   vm._watchers.push(this)
-  this.expression = expression
+  this.expression = isFn ? '' : expOrFn
   this.cb = cb
   this.id = ++uid // uid for batching
   this.active = true
@@ -38,9 +39,14 @@ function Watcher (vm, expression, cb, options) {
   this.deps = []
   this.newDeps = []
   // parse expression for getter/setter
-  var res = expParser.parse(expression, options.twoWay)
-  this.getter = res.get
-  this.setter = res.set
+  if (isFn) {
+    this.getter = expOrFn
+    this.setter = undefined
+  } else {
+    var res = expParser.parse(expOrFn, options.twoWay)
+    this.getter = res.get
+    this.setter = res.set
+  }
   this.value = this.get()
 }
 

test/unit/specs/api/data_spec.js

@@ -104,6 +104,26 @@ describe('Data API', function () {
     })
   })
 
+  it('function $watch', function (done) {
+    var spy = jasmine.createSpy()
+    // test immediate invoke
+    var unwatch = vm.$watch(function () {
+      return this.a + this.b.c
+    }, spy, false, true)
+    expect(spy).toHaveBeenCalledWith(3, undefined)
+    vm.a = 2
+    nextTick(function () {
+      expect(spy).toHaveBeenCalledWith(4, 3)
+      // unwatch
+      unwatch()
+      vm.a = 3
+      nextTick(function () {
+        expect(spy.calls.count()).toBe(2)
+        done()
+      })
+    })
+  })
+
   it('deep $watch', function (done) {
     var oldB = vm.b
     var spy = jasmine.createSpy()
@@ -164,4 +184,4 @@ function leftHandThrows () {
   } catch (e) {
     return true
   }
-}
+}

test/unit/specs/watcher_spec.js

@@ -313,6 +313,22 @@ describe('Watcher', function () {
     })
   })
 
+  it('watch function', function (done) {
+    var watcher = new Watcher(vm, function () {
+      return this.a + this.b.d
+    }, spy)
+    expect(watcher.value).toBe(5)
+    vm.a = 2
+    nextTick(function () {
+      expect(spy).toHaveBeenCalledWith(6, 5)
+      vm.b = { d: 2 }
+      nextTick(function () {
+        expect(spy).toHaveBeenCalledWith(4, 6)
+        done()
+      })
+    })
+  })
+
   it('teardown', function (done) {
     var watcher = new Watcher(vm, 'b.c', spy)
     watcher.teardown()
@@ -348,4 +364,4 @@ describe('Watcher', function () {
     expect(hasWarned(_, 'Error when evaluating setter')).toBe(true)
   })
 
-})
+})