人人都能看懂的鸿蒙 “JS 小程序” 数据绑定原理 | 解读鸿蒙源码

else if (typeof value === 'function') {
        vm[key] = value.bind(vm);
}

/**
 * proxy data
 * @param {ViewModel} target - 即 vm 实例
 * @param {Object} source - 即 data
 * @param {String} key - data 中的 key
 */
function proxy(target, source, key) {
  Object.defineProperty(target, key, {
    enumerable: false,
    configurable: true,
    get() {
      return source[key];
    },
    set(value) {
      source[key] = value;
    }
  });
}

// src/core/components/component.cpp 
/**
 * check if the pass-in attrValue is an Expression, if it is, calculate it and bind watcher instance.
 * if it's not, just return the passed-in attrValue itself.
 */
jerry_value_t Component::ParseExpression(jerry_value_t attrKey, jerry_value_t attrValue)
{
    jerry_value_t options = jerry_create_object();
    JerrySetNamedProperty(options, ARG_WATCH_EL, nativeElement_);
    JerrySetNamedProperty(options, ARG_WATCH_ATTR, attrKey);
    jerry_value_t watcher = CallJSWatcher(attrValue, WatcherCallbackFunc, options);
    jerry_value_t propValue = UNDEFINED;
    if (IS_UNDEFINED(watcher) || jerry_value_is_error(watcher)) {
        HILOG_ERROR(HILOG_MODULE_ACE, "Failed to create Watcher instance.");
    } else {
        InsertWatcherCommon(watchersHead_, watcher);
        propValue = jerryx_get_property_str(watcher, "_lastValue");
    }
    jerry_release_value(options);
    return propValue;
}

// src/core/base/js_fwk_common.h
struct Watcher : public MemoryHeap {
    ACE_DISALLOW_COPY_AND_MOVE(Watcher);
    Watcher() : watcher(jerry_create_undefined()), next(nullptr) {}
    jerry_value_t watcher;
    struct Watcher *next;
};

// src/core/base/memory_heap.cpp
void *MemoryHeap::operator new(size_t size)
{
    return ace_malloc(size);
}

// packages/runtime-core/src/core/index.js
ViewModel.prototype.$watch = function(getter, callback, meta) {
  return new Observer(this, getter, callback, meta);
};

Observer.prototype._get = function() {
  try {
    ObserverStack.push(this);
    return this._getter.call(this._ctx);
  } finally {
    ObserverStack.pop();
  }
};

let _str = '';
function Abc() {}
Abc.prototype.hello = function() {
  try {
    _str += 'try';
    return _str + 'return';
  } catch (ex) {
    console.log(ex);
  } finally {
    _str += 'finally';
  }
};
const abc = new Abc();
const result = abc.hello();
console.log('[result]', result, _str);

[result] tryreturn tryfinally

Observer.prototype.update = function() {
  const lastValue = this._lastValue;
  const nextValue = this._get();
  const context = this._ctx;
  const meta = this._meta;
  if (nextValue !== lastValue || canObserve(nextValue)) {
    this._fn.call(context, nextValue, lastValue, meta);
    this._lastValue = nextValue;
  }
};

// packages/runtime-core/src/observer/utils.js 
export const canObserve = target => typeof target === 'object' && target !== null;

Observer.prototype.subscribe = function(subject, key) {
  const detach = subject.attach(key, this);
  if (typeof detach !== 'function') {
    return void 0;
  }
  if (!this._detaches) {
    this._detaches = [];
  }
  this._detaches.push(detach);
};

Observer.prototype.unsubscribe = function() {
  const detaches = this._detaches;
  if (!detaches) {
    return void 0;
  }
  while (detaches.length) {
    detaches.pop()(); // 注意此处的立即执行
  }
};

/**
 * observe object
 * @param {any} target the object to be observed
 * @param {String} key the key to be observed
 * @param {any} cache the cached value
 */
function hijack(target, key, cache) {
  const subject = target[SYMBOL_OBSERVABLE]; // "__ob__"
  Object.defineProperty(target, key, {
    enumerable: true,
    get() {
      const observer = ObserverStack.top();
      if (observer) {
        console.log('[topObserver.subscribe in Subject::hijack]');
        observer.subscribe(subject, key);
      }
    ...
      return cache;
    },
    set(value) {
      cache = value;
      subject.notify(key);
    },
  });
}

Subject.prototype.notify = function (key) {
  ...
  this._obsMap[key].forEach((observer) => observer.update());
};

export const ObserverStack = {
  stack: [],
  push(observer) {
    this.stack.push(observer);
  },
  pop() {
    return this.stack.pop();
  },
  top() { // 实际上是将数组“队尾”当作栈顶方向的
    return this.stack[this.stack.length - 1];
  }
};

// packages/runtime-core/src/__test__/index.test.js
  test.only('04_watch_basic_usage', (done) => {
    const vm = new ViewModel({
      data: function () {
        return { count: 1 };
      },
      increase() {
        ++this.count;
      },
      decrease() {
        --this.count;
      },
    });
    console.log('test step 1 =========================');
    expect(vm.count).toBe(1);
    console.log('test step 2 =========================');
    const watcher = vm.$watch(
      () => vm.count,
      (value) => {
        expect(value).toBe(2);
        watcher.unsubscribe();
        done();
      }
    );
    console.log('test step 3 =========================');
    vm.increase();
  });