setState 调用了 this.updater.enqueueSetState 方法，enqueueSetState 主要是创建 update，然后将它放入更新队列，最后进入调度阶段。
Component.prototype.setState = function(partialState, callback) {
// partialState 可以为对象，或者一个返回值为对象的函数
invariant(
typeof partialState === 'object' ||
typeof partialState === 'function' ||
partialState == null,
'setState(...): takes an object of state variables to update or a ' +
'function which returns an object of state variables.',
);
this.updater.enqueueSetState(this, partialState, callback, 'setState');
};
enqueueSetState(inst, payload, callback) {
const fiber = getInstance(inst);
const currentTime = requestCurrentTime();
const expirationTime = computeExpirationForFiber(currentTime, fiber);
const update = createUpdate(expirationTime);
update.payload = payload; // 要更新的内容
if (callback !== undefined && callback !== null) {
update.callback = callback;
}
flushPassiveEffects();
enqueueUpdate(fiber, update);
scheduleWork(fiber, expirationTime);
}
forceUpdate 与 setState 几乎相同，不同是 forceUpdate 的 update.tag 为 ForceUpdate，且没有 payload 这个参数。
Component.prototype.forceUpdate = function(callback) {
this.updater.enqueueForceUpdate(this, callback, 'forceUpdate');
};
enqueueForceUpdate(inst, callback) {
const fiber = getInstance(inst);
const currentTime = requestCurrentTime();
const expirationTime = computeExpirationForFiber(currentTime, fiber);
const update = createUpdate(expirationTime);
update.tag = ForceUpdate;
if (callback !== undefined && callback !== null) {
update.callback = callback;
}
flushPassiveEffects();
enqueueUpdate(fiber, update);
scheduleWork(fiber, expirationTime);
}
