useState
react对useState进行了封装，调用了mountState。
function useState<S>(
initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
currentHookNameInDev = 'useState';
mountHookTypesDev();
const prevDispatcher = ReactCurrentDispatcher.current;
ReactCurrentDispatcher.current = InvalidNestedHooksDispatcherOnMountInDEV;
try {
return mountState(initialState);
} finally {
ReactCurrentDispatcher.current = prevDispatcher;
}
}
mountState
如果initialState是函数还可以执行。
生成一个dispatch方法，通过闭包绑定当前states。
把初始值存到memoizedState上。这个memoizedState绑定到fiber树上。用来存储state。
function mountState<S>(
initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
// 把hooks加入queue，实际上是为了保证执行顺序。
const hook = mountWorkInProgressHook();
if (typeof initialState === 'function') {
initialState = initialState();
}
hook.memoizedState = hook.baseState = initialState;
const queue = (hook.queue = {
last: null,
dispatch: null,
lastRenderedReducer: basicStateReducer,
lastRenderedState: (initialState: any),
});
const dispatch: Dispatch<
BasicStateAction<S>,
> = (queue.dispatch = (dispatchAction.bind(
null,
// Flow doesn't know this is non-null, but we do.
((currentlyRenderingFiber: any): Fiber),
queue,
): any));
return [hook.memoizedState, dispatch];
}
memoizedState
react其实不知道我们调用了几次useState。
所以还是在memoizedState上动手脚，这个处理体现在mountWorkInProgressHook
memoizedState: {
baseState,
next,
baseUpdate,
queue,
memoizedState
}
memoizedState.next就是下一次useState的hook对象。
hook1 === Fiber.memoizedState
state1 === hook1.memoizedState
state2 = hook1.next.memoizedState
setstate
mountState函数返回的是 return [hook.memoizedState, dispatch];
dispatch通过闭包就可以处理state。
更新
useState在更新的时候是调用的updateState，这个函数其实是封装的updateReducer。
function renderWithHooks(){
ReactCurrentDispatcher.current =
nextCurrentHook === null
? HooksDispatcherOnMount
: HooksDispatcherOnUpdate;
};
HooksDispatcherOnMount: {
useState: mountState,
}
HooksDispatcherOnUpdate: {
useState: updateState,
}
updateReducer
可以看到updateReducer把新的fiber中的state值更新，返回新的值。然后后续走渲染流程。（之前写过reat 的渲染流程）
还可以看到这有个循环update = update.next; while (update !== null && update !== first);
这就是hooks的batchUpdate。
function updateReducer<S, I, A>(
reducer: (S, A) => S,
initialArg: I,
init?: I => S,
): [S, Dispatch<A>] {
const hook = updateWorkInProgressHook();
const queue = hook.queue;
queue.lastRenderedReducer = reducer;
// ...
// The last update in the entire queue
const last = queue.last;
// The last update that is part of the base state.
const baseUpdate = hook.baseUpdate;
const baseState = hook.baseState;
// Find the first unprocessed update.
let first;
if (baseUpdate !== null) {
if (last !== null) {
// For the first update, the queue is a circular linked list where
// `queue.last.next = queue.first`. Once the first update commits, and
// the `baseUpdate` is no longer empty, we can unravel the list.
last.next = null;
}
first = baseUpdate.next;
} else {
first = last !== null ? last.next : null;
}
if (first !== null) {
let newState = baseState;
let newBaseState = null;
let newBaseUpdate = null;
let prevUpdate = baseUpdate;
let update = first;
let didSkip = false;
do {
const updateExpirationTime = update.expirationTime;
if (updateExpirationTime < renderExpirationTime) {
// Priority is insufficient. Skip this update. If this is the first
// skipped update, the previous update/state is the new base
// update/state.
if (!didSkip) {
didSkip = true;
newBaseUpdate = prevUpdate;
newBaseState = newState;
}
// Update the remaining priority in the queue.
if (updateExpirationTime > remainingExpirationTime) {
remainingExpirationTime = updateExpirationTime;
}
} else {
markRenderEventTimeAndConfig(
updateExpirationTime,
update.suspenseConfig,
);
// Process this update.
if (update.eagerReducer === reducer) {
// If this update was processed eagerly, and its reducer matches the
// current reducer, we can use the eagerly computed state.
newState = ((update.eagerState: any): S);
} else {
const action = update.action;
newState = reducer(newState, action);
}
}
prevUpdate = update;
update = update.next;
} while (update !== null && update !== first);
if (!didSkip) {
newBaseUpdate = prevUpdate;
newBaseState = newState;
}
// Mark that the fiber performed work, but only if the new state is
// different from the current state.
if (!is(newState, hook.memoizedState)) {
markWorkInProgressReceivedUpdate();
}
hook.memoizedState = newState;
hook.baseUpdate = newBaseUpdate;
hook.baseState = newBaseState;
queue.lastRenderedState = newState;
}
const dispatch: Dispatch<A> = (queue.dispatch: any);
return [hook.memoizedState, dispatch];
}