研究2

從- (void)_frameOrBoundsChanged裡面還沒看到跟WKContentView或WKScrollView相關的內容，研究一下後再從[self _scheduleVisibleContentRectUpdate]繼續往下看：

- (void)_scheduleVisibleContentRectUpdate
{
    // For visible rect updates not associated with a specific UIScrollView, just consider our own scroller.
    [self _scheduleVisibleContentRectUpdateAfterScrollInView:_scrollView.get()];
}

- (void)_scheduleVisibleContentRectUpdateAfterScrollInView:(UIScrollView *)scrollView
{
    _visibleContentRectUpdateScheduledFromScrollViewInStableState = [self _scrollViewIsInStableState:scrollView];

    if (_hasScheduledVisibleRectUpdate)
        return;

    _hasScheduledVisibleRectUpdate = YES;

#if __IPHONE_OS_VERSION_MIN_REQUIRED >= 110000
    CATransactionPhase transactionPhase = [CATransaction currentPhase];
    if (transactionPhase == kCATransactionPhaseNull || transactionPhase == kCATransactionPhasePreLayout) {
        [self _addUpdateVisibleContentRectPreCommitHandler];
        return;
    }
#endif

    dispatch_async(dispatch_get_main_queue(), [retainedSelf = retainPtr(self)] {
        WKWebView *webView = retainedSelf.get();
        if (![webView _isValid])
            return;
        [webView _addUpdateVisibleContentRectPreCommitHandler];
    });
}

- (void)_addUpdateVisibleContentRectPreCommitHandler
{
    auto retainedSelf = retainPtr(self);
    [CATransaction addCommitHandler:[retainedSelf] {
        WKWebView *webView = retainedSelf.get();
        if (![webView _isValid])
            return;
        [webView _updateVisibleContentRects];
        webView->_hasScheduledVisibleRectUpdate = NO;
    } forPhase:kCATransactionPhasePreCommit];
}

看到[webView _updateVisibleContentRects]從function名稱推斷這可能是畫面更新的關鍵，於是繼續往下看

未完繼續...