我想在Vc收到通知时改变第一个单元格的背景色。我使用了像blow这样的代码来实现这一点，但它总是得到nil：

@objc func getNotification() {
        let indexPath = IndexPath(row: 0, section: 0)
        if let cell = collectionView.cellForItem(at: indexPath) as? TestCell {
            cell.changeBackgroundColor()
        }
 }

如果我打印visibelCells，我得到了6个不同的单元格对象！（它只在屏幕上显示3个，并且models.count也是3），下面是我打印visibelCells的日志。

>>> visibleCells:  <TestCell: 0x7fccbb6a5560; baseClass = UICollectionViewCell; frame = (58969.7 18.4; 353.7 115.2); transform = [0.90000000000000002, 0, 0, 0.90000000000000002, 0, 0]; alpha = 0.6; layer = <CALayer: 0x600000ccd9a0>>
>>> visibleCells:  <TestCell: 0x7fccbb74c2b0; baseClass = UICollectionViewCell; frame = (58950 0; 393 128); layer = <CALayer: 0x600000cd8a20>>
>>> visibleCells:  <TestCell: 0x7fccbe06c560; baseClass = UICollectionViewCell; frame = (58989.3 24.8; 314.4 102.4); transform = [0.80000000000000004, 0, 0, 0.80000000000000004, 0, 0]; alpha = 0.2; layer = <CALayer: 0x600000cdd3e0>>
>>> -----------------------
>>> visibleCells:  <TestCell: 0x7fccbb6a5560; baseClass = UICollectionViewCell; frame = (58969.7 18.4; 353.7 115.2); transform = [0.90000000000000002, 0, 0, 0.90000000000000002, 0, 0]; alpha = 0.6; layer = <CALayer: 0x600000ccd9a0>>
>>> visibleCells:  <TestCell: 0x7fccbb74c2b0; baseClass = UICollectionViewCell; frame = (58950 0; 393 128); layer = <CALayer: 0x600000cd8a20>>
>>> visibleCells:  <TestCell: 0x7fccbe06c560; baseClass = UICollectionViewCell; frame = (58989.3 24.8; 314.4 102.4); transform = [0.80000000000000004, 0, 0, 0.80000000000000004, 0, 0]; alpha = 0.2; layer = <CALayer: 0x600000cdd3e0>>
>>> -----------------------
>>> visibleCells:  <TestCell: 0x7fccbb6a5560; baseClass = UICollectionViewCell; frame = (59343 0; 393 128); layer = <CALayer: 0x600000ccd9a0>>
>>> visibleCells:  <TestCell: 0x7fccbb663b90; baseClass = UICollectionViewCell; frame = (59382.3 24.8; 314.4 102.4); transform = [0.80000000000000004, 0, 0, 0.80000000000000004, 0, 0]; alpha = 0.2; layer = <CALayer: 0x600000cd6d40>>
>>> visibleCells:  <TestCell: 0x7fccbe06c560; baseClass = UICollectionViewCell; frame = (59362.7 18.4; 353.7 115.2); transform = [0.90000000000000002, 0, 0, 0.90000000000000002, 0, 0]; alpha = 0.6; layer = <CALayer: 0x600000cdd3e0>>
>>> -----------------------
>>> visibleCells:  <TestCell: 0x7fccbb74c2b0; baseClass = UICollectionViewCell; frame = (59775.3 24.8; 314.4 102.4); transform = [0.80000000000000004, 0, 0, 0.80000000000000004, 0, 0]; alpha = 0.2; layer = <CALayer: 0x600000cd8a20>>
>>> visibleCells:  <TestCell: 0x7fccbb663b90; baseClass = UICollectionViewCell; frame = (59755.7 18.4; 353.7 115.2); transform = [0.90000000000000002, 0, 0, 0.90000000000000002, 0, 0]; alpha = 0.6; layer = <CALayer: 0x600000cd6d40>>
>>> visibleCells:  <TestCell: 0x7fccbe06c560; baseClass = UICollectionViewCell; frame = (59736 0; 393 128); layer = <CALayer: 0x600000cdd3e0>>
>>> -----------------------
>>> visibleCells:  <TestCell: 0x7fccbb55f6f0; baseClass = UICollectionViewCell; frame = (58969.7 18.4; 353.7 115.2); transform = [0.90000000000000002, 0, 0, 0.90000000000000002, 0, 0]; alpha = 0.6; layer = <CALayer: 0x600000cc84c0>>
>>> visibleCells:  <TestCell: 0x7fccbb55a1e0; baseClass = UICollectionViewCell; frame = (58950 0; 393 128); layer = <CALayer: 0x600000d68000>>
>>> visibleCells:  <TestCell: 0x7fccbe06c560; baseClass = UICollectionViewCell; frame = (58989.3 24.8; 314.4 102.4); transform = [0.80000000000000004, 0, 0, 0.80000000000000004, 0, 0]; alpha = 0.2; layer = <CALayer: 0x600000cdd3e0>>
>>> -----------------------
>>> visibleCells:  <TestCell: 0x7fccbb55f6f0; baseClass = UICollectionViewCell; frame = (59343 0; 393 128); layer = <CALayer: 0x600000cc84c0>>
>>> visibleCells:  <TestCell: 0x7fccbb6a5560; baseClass = UICollectionViewCell; frame = (59382.3 24.8; 314.4 102.4); transform = [0.80000000000000004, 0, 0, 0.80000000000000004, 0, 0]; alpha = 0.2; layer = <CALayer: 0x600000ccd9a0>>
>>> visibleCells:  <TestCell: 0x7fccbe06c560; baseClass = UICollectionViewCell; frame = (59362.7 18.4; 353.7 115.2); transform = [0.90000000000000002, 0, 0, 0.90000000000000002, 0, 0]; alpha = 0.6; layer = <CALayer: 0x600000cdd3e0>>
>>> -----------------------
>>> visibleCells:  <TestCell: 0x7fccbb55a1e0; baseClass = UICollectionViewCell; frame = (59775.3 24.8; 314.4 102.4); transform = [0.80000000000000004, 0, 0, 0.80000000000000004, 0, 0]; alpha = 0.2; layer = <CALayer: 0x600000d68000>>
>>> visibleCells:  <TestCell: 0x7fccbb6a5560; baseClass = UICollectionViewCell; frame = (59755.7 18.4; 353.7 115.2); transform = [0.90000000000000002, 0, 0, 0.90000000000000002, 0, 0]; alpha = 0.6; layer = <CALayer: 0x600000ccd9a0>>
>>> visibleCells:  <TestCell: 0x7fccbe06c560; baseClass = UICollectionViewCell; frame = (59736 0; 393 128); layer = <CALayer: 0x600000cdd3e0>>
>>> -----------------------
>>> visibleCells:  <TestCell: 0x7fccbb663b90; baseClass = UICollectionViewCell; frame = (58969.7 18.4; 353.7 115.2); transform = [0.90000000000000002, 0, 0, 0.90000000000000002, 0, 0]; alpha = 0.6; layer = <CALayer: 0x600000cd6d40>>
>>> visibleCells:  <TestCell: 0x7fccbb74c2b0; baseClass = UICollectionViewCell; frame = (58950 0; 393 128); layer = <CALayer: 0x600000cd8a20>>
>>> visibleCells:  <TestCell: 0x7fccbe06c560; baseClass = UICollectionViewCell; frame = (58989.3 24.8; 314.4 102.4); transform = [0.80000000000000004, 0, 0, 0.80000000000000004, 0, 0]; alpha = 0.2; layer = <CALayer: 0x600000cdd3e0>>
>>> -----------------------
>>> visibleCells:  <TestCell: 0x7fccbb663b90; baseClass = UICollectionViewCell; frame = (59343 0; 393 128); layer = <CALayer: 0x600000cd6d40>>
>>> visibleCells:  <TestCell: 0x7fccbb55f6f0; baseClass = UICollectionViewCell; frame = (59382.3 24.8; 314.4 102.4); transform = [0.80000000000000004, 0, 0, 0.80000000000000004, 0, 0]; alpha = 0.2; layer = <CALayer: 0x600000cc84c0>>
>>> visibleCells:  <TestCell: 0x7fccbe06c560; baseClass = UICollectionViewCell; frame = (59362.7 18.4; 353.7 115.2); transform = [0.90000000000000002, 0, 0, 0.90000000000000002, 0, 0]; alpha = 0.6; layer = <CALayer: 0x600000cdd3e0>>
>>> -----------------------