NSCollectionViewAttributes未與自定義NSFlowLayout子類正確應用

我目前正在NSCollectionView使用自定義布局。為此，我NSCollectionViewFlowLayout對我的專案進行了子類化以頂部對齊（它們都有固定的寬度，這使得演算法非常容易）。我現在遇到的問題是只有我的集合視圖中的前九個專案才能正確顯示，即頂部對齊。

當最初顯示集合視圖時，這前九個專案至少部分可見。向下滾動集合視圖時出現的其他專案將在沒有任何頂部對齊的情況下繪制，只是垂直居中每行作為NSCollectionViewFlowLayout. 通過廣泛的日志記錄，我可以驗證我的布局僅提供NSCollectionViewLayoutAttributes正確修改的頂??部對齊。

當我調整包含 的視窗的NSCollectionView大小，從而調整集合視圖本身的大小時，它的可見部分中的所有專案都會突然正確顯示。

我錯過了什么？

這是我的自定義子類：

#import "MyCollectionViewLayout.h"

@interface MyCollectionViewLayout ()

@property (nonatomic, strong) NSMutableDictionary<NSIndexPath *, NSCollectionViewLayoutAttributes *> *attributesCache;
@property NSInteger numberOfItemsPerRow;
@property NSInteger numberOfItemsTotal;

@end


@implementation MyCollectionViewLayout

- (void)prepareLayout {
    NSLog(@"Preparing layout");
    [super prepareLayout];
    self.itemSize = CGSizeMake(100, 138);
    self.minimumInteritemSpacing = 5;
    self.minimumLineSpacing = 5;
    self.sectionInset = NSEdgeInsetsMake(10, 10, 10, 10);
    self.attributesCache = @{}.mutableCopy;
    self.numberOfItemsTotal = [self.collectionView.dataSource collectionView:self.collectionView numberOfItemsInSection:0];
    
    NSInteger numberOfItemsPerRow = 1;
    CGFloat computedSize = self.itemSize.width;
    CGFloat usableWidth = self.collectionView.frame.size.width - (self.sectionInset.right   self.sectionInset.left);
    repeat: {
        computedSize  = self.minimumInteritemSpacing   self.itemSize.width;
        if (computedSize < usableWidth) {
            numberOfItemsPerRow  ;
            goto repeat;
        }
    }
    self.numberOfItemsPerRow = numberOfItemsPerRow;
}

#pragma mark NSCollectionViewFlowLayout override

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
    NSLog(@"Getting layout attributes for rect: %f, %f, %f, %f", rect.origin.x, rect.origin.y, rect.size.width, rect.size.height);
    NSArray *attributesArray = [super layoutAttributesForElementsInRect:rect].mutableCopy;
    for (int i = 0; i < attributesArray.count; i  ) {
        NSCollectionViewLayoutAttributes *attributes = attributesArray[i];
        NSLog(@"Forwarding attribute request for %@", attributes.indexPath);
        attributes = [self layoutAttributesForItemAtIndexPath:attributes.indexPath];
    }
    return attributesArray;
}

- (NSCollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
    if (!self.attributesCache[indexPath]) NSLog(@"");
    NSLog(@"Getting layout attributes for %@", indexPath);
    if (!self.attributesCache[indexPath]) {
        NSLog(@ "Not cached yet, caching full row");
        [self computeAndCacheAttributesForRowContaining:indexPath];
    }
    return self.attributesCache[indexPath];
}

#pragma mark Private instance methods

- (void)computeAndCacheAttributesForRowContaining:(NSIndexPath *)indexPath {
    NSDictionary<NSIndexPath *, NSCollectionViewLayoutAttributes *> *allAttributesInRowByPath = [self getAllAttributesInRowContaining:indexPath];
    CGFloat minY = CGFLOAT_MAX;
    for (NSIndexPath *path in allAttributesInRowByPath) {
        if (allAttributesInRowByPath[path].frame.origin.y < minY) {
            minY = allAttributesInRowByPath[path].frame.origin.y;
        }
    }
    for (NSIndexPath *path in allAttributesInRowByPath) {
        if (indexPath.item == 9) {
            
        }
        NSLog(@"Changing frame for indexPath %@", path);
        NSRect frame = allAttributesInRowByPath[path].frame;
        NSLog(@"Previous y Position: %f", frame.origin.y);
        NSLog(@"New y Position:      %f", minY);
        frame.origin.y = minY;
        allAttributesInRowByPath[path].frame = frame;
    }
    [self.attributesCache addEntriesFromDictionary:allAttributesInRowByPath];
}

- (NSDictionary<NSIndexPath *, NSCollectionViewLayoutAttributes *> *)getAllAttributesInRowContaining:(NSIndexPath *)indexPath {
    NSMutableDictionary<NSIndexPath *, NSCollectionViewLayoutAttributes *> *attributesToReturn = @{}.mutableCopy;
    NSInteger index = indexPath.item;
    NSInteger firstIndex = index - (index % self.numberOfItemsPerRow);
    NSIndexPath *path;
    for (index = firstIndex; (index < firstIndex   self.numberOfItemsPerRow) && (index < self.numberOfItemsTotal); index  ) {
        path = [NSIndexPath indexPathForItem:index inSection:indexPath.section];
        [attributesToReturn setObject:[super layoutAttributesForItemAtIndexPath:path].copy forKey:path];
    }
    return attributesToReturn.copy;
}

@end

uj5u.com熱心網友回復：

您缺少attributesArray[i] = attributes;并layoutAttributesForElementsInRect :回傳未修改的屬性。

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
    NSLog(@"Getting layout attributes for rect: %f, %f, %f, %f", rect.origin.x, rect.origin.y, rect.size.width, rect.size.height);
    NSMutableArray *attributesArray = [super layoutAttributesForElementsInRect:rect].mutableCopy;
    for (int i = 0; i < attributesArray.count; i  ) {
        NSCollectionViewLayoutAttributes *attributes = attributesArray[i];
        NSLog(@"Forwarding attribute request for %@", attributes.indexPath);
        attributes = [self layoutAttributesForItemAtIndexPath:attributes.indexPath];
        attributesArray[i] = attributes; // <---
    }
    return attributesArray;
}

標籤：可可 应用套件 nscollectionview nscollectionview布局 nscollectionviewflowlayout

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