NSScrollView Infinite / Infinite Scroll | reuse subview

Question

I am looking for a way to implement something like reusable cells for UI/NSTableView, but for NSScrollView. I basically want the same as WWDC 2011 video "Session 104 - Advanced Scroll View Techniques", but for Mac.

I have several problems with this. First: it NSScrollViewdoes not -layoutSubviews. I tried using -adjustScrollinstead, but could not install another contentOffset:

- (NSRect)adjustScroll:(NSRect)proposedVisibleRect {
    if (proposedVisibleRect.origin.x > 600) {
        //  non of them work properly
        // proposedVisibleRect.origin.x = 0;
        // [self setBoundsOrigin:NSZeroPoint];
        // [self setFrameOrigin:NSZeroPoint];
        // [[parentScrollView contentView] scrollPoint:NSZeroPoint];
        // [[parentScrollView contentView] setBoundsOrigin:NSZeroPoint];
    }
    return proposedVisibleRect;
}

The next thing I tried was to set a really huge view of the content using widthmillions of pixels (which actually works compared to iOS!), But now the question is, how to set up a reuse pool? <w> Is it better to move subitems when scrolling to a new position or delete all subitems and insert them again? and how and where should I do it?

Answer 1

As far as I can tell, -adjustScroll:this is not the place where you want to use scroll events, because it is not called universal. I think that -reflectScrolledClipView:is probably the best connection point.

, . scrollView documentView "", , , "" , . , . ( , , , .) :

// For the header file
@interface SOReuseScrollView : NSScrollView
@end

// For the implementation file
@interface SOReuseScrollView () // Private

- (void)p_updateTiles;
@property (nonatomic, readonly, retain) NSMutableArray* p_reusableViews;

@end

// Just a small diagnosting view to convince myself that this works.
@interface SODiagnosticView : NSView
@end

@implementation SOReuseScrollView

@synthesize p_reusableViews = mReusableViews;

- (void)dealloc
{
    [mReusableViews release];
    [super dealloc];
}

- (NSMutableArray*)p_reusableViews
{
    if (nil == mReusableViews)
    {
        mReusableViews = [[NSMutableArray alloc] init];
    }
    return mReusableViews;
}

- (void)reflectScrolledClipView:(NSClipView *)cView
{
    [super reflectScrolledClipView: cView];
    [self p_updateTiles];
}

- (void)p_updateTiles
{
    // The size of a tile...
    static const NSSize gGranuleSize = {250.0, 250.0};

    NSMutableArray* reusableViews = self.p_reusableViews;
    NSRect documentVisibleRect = self.documentVisibleRect;

    // Determine the needed tiles for coverage
    const CGFloat xMin = floor(NSMinX(documentVisibleRect) / gGranuleSize.width) * gGranuleSize.width;
    const CGFloat xMax = xMin + (ceil((NSMaxX(documentVisibleRect) - xMin) / gGranuleSize.width) * gGranuleSize.width);
    const CGFloat yMin = floor(NSMinY(documentVisibleRect) / gGranuleSize.height) * gGranuleSize.height;
    const CGFloat yMax = ceil((NSMaxY(documentVisibleRect) - yMin) / gGranuleSize.height) * gGranuleSize.height;

    // Figure out the tile frames we would need to get full coverage
    NSMutableSet* neededTileFrames = [NSMutableSet set];
    for (CGFloat x = xMin; x < xMax; x += gGranuleSize.width)
    {
        for (CGFloat y = yMin; y < yMax; y += gGranuleSize.height)
        {
            NSRect rect = NSMakeRect(x, y, gGranuleSize.width, gGranuleSize.height);
            [neededTileFrames addObject: [NSValue valueWithRect: rect]];
        }
    }

    // See if we already have subviews that cover these needed frames.
    for (NSView* subview in [[[self.documentView subviews] copy] autorelease])
    {
        NSValue* frameRectVal = [NSValue valueWithRect: subview.frame];

        // If we don't need this one any more...
        if (![neededTileFrames containsObject: frameRectVal])
        {
            // Then recycle it...
            [reusableViews addObject: subview];
            [subview removeFromSuperview];
        }
        else
        {
            // Take this frame rect off the To-do list.
            [neededTileFrames removeObject: frameRectVal];
        }
    }

    // Add needed tiles from the to-do list
    for (NSValue* neededFrame in neededTileFrames)
    {
        NSView* view = [[[reusableViews lastObject] retain] autorelease];
        [reusableViews removeLastObject];

        if (nil == view)
        {
            // Create one if we didnt find a reusable one.
            view = [[[SODiagnosticView alloc] initWithFrame: NSZeroRect] autorelease];
            NSLog(@"Created a view.");
        }
        else 
        {
            NSLog(@"Reused a view.");
        }

        // Place it and install it.
        view.frame = [neededFrame rectValue];
        [view setNeedsDisplay: YES];        
        [self.documentView addSubview: view];
    }
}

@end

@implementation SODiagnosticView

- (void)drawRect:(NSRect)dirtyRect
{
    // Draw a red tile with a blue border.
    [[NSColor blueColor] set];
    NSRectFill(self.bounds);

    [[NSColor redColor] setFill];
    NSRectFill(NSInsetRect(self.bounds, 2,2));    
}

@end

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