Question

I have custom mapView (I named it JMMapView, which is inherited from UIView) where I created UIImageView in UIScrollView. It does basic stuff as zooming, scrolling, adding pinpoints and so on. But the main problem is that it loads one Image stored in Documents folder (images are downloaded at runtime and stored permanent in documents folder) and takes lots of memory. For example, all images are from 100kb to 800kb, but loading only one image memory is taken from 20mb to 40mb. What I am doing wrong? Why it takes so much memory? I use ARC and images are all PNG with size like 2000×2000.

DataController.m

+(NSData*) getDataOfUrlString:(NSString*)urlString {
    NSString *fullLocalUrlString = [self formLocalDocumentsUrlWithPath:urlString];
    NSString *remoteUrl;
    NSData *data = [NSData dataWithContentsOfFile:fullLocalUrlString];
    if (!data) {
        NSString *remoteUrlString = [WSController formRequestURLWithMethod:urlString];
        remoteUrl = remoteUrlString;
        data = [NSData dataWithContentsOfURL:[NSURL URLWithString:remoteUrlString]];
        if (data) {
            [self createFoldersPathForFileWithFullUrlString:fullLocalUrlString];
            [data writeToFile:fullLocalUrlString atomically:YES];
        } else {
            WLog([NSString stringWithFormat:@"data was not even downloaded from url:%@", remoteUrlString]);
        }
    }

    if (!data) {
        DLog(@"loaded data:%@, from urlString:%@", data ? @"YES" : @"NO", fullLocalUrlString);
        DLog(@"Can't download from remote url:%@\n", remoteUrl);
    }
    return data;
}

MapPreviewViewController.m

//In background with @autoreleasepool
NSData* data = [DataController getDataOfUrlString:abstractUrl];
UIImage *image = nil;
if (data) {
     DLog(@"returning map image with path:%@", abstractUrl);
     image = [UIImage imageWithData:data];
}
 //On Main thread
if (image && !mapView) {
     mapView = [[JMMapView alloc] initWithImage:image fitSize:fitSize];
     [mapView setViewController:self];
     [self.view addSubview mapView];
}

JMMapView : UIView

#define KEY_BUTTON @"buttonKey"
#define KEY_COORDINATES @"coordinatesKey"
#define KEY_OBJECT @"jmObjectKey"

@interface JMMapView () {
    UIImageView *mapView;
    CGSize fitSize;
    NSArray *pinpoints;
    NSArray *directions;
    UIScrollView *scrollView;
    CGPoint pinchCenter;
    NSDictionary *currentPoint;

    NSDictionary *openPinpoint;
    UIViewController *activeViewController;

    FPPopoverController *popController;

}

@end

@implementation JMMapView

#pragma mark - initialization
-(id)initWithImage:(UIImage *)image fitSize:(CGSize)newSize {

    self = [self initWithFrame:CGRectMake(0, NAVIGATION_BAR_HEIGHT - 44, newSize.width, newSize.height)];
    if (self) {
        [self initMapViewWithImage:image fitSize:newSize];
    }
    return self;
}

-(void) initMapViewWithImage:(UIImage*)mapImage fitSize:(CGSize)newFitSize {

    mapView = [[UIImageView alloc] initWithImage:mapImage];
    [self finishInitWithFitSize:newFitSize];
}

-(void) finishInitWithFitSize:(CGSize)newFitSize {
    mapView.frame = [Utilities reframe:mapView.frame toFitSize:newFitSize];

    CGRect scrollFrame = CGRectMake(0, 0, newFitSize.width, newFitSize.height);
    scrollView = [[UIScrollView alloc] initWithFrame:scrollFrame];
    [scrollView setContentSize:newFitSize];
    [mapView setCenter:CGPointMake(scrollView.frame.size.width/2, scrollView.frame.size.height/2)];
    [mapView setUserInteractionEnabled:YES];

    [scrollView addSubview:mapView];
    [self addSubview:scrollView];


    fitSize = newFitSize;
    [self initZooming];

}
//gesture recognizer
...
//

#pragma mark - private methods
//pinch handling
..
//
//Add pinpoint
-(void) addPinpointToArray:(UIButton*)pin position:(JMMapPosition *)position pinpointType:(PINPOINT_TYPE)pinType object:(id)object{
    NSDictionary *dict = [NSDictionary dictionaryWithObjectsAndKeys:
                          pin, KEY_BUTTON,
                          position, KEY_COORDINATES,
                          object, KEY_OBJECT,
                          nil];
    switch (pinType) {
        case PINPOINT_TYPE_CHILD_OBJECT:
        {
            if (!pinpoints) {
                pinpoints = [[NSArray alloc]  initWithObjects:dict, nil];
            } else {
                NSMutableArray *arrayM = [NSMutableArray arrayWithArray:pinpoints];
                [arrayM addObject:dict];
                pinpoints = [NSArray arrayWithArray:arrayM];
            }
            break;
        }

        case PINPOINT_TYPE_DIRECTION:
        {
            if (!directions) {
                directions = [[NSArray alloc]  initWithObjects:dict, nil];
            } else {
                NSMutableArray *arrayM = [NSMutableArray arrayWithArray:directions];
                [arrayM addObject:dict];
                directions = [NSArray arrayWithArray:arrayM];
            }
            break;
        }
        case PINPOINT_TYPE_OBJECT:
            currentPoint = dict;
            break;
        default:
            break;
    }
}


#pragma mark - public methods
-(void)setViewController:(UIViewController *)newController {
    if (activeViewController != newController) {
        activeViewController = newController;
    }
}



-(void)addPinpointWithObject:(id)object mapPosition:(JMMapPosition*)position pinpointType:(PINPOINT_TYPE)pinType
 {
    static UIImage *greenPin = nil;
    static UIImage *redPin = nil;
    static UIImage *directionPin = nil;
    if (greenPin == nil) {

        greenPin = [UIImage imageNamed:@"pin_green"];
        redPin = [UIImage imageNamed:@"pin"];
        //todo direction pin
        directionPin = [UIImage imageNamed:@"pin_direction"];
    }
    UIImage *image = nil;
    switch (pinType) {
        case PINPOINT_TYPE_CHILD_OBJECT:
            image = greenPin;
            break;

        case PINPOINT_TYPE_OBJECT:
            image = redPin;
            break;

        case PINPOINT_TYPE_DIRECTION:
            image = directionPin;
        default:
            break;
    }
    CGFloat width = image.size.width;
    CGFloat height = image.size.height;
    UIButton *button = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, width, height)];
     [self setButton:button positionToCoords:position];
    [button setBackgroundImage:image forState:UIControlStateNormal];

    [mapView addSubview:button];

    if (pinType == PINPOINT_TYPE_CHILD_OBJECT) {
        [button setTag:[pinpoints count]];
        [button addTarget:self action:@selector(pinpointPressed:) forControlEvents:UIControlEventTouchUpInside];
    } else if (pinType == PINPOINT_TYPE_DIRECTION) {
        [button setTag:[directions count]];
        [button addTarget:self action:@selector(directionPressed:) forControlEvents:UIControlEventTouchUpInside];

    }

    //add pinpoint to register
    [self addPinpointToArray:button position:position pinpointType:pinType object:object];
}

-(void) destroyAllMapView {
    //[self dismissPopover];
    [self setViewController:nil];
    for (NSDictionary *dict in pinpoints) {
        @autoreleasepool {
            UIButton *button = [dict objectForKey:KEY_BUTTON];
            [button removeTarget:nil action:NULL forControlEvents:UIControlEventAllEvents];
            //[button removeFromSuperview];

        }
    }
    [self setNilToEveryGlobalObject];
}

Answer 1

The image may be 100-800kb while compressed as a PNG, but it must be fully decompressed in memory.

This means a 2000×2000 image with alpha transparency (RGBA) will be 2000x2000x4 bytes, or roughly 15 megabytes. This is why the images are using up so much memory.

The first thing you should do is make sure the images do not have an alpha channel, and that the view they are displayed in has opaque set to YES to disable any transparency/blending.

You probably should use a tile-based approach as well. Apple has good sample code to demonstrate this with their PhotoScroller sample project.