Ancient Programming

What I encounter in my software part of life is in danger of being commented upon here

Archive for April, 2012

Animating the drag drop example

Posted by Jacob von Eyben on 9th April 2012

As an update to my original ios drag drop post I have added an animation to the snap back to original position.

The changes is made to the DragContext.m file where the snapToOrignalPostion is changed to look like the following:

- (void)snapToOriginalPosition {
    [UIView animateWithDuration:0.3 animations:^() {
        CGPoint originalPointInSuperView = [_draggedView.superview convertPoint:_originalPosition fromView:_originalView];
        _draggedView.frame = CGRectMake(originalPointInSuperView.x, originalPointInSuperView.y, _draggedView.frame.size.width, _draggedView.frame.size.height);
    } completion:^(BOOL finished) {
        _draggedView.frame = CGRectMake(_originalPosition.x, _originalPosition.y, _draggedView.frame.size.width, _draggedView.frame.size.height);
        [_draggedView removeFromSuperview];
        [_originalView addSubview:_draggedView];
    }];
}

First we animate the dragged object back to the original position (in the super view containing all drag objects and drop areas). When the animation is completed, we change the superview.

Posted in animation, how to, ios, ipad, iphone, mac, objective-c | 1 Comment »

Drag and drop between multiple UIViews in iOS

Posted by Jacob von Eyben on 5th April 2012

This is an example showing how to drag and and drop UIViews between other UIViews in iOS.

Illustrates the drag and drop UIView stack

Illustrates the drag and drop UIView stack

The example is broken into the following four and a half step.

Example broken into steps
1. register a UIPanGestureRecognizer to be able to get drag panning eventsĀ  (in our case drag events).
2. detect if the panning started above the view we would like to drag.
3. move the view according to the panning - which means removing the object from the view we would like to drag it from.
4. when dragging ends, drop the view onto the view below the dragged object.
4.a. optionally reset the dragged object, if the object is dragged onto a view which we don’t consider as a valid drop view.


In my example I have created a more or less generic DragDropManager, which is responsible for actually handling the UIViews objects we allow to drag ‘n drop and the UIVeiws we allow to drop onto.

Step 1
The manager registers the draggable objects and drop areas and takes care of detecting when a object should be dragged and dropped. This is done by receiving pan events from the UIPanGestureRecognizer. I recommend registering the gesture recognizer to a UIView having a frame covering the entire drag and drop area, because if you bind the recognizer to the actual draggable objects, you will loose the pan sequence as soon as you change super view for the dragged object (and changing super view is the hole point of dragging objects between UIViews).

@implementation DragDropBetweenViewsViewController
...
- (void)viewDidLoad {
    [super viewDidLoad];
    _viewA = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 320, 200)];
    [_viewA setBackgroundColor:[UIColor greenColor]];
    _viewA.tag = 1;
    _viewB = [[UIView alloc] initWithFrame:CGRectMake(0, 220, 320, 200)];
    [_viewB setBackgroundColor:[UIColor yellowColor]];
    _viewB.tag = 2;

    [[self view] addSubview:_viewA];
    [[self view] addSubview:_viewB];
    //[[self view] addSubview:_viewB];

    //add elements to drag and drop
    UIView *dragDropView1 = [[[UIView alloc] initWithFrame:CGRectMake(100, 100, 50, 50)] autorelease];
    UIView *dragDropView2 = [[[UIView alloc] initWithFrame:CGRectMake(100, 100, 50, 50)] autorelease];
    [_viewA addSubview:dragDropView1];
    [_viewB addSubview:dragDropView2];
    [dragDropView1 setBackgroundColor:[UIColor redColor]];
    [dragDropView2 setBackgroundColor:[UIColor blueColor]];
    NSMutableArray *draggableSubjects = [[NSMutableArray alloc] initWithObjects:dragDropView1, dragDropView2, nil];
    NSMutableArray *droppableAreas = [[NSMutableArray alloc] initWithObjects:_viewA, _viewB, nil];
    _dragDropManager = [[DragDropManager alloc] initWithDragSubjects:draggableSubjects andDropAreas:droppableAreas];
    [draggableSubjects release];
    [droppableAreas release];

    UIPanGestureRecognizer * uiTapGestureRecognizer = [[UIPanGestureRecognizer alloc] initWithTarget:_dragDropManager action:@selector(dragging:)];
    [[self view] addGestureRecognizer:uiTapGestureRecognizer];
    [uiTapGestureRecognizer release];
}
...
@end

Step 2
The manager switches on the different states of the UIPanGestureRecognizer and detects if a panningĀ  starts on one of our dragSubjects. If so we store the DragContext containing the dragged view and the starting point (to be able to snap back to the offset in case the drag ends outside a drop zone). At the same time we change superview for the dragged object, by attaching it to the UIView spanning the entire area. This is done to ensure our UIView is shown above all other subviews and hence is visible during the entire drag drop session.

Step 3
During the drag process we simply move the object according to the coordinates of the UIGestureRecognizer.

Step 4
When the panning ends (the UIGestureRecognizer is in state UIGestureRecognizerStateEnded), we check to see if the dropped view is above any registered drop areas. If so, we add the dragged UIView as a subview of the detected drop area.

Step 4.a.
If we don’t recognize a valid drop area beneath our dragged view, we snap the dragged UIView back to its original superview and position.


The DragManager code can be seen here:

@implementation DragDropManager {

    NSArray *_dragSubjects;
    NSArray *_dropAreas;
    DragContext *_dragContext;
}

@synthesize dragContext = _dragContext;
@synthesize dropAreas = _dropAreas;

- (id)initWithDragSubjects:(NSArray *)dragSubjects andDropAreas:(NSArray *)dropAreas {
    self = [super init];
    if (self) {
        _dropAreas = [dropAreas retain];
        _dragSubjects = [dragSubjects retain];
        _dragContext = nil;
    }

    return self;
}

- (void)dealloc {
    [_dragSubjects release];
    [_dragContext release];
    [_dropAreas release];
    [super dealloc];
}

- (void)dragObjectAccordingToGesture:(UIPanGestureRecognizer *)recognizer {
    if (self.dragContext) {
        CGPoint pointOnView = [recognizer locationInView:recognizer.view];
        self.dragContext.draggedView.center = pointOnView;
    }
}

- (void)dragging:(id)sender {
    UIPanGestureRecognizer *recognizer = (UIPanGestureRecognizer *) sender;
    switch (recognizer.state) {
        case UIGestureRecognizerStateBegan: {
            for (UIView *dragSubject in _dragSubjects) {
                CGPoint pointInSubjectsView = [recognizer locationInView:dragSubject];
                BOOL pointInSideDraggableObject = [dragSubject pointInside:pointInSubjectsView withEvent:nil];
                NSLog(@"point%@ %@ subject%@", NSStringFromCGPoint(pointInSubjectsView), pointInSideDraggableObject ? @"inside" : @"outside", NSStringFromCGRect(dragSubject.frame));
                if (pointInSideDraggableObject) {
                    NSLog(@"started dragging an object");
                    self.dragContext = [[[DragContext alloc] initWithDraggedView:dragSubject] autorelease];
                    [dragSubject removeFromSuperview];
                    [recognizer.view addSubview:dragSubject];
                    [self dragObjectAccordingToGesture:recognizer];
                } else {
                    NSLog(@"started drag outside drag subjects");
                }
            }
            break;
        }
        case UIGestureRecognizerStateChanged: {
            [self dragObjectAccordingToGesture:recognizer];
            break;
        }
        case UIGestureRecognizerStateEnded: {
            if (self.dragContext) {
                UIView *viewBeingDragged = self.dragContext.draggedView;
                NSLog(@"ended drag event");
                CGPoint centerOfDraggedView = viewBeingDragged.center;
                BOOL droppedViewInKnownArea = NO;
                for (UIView *dropArea in self.dropAreas) {
                    CGPoint pointInDropView = [recognizer locationInView:dropArea];
                    NSLog(@"tag %li pointInDropView %@ center of dragged view %@", dropArea.tag, NSStringFromCGPoint(pointInDropView), NSStringFromCGPoint(centerOfDraggedView));
                    if ([dropArea pointInside:pointInDropView withEvent:nil]) {
                        droppedViewInKnownArea = YES;
                        NSLog(@"dropped subject %@ on to view tag %li", NSStringFromCGRect(viewBeingDragged.frame), dropArea.tag);
                        [viewBeingDragged removeFromSuperview];
                        [dropArea addSubview:viewBeingDragged];
                        //change origin to match offset on new super view
                        viewBeingDragged.frame = CGRectMake(pointInDropView.x - (viewBeingDragged.frame.size.width / 2), pointInDropView.y - (viewBeingDragged.frame.size.height / 2), viewBeingDragged.frame.size.width, viewBeingDragged.frame.size.height);
                    }
                }

                if (!droppedViewInKnownArea) {
                    NSLog(@"release draggable object outside target views - snapping back to last known location");
                    [self.dragContext snapToOriginalPosition];
                }

                self.dragContext = nil;
            } else {
                NSLog(@"Nothing was being dragged");
            }
            break;
        }
    }
}
@end

The DragContext containing the dragged view and the original position and view

#import "DragContext.h"

@implementation DragContext {

    UIView *_draggedView;
    CGPoint _originalPosition;
    UIView *_originalView;
}
@synthesize draggedView = _draggedView;

- (id)initWithDraggedView:(UIView *)draggedView {
    self = [super init];
    if (self) {
        _draggedView = [draggedView retain];
        _originalPosition = _draggedView.frame.origin;
        _originalView = [_draggedView.superview retain];
    }

    return self;
}

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

- (void)snapToOriginalPosition {
    [_draggedView removeFromSuperview];
    [_originalView addSubview:_draggedView];
    _draggedView.frame = CGRectMake(_originalPosition.x, _originalPosition.y, _draggedView.frame.size.width, _draggedView.frame.size.height);
}
@end

Room for improvements
- animations should be added to show more smooth look and feel.. This blog post demonstrates how to add animations to the snap back to orignial position.
- Consider the visible order of the views when selecting draggable UIView, if multiple draggable UIViews is stacked upon each other.

Posted in how to, ios, ipad, iphone, objective-c | 18 Comments »