I want to draw a simple line on the iPhone by touching and dragging across the screen. I managed to do that by subclassing UIView and change the default drawRect: method. At the same time in my viewcontroller I detect the touch event and call the [myView setNeedsDisplay] when necessary. The problem is that when I try to draw the second line the previous line disappears. Is there a way to keep the previous line on the screen?
Any input will be very much appreciated.
The usual method is to use CGBitmapContextCreate(). Create it in -init/-init-WithFrame:/etc and call CGContextRelease() in -dealloc. I'm not sure how you handle the 2x scale of the "retina display" with CGBitmapContextCreate().
(UIGraphicsBeginImageContext() is easier, but it might not be safe to do UIGraphicsBeginImageContext(); myContext = CFRetain(UIGraphicsGetCurrentContext()); UIGraphicsEndImageContext();.)
Then do something like
#import <QuartzCore/CALayer.h>
-(void)displayLayer:(CALayer*)layer
{
UIGraphicsPushContext(mycontext);
... Update the bitmap context by drawing a line...
UIGraphicsPopContext();
CGImageRef cgImage = CGBitmapContextCreateImage(mycontext);
layer.contents = (id)cgImage;
CFRelease(cgImage);
}
I've used -displayLayer: (a CALayer delegate function; a UIView is its layer's delegate) instead of -drawRect: for efficiency: if you use -drawRect:, CALayer creates a second context (and thus a second copy of the bitmap).
Alternatively, you might have luck with CGLayer. I've never seen a reason to use it instead of a bitmap context; it might be more efficient in some cases.
Alternatively, you might get away with setting self.clearsContextBeforeDrawing = NO, but this is very likely to break. UIKit (or, more accurately, CoreAnimation) expects you to draw the whole view contained in the clip rect (that's the "rect" argument of -drawRect:; it's not guaranteed to be the bounds). If your view goes offscreen, CoreAnimation might decide that it wants the memory back. Or CoreAnimation might only draw the part of the view that's on-screen. Or CoreAnimation might do double-buffered drawing, causing your view to appear to flip between two states.
If you use drawRect: to draw, then you need to draw the whole area. So you need to store not only the data for the latest part but everything.
As an alternative, you might draw directly into a bitmap, or generate dynamically subviews for your lines (makes only sense for very limited drawing (i.e. some few vector-based stuff).
Related
I have two square images in my UIVIew. Once I drag my finger from one image to another image I want to draw a straight line between them.
I have handled touchesMoved method to check when my touchLocation reaches the frame of either of the images. So I have handled the logic part of when to start drawing and between which two points.
I just cant figure out how to do that using (void)drawRect:(CGRect)rect. For one thing, I addded an NSlog in my drawRect and I wrote code to draw a line between two lines, even that's not happening.
I checked this question too, but I want to continue to drag and draw lines between multiple points.
- (void)drawRect:(CGRect)rect
{
NSLog(#"Draw Rect Entered");
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSaveGState(context);
CGContextSetStrokeColorWithColor(context, [[UIColor blackColor]CGColor]);
CGContextSetLineWidth(context, 1.0);
CGContextMoveToPoint(context, 0, 0);
CGContextAddLineToPoint(context, 20, 20);
CGContextStrokePath(context);
}
In order for the view to update you need to call -setNeedsDisplay or -setNeedsDisplayInRect: on the view that needs to be redrawn. You should do this in the touch handler once you have determined you need to draw the line. This will trigger the -drawRect: method on the view- you don't call -drawRect directly. For more information, read the UIView docs. I don't know whether you want your line to exist permanently once created- if there's a possibility it might be deleted, I'd add an if statement to -drawRect: controlled by a boolean determining whether to draw the line or not. Then you can easily switch it off if necessary.
Based on your comments below, I'd create an ivar array in which to store the points for the currently traced line and then loop over them in -drawRect:, using the Core graphics functions to draw the line a segment at a time. That way the new segment and all previous segments will be drawn with each redraw. Since CGPoint isn't an obj-c object, you'll either have to create a wrapper class or use obj-C++ and std::vector. You could also use a fixed size C-Array, (you mention you have precisely 10 points)- in that case you'd have to preset the unused coordinates to some arbitrary value defined as invalid (e.g. -500,-500) and add logic to not draw line segments if those values are encountered.
Minor point- I wouldn't hardcode the line coordinates - instead you could derive them from the images' frame property. That way your code will be more readable and won't break if you change the images or resize them.
There are two issues I have run into that could cause this problem:
1) Make sure you are calling setNeedsDisplay on the main thread
2) If you are working with multiple views make sure you are calling setNeedsDisplay against the correct one.
It is easy to work with one view and forget to call setNeedsDisplay against the parent view.
I'm well aware that there's no nice way to render a small part of a UIView to an image, besides rendering the whole view and cropping. (VERY expensive on something like an iPad 3, where the resulting image is huge). See here for a description of the renderInContext method (there's no alternatives).
The part of the view that I want to render can't be predetermined, meaning I can't set up the view hierarchy for the small section to be it's own UIView (and therefore CALayer).
My Idea...
I had an idea, but I need some direction if I'm going to succeed. What if I create a category on UIView (or CALayer) that adds a method along the lines of:
[UIView renderSubframe:(CGFrame)frame];
How? Well, I'm thinking that if a dummy view the size of the sub frame was created, then the view could be shifted temporarily onto this. Then the dummy view's layer could call renderInContext:, resulting in an efficient and fast way of getting views.
So...
I'm really not that up to speed with CALayer/Quartz core stuff... Will this have any chance of working? If not, what's another way I could achieve the same thing - what's the most efficient way I could achieve the same result (or has anyone else already faced this problem and come up with a different solution)?
Here's the category I ended up writing. Not as hard as I first thought, thanks to a handy CGAffineTransform.
#import "UIView+RenderSubframe.h"
#import <QuartzCore/QuartzCore.h>
#implementation UIView (RenderSubframe)
- (UIImage *) renderWithBounds:(CGRect)frame {
CGSize imageSize = frame.size;
UIGraphicsBeginImageContextWithOptions(imageSize, NO, 0.0);
CGContextRef c = UIGraphicsGetCurrentContext();
CGContextConcatCTM(c, CGAffineTransformMakeTranslation(-frame.origin.x, -frame.origin.y));
[self.layer renderInContext:c];
UIImage *screenshot = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return screenshot;
}
#end
A couple of different ways you might do this:
specify a CGContextClipToRect for your context before calling renderInContext;
use:
setNeedsDisplayInRect:
Marks the region within the specified rectangle as needing to be updated.
- (void)setNeedsDisplayInRect:(CGRect)theRect
This would make the rendering happen only in the specified rect. I am not sure though whether this would work as per your requirement.
I think that the first option should work seamlessly.
I need to draw incrementally in a subclassed UIView, but the view is cleared every time I call [self setNeedsDisplay].
I'm doing this:
- (id)initWithCoder:(NSCoder *)aDecoder {
if ((self = [super initWithCoder:aDecoder])) {
self.clearsContextBeforeDrawing = FALSE;
}
return self;
}
Am I missing something? How do I stop UIView from clearing?
If you call setNeedsDisplay it will call drawRect with the full bounds.
If you are drawing incrementally (portions of the view), you need to call:
[self setNeedsDisplayInRect:rect];
Of course that means your drawRect needs to be able to handle just drawing a portion of the view.
More info here: Drawing incrementally in a UIView (iPhone)
Also, you're setting it to FALSE and it's typically NO in objective-c. One resolves to 0 and the other to (BOOL)0 at compile time. A bit odd but not causing issues:
Is there a difference between YES/NO,TRUE/FALSE and true/false in objective-c?
Sorry.
drawRect: is not guaranteed use the saved contents of any previous drawing of a regular UIView. The framebuffer is likely behind an opaque path to the GPU, and can be thought of as essentially write only. So you always have to be able to recreate the entire view if you implement a drawRect for a regular UIView.
But you can draw into another context including previous graphics contents if you create your own bitmap drawing context, and draw into that context instead of a UIView. You can then convert that bitmap context into an image and display that image as the content of the CALayer of a UIView.
I derived a class from UIView, only to realize that there are real limitations on its size due to memory. I have this UIView inside UIScrollView.
Is there a way for me to put something inside a scroll view that is not a UIView-derived class but into which I can still draw, and which can be very very large?
I don't mind having to respond to expose-rectangle events, like one does when using conventional windowing systems.
Thanks.
The things inside of a UIScrollView must be UIViews, which are size-restricted for memory reasons. UIView maintains a bitmapped backing store for performance reasons, so it has to allocate memory proportional to its size.
The usual way that you handle this is to generate several UIViews and swap them out as the user scrolls around. The other version of that is to use CATiledLayer. Neither of those give you the "giant canvas" drawing model, though. It's up to you to break things up and draw them as needed. This is the usual approach, though.
If you really want a giant canvas, my recommendation would be a CGPDFContext. There is rich existing support for these, particularly using UIWebView (remember, you can open data: URIs to avoid reading files from disk). And you can draw parts of them directly by applying affine transforms and then CGContextDrawPDFPage. CGBitmapContext is another approach, but it could require a lot more memory for a small amount of drawing.
So you have a UIView inside a UIScrollView, but you want your UIView to have very large bounds (i.e., so it matches the size of your UIScrollView's contentSize). But you don't want to draw the entire UIView every time it needs displaying, nor can you fit its entire contents in memory at once.
Make your UIView uses a CAScrollLayer backing, as follows:
// MyCustomUIView.m
+ (Class) layerClass
{
return [CAScrollLayer class];
}
Add a method to update the scroll position when the user scrolls the UIScrollView containing your UIView:
// MyCustomUIView.m
- (void) setScrollOffset:(CGPoint)scrollOffset
{
CAScrollLayer *scrollLayer = (CAScrollLayer*)self.layer;
[scrollLayer scrollToPoint:scrollOffset];
}
Ensure that when you draw your UIView, you only draw the portions contained in the CGRect provided to you:
- (void)drawRect:(CGRect)rect
{
// Only draw stuff that lies inside 'rect'
// CGRectIntersection might be handy here!
}
Now, in your UIScrollViewDelegate, you'll need to notify your CAScrollLayer backed view when the parent UIScrollView updates:
// SomeUIScrollViewDelegate.m
- (void) scrollViewDidScroll:(UIScrollView *)scrollView
{
// Offset myCustomView within the scrollview so that it is always visible
myCustomView.frame = CGRectMake(scrollView.contentOffset.x,
scrollView.contentOffset.y,
scrollView.bounds.size.width,
scrollView.bounds.size.height);
// "Scroll" myCustomView so that the correct portion is rendered
[myCustomView setScrollOffset:self.contentOffset];
// Tell it to update its display
[myCustomView setNeedsDisplay];
}
You can also use CATiledLayer, which is easier because you do not have to track the scroll position — instead your drawRect method will be called with each tile as-needed. However this will cause your view to fade in slowly. It might be desirable if you intend to cache parts of your view and don't mind the slow updates.
I have read what I believe to be the relevant parts of the Quartz 2D Programming Guide, but cannot find an answer to the following (they don't seem to talk a lot about iOS in the document):
My application displays a drawing in a UIView. Every now and then I have to update the drawing in some way, e.g. change the fill colour of one of the shapes (I keep CGPathRefs to the important shapes to be able to redraw them with a different fill colour later). As described in the Section "Drawing With a CGLayer" on page 169 of the aforementioned document, I was thinking of drawing the entire drawing into a CGContext that I would obtain from a CGLayer, like so:
CGContextRef offscreenContext = CGLayerGetContext(offscreenLayer);
Then I could do my updating off-screen into the CGContext and draw the CGLayer into my UIView in the UIView's drawRect: method, like so:
CGContextDrawLayerAtPoint(viewContext, CGPointZero, offscreenLayer);
The problem I am having is, where do I get my CGLayer from? My understanding is I have to make it using CGLayerCreateWithContext and supply a CGContext as a parameter from which it inherits most of it's properties. Obviously, the right context would be the context of the UIView, that I am getting with
CGContextRef viewContext = UIGraphicsGetCurrentContext();
but if I am not mistaken, I can only get that within the drawRect: method and it is not valid to assume that the context I am given there will be the same one next time the method is called, i.e. I can only use that CGContext locally within the method.
So, how can I get a CGContext that I can use to initialise my CGLayer to create an offscreen CGContext to draw into and then draw the entire layer back into my UIView's CGContext?
PS: While you're at it; if anything above does not make sense or is not sane, please let me know. I am just starting to get my head around Quartz 2D.
First of all, if you are doing it from in an iOS environment, I think you are right. The documentation clearly said that the only way to obtain a CGContextRef is by
CGContextRef ctx = UIGraphicGetCurrentContext();
Then you use that context for creating the CGLayer with
CGLayerRef layer = CGLayerCreateWithContext(ctx, (CGSize){0,0}, NULL);
And if you want to draw on that layer, you have to draw it with the context you get from the layer. (It is somewhat different from the context you passed in earlier to create the CGLayer). Im guessing the CGLayerCreateWithContext saves the information it can get from the context passed in, but not everything. (One of the example is the ColorSpace information, you have to re-specify when you fill something with the context from CGLayer).
You can get the CGLayer context reference from the CGLayerGetContext() function and use that to draw.
CGContextRef layerCtx = CGLayerGetContext(layer);
CGContextBeginPath(layerCtx);
CGContextMoveToPoint(layerCtx, -10, 10);
CGContextAddLineToPoint(layerCtx, 100, 10);
CGContextAddLineToPoint(layerCtx, 100, 100);
CGContextClosePath(layerCtx);
One point that I found out is when you draw something offscreen, it automatically clips the thing offscreen. (make sense, so it doesnt draw things that is not seen) but when you move the layer (using the matrix transformation). The clipped path is not showing (missing).
One last thing, if you save the reference to a layer into a variable and later on you want to draw it, you can use CGContextDrawLayerAtPoint() method like
CGContextDrawLayerAtPoint(ctx, (CGPoint) {newPointX, newPointY}, layer);
It will sort of "stampt" or "draw" the layer at that newPointX and new PointY coordinate.
I hope that answer your question, if its not please let me know.