I'm using this to move my image:
meteorDisplayLink = [CADisplayLink displayLinkWithTarget:self selector:#selector(moveImage:)];
Here is my moveImage: method:
-(void) moveImage:(CADisplayLink *)sender{
CGPoint img;
imgLocation = imgImageView.center;
if (img < 100) {
img.y++;
}
else{
imgLocation.y = 0;
}
imgImageView.center = imgLocation;
}
This updates the location of my image on screen no matter what. Why does setNeedsDisplay have no effect here? What is the point of setNeedsDisplay?
I assume imgImageView is an instance of UIImageView. If it's not an instance of UIImageView, edit your question to tell us what it is.
Changing the center property of a view never requires your app to redraw the view. The display server (which is a separate process named backboardd in iOS 6 and springboard in earlier versions) has a copy of your view's pixel data already. When you change a view's center, the view just informs the display server of its new position, and the display server copies the view's pixel data to the appropriate parts of the screen.
Thus changing the center property of a view never requires you to send setNeedsDisplay.
More specifically, UIImageView doesn't even implement drawRect:. Instead, UIImageView just sends its image's pixel data to the display server when you set its image. So even if you send setNeedsDisplay to a UIImageView, it still won't run any drawRect: method.
By the way, you can do this in your own UIView subclasses too, by setting self.layer.contents to a CGImageRef instead of implementing drawRect:. You will need to #import <QuartzCore/QuartzCore.h> to do this. Of course, UIImageView does other things too, like handling animated and resizable images.
When you set properties that affect the layout of an object, setNeedsDisplay will be called internally. You need to call it yourself for custom properties, etc, that don't automatically call it.
setNeedsDisplay only applies to the contents of a layer or view. I.e., if the contents of a view has changed and needs to be redrawn, you would use setNeedsDisplay to tell the view that it needs to be redrawn and that would trigger its drawRect method.
Transformations on a view, such as position, scale, and rotation, etc., are handled directly by the GPU and do not require the view to be redrawn; therefore, setNeedsDisplay does not apply when you're simply moving or transforming a view.
Related
On Android I have item views with potentially large widths on a horizontal scrolling list. The view loads and draws chunks "images" as parts of the view become visible on the list. This is an optimization to avoid drawing all images at once as it would be wasteful and slow. The content drawn is essentially an audio waveform. Do to the ways things need to work I can't split the chunks as individual view items on a list. This strategy works perfect because of how android drawing architecture works.
Now I'm trying to use a similar pattern in iOS but I'm having some trouble and I'm not too sure how to come up with a solution.
In iOS I'm are using a UICollectionView which draws large width Cells and we need that same optimization of loading and drawing only the visible chunks.
Solution 1:
Check what parts of the UIView is visible and draw only those visible chunks. The problem with this solution is that as the UICollectionView scrolls the UIView doesn't draw the next chucks that are becoming visible. Here are examples of what I'm talking about.
UIView loads initial chunks
These can be seen by their different chunk colors:
Scroll a bit
Black is shown and no content is loaded because there is no hint that the view needs to draw again so we can't load the next visible chunk.
Solution 2:
Use custom UIView backed by a CATiledLayer. This works perfect because it draws the tiles as they become visible while scrolling the UICollectionView.
The problem is that drawing happens on the background thread or on the next drawing cycle if shouldDrawOnMainThread is defined. This brings issues when the UIView is resized or my internal zoom logic kicks. Things shift because the drawing cycle is not synchronized to the view resizing.
So how could I get notified like CATiledLayer that a part is becoming visible and I could draw normally like a CALayer backed UIView?
UPDATE 1
I'm looking into using preferredLayoutAttributesFittingAttributes as a way to check if a new chunk need to be drawn. This is called every time the cell is moved into a new position because of scrolling. Hopefully, this isn't a bad idea. :)
- (UICollectionViewLayoutAttributes *)preferredLayoutAttributesFittingAttributes:(UICollectionViewLayoutAttributes *)layoutAttributes
UPDATE 2
After much testing and playing around. Using solution 1 is not an option. When a UIView reaches a huge width memory usage goes off the roof while using CATiledLayer memory usage is at minimal as I guess one would expect. :/
I don't know if you can completely turn off the effect. You can soften it by setting the fadeDuration to 0. To do this, you must create a subclass of CATiledLayer and overwrite the class method fadeDuration.
You can also try to implement shouldDrawOnMainThread class method, but this is a private method, and you're risking a rejection on the App Store:
#implementation MyTiledLayer
+(CFTimeInterval)fadeDuration {
return 0.0;
}
+ (BOOL)shouldDrawOnMainThread {
return YES;
}
#end
or in Swift:
class MyTiledLayer: CATiledLayer {
override class func fadeDuration() -> CFTimeInterval {
return 0.0
}
class func shouldDrawOnMainThread() -> Bool {
return true;
}
}
If you want to implement your own tiled layer instead, you should waive of the tiles and try to compute the visible part (section and zoom level) of the view. Than you can draw only this part as layer whole content.
Apparently, there is no easy way to re-draw the view when the scrollview is scrolling. But you may implement scrollViewDidScroll:, and trigger the redrawing manually. The visibleRect method always returns the complete area of the layer, but you can use
CGRect theVisibleRect = CGRectIntersection(self.frame, self.superlayer.bounds);
or in Swift
let theVisibleRect = frame.intersection(superlayer.bounds)
to determine the visible area of the layer.
Was hoping someone could help with this please? I've scanned through the forum and apple docs and can't find anything that matches my problem exactly.
I'm trying to animate a custom 'Composite' UIView derived class (with overidden drawRect) by changing a custom colour property (not one of the standard view properties like centre, alpha, backgroundColour etc), calling setNeedsDisplay and redrawing the view using drawRect over X seconds duration to produce a slow colour change from say, red to blue.
At the moment, all Composite::drawRect does internally is clear the context (the view has a default background colour set to clear) and fill the view using the colour provided. This works fine the first time it's called - the view appears on screen in the correct red colour.
My problem is that when I try to animate the view by updating the colour property, even though drawRect is being called everytime I call setNeedsDisplay and the colour being fed into the view instance is correct the view doesn't update on screen until I stop calling setNeedsDisplay (or rather, stop updating the custom view colour property). It's like every call to setNeedsDisplay pushes the view to the back of a list so it doesn't get updated on screen until it stops getting called (even though drawRect is being called).
For example - trying to change the colour from red to blue over 10 seconds, the view stays red for 10 seconds, then turns to blue in a single frame at the end of the property changing (and presumably when I stop calling setNeedsDisplay) rather than a slow fade between the two over the time duration.
Any ideas what's going on? I'm guessing I need to animate this using apple's own animation property stuff but I'd really prefer not to - I'm trying to make this cross platform if possible and I don't see why my own method shouldn't work.
The basics:
In case it matters, my composite view instance is a subview of an UIImageView. (I know imageview drawRect doesn't get called but I can't imagine this is a problem for it's subviews - after all, my Composite::drawRect is definitely being called).
I'm setting the right colour on my view instance - I've debug printed this to be sure - so the colour provided to the view goes from red to blue over however many seconds / frames.
I'm setting setNeedsDispay and drawRect is being called every time - again, debug printing confirms this.
This is happening over multiple frames and the app goes through the drawing update at the end of every frame - I.e I'm not blocking the render thread or anything - the app runs fine, it just doesn't update the view on screen even though its drawRect is being called, until I stop manipulating the property. Someone mentioned a similar problem where they were blocking the render thread in a single frame but I'm definitely not doing anything like that.
I've tried messing around with the following but they all produced the same results (i.e. view changed from red to blue in a single frame rather than a slow change over).
Updating the colour every frame but only calling setNeedsDisplay on my view every Xth frame (5, 10, 20, 30 etc)
Updating the colour every frame but not calling setNeedsDisplay unless the corresponding drawRect call from the last setNeedsDisplay has happened (i.e. set a flag to false in SetColour func, set same flag to true in drawRect, test against it when decided to call setNeedsDisplay) - same results.
setting the view bounds with ContentMode set to redraw to force an update - same results
tried calling setNeedsDisplay on the parent image view too - no change.
I'm guessing that this just isn't possible - can anyone explain why its not updating the v view onscreen even though drawRect is being called? Any ideas how I should do this? I don't think I would be able to do what I want to do if I tried animated one of the default view properties using apple's internal animation system but if I've got to use that....
Thanks!
:-)
I am working on a graphing application which takes in a stream of data and displays it. It turns out that drawing a lot of line segments in UIView (Core Graphics) is a very slow. I'm trying to speed it up by only drawing a portion of the screen that has new data.
I currently use this function to redraw the screen (using Xamarin/monotouch)
NSTimer.CreateRepeatingScheduledTimer (TimeSpan.FromSeconds (0.1), delegate {
pane.SetNeedsDisplay ();
});
where pane is my UIView object. The pane object knows itself what part of the screen needs to be redrawn. I think there's another method that specifies a region of the screen to redraw - is that the right way to go? Or some other solution?
Also, the code draws a set of grids. These never change. Is there a way to save them to some layer or something so that I can just add it to the background?
The alternative is to go with openGL though drawing lines doesn't look as good and I'll have to implement a lot of extra code to make it look right.
Thanks!
For the graph data, you should be using CoreGraphics directly to render the lines in your UIView subclass' drawRect: method. Make sure that method only redraws the data that overlaps with the CGRect argument. The method setNeedsDisplay marks the entire UIView as needing a redraw, so instead of using that when your graph data changes, you should use the UIView's method setNeedsDisplayInRect: to indicate that there's new data in that region of the UIView that will call drawRect: to be called, which will only redraw that portion (if you've implemented it right).
Regarding the background grid, you can render the grid to a UIImage then display that UIImage behind your view that draws the graph. You'll want to look at the documentation for UIGraphicsBeginImageContext, CALayer renderInContext:, UIGraphicsGetImageFromCurrentImageContext, and UIGraphicsEndImageContext.
You can use this variant:
void SetNeedsDisplayInRect (RectangleF rect);
That will only request that the specified region be redrawn.
When creating an iOS app, I'm confused as to when exactly I need to call setNeedsDisplay? I know that it has something to do with updating/redrawing the UI; however, do I need to call this every time I change any of my views?
For example, do I need to call it:
After programatically changing the text in a text field
When changing the background of a view?
When I make changes in viewDidLoad?
How about in viewDidAppear?
Could someone give me some general guidelines regarding when to use this method?
You should only be calling setNeedsDisplay if you override drawRect in a subclass of UIView which is basically a custom view drawing something on the screen, like lines, images, or shapes like a rectangle.
So you should call setNeedsDisplay when you make changes to few variables on which this drawing depends and for view to represent that change , you need to call this method which internally will give a call to drawRect and redraw the components.
When you add an imageView or a UIButton as a subview or make changes to any subview, you need not call this method.
Example:
You have a view that shows a moving circle, either you touch and move it, or may be timer based animation.
Now for this, you will need a custom view that draws a circle at given center and with given radius.
These are kept as instance variables which are modified to move the circle by changing its center or make it bigger by increasing radius of it.
Now in this case either you will modify these variables(centre or radius) in a loop and timer Or may be by your fingers in touchesEnded and touchesMoved methods.
To reflect the change in this property you need to redraw this view for which you will call setNeedsDisplay.
You only really need to call -setNeedsDisplay on UIView subclasses that draw their contents with -drawRect:.
For labels and other standard controls, changing the text will automatically cause the label to redraw so you don't need to do this yourself.
setNeedsDisplay: should be called when you want to refresh your view explicitly. It just sets an internal flag, and the iOS UI system will call drawRect: at an appropriate time later.
It sounds like it should be always called when you updating any property which may change the presentation. But it's not. Almost all the standard UI controls already handled that. I believe whenever you modify the properties of standard UI components (views), setNeedsDisplay: would be triggered internally, and the affected region will be redrawn. (In all the situations you listed)
However, if you create your own view, implement its own drawRect:, and want to update that when something has been changed, you must call setNeedsDisplay: explicitly.
I think #Amogh Talpallikar make it clear. And I just wanna discuss one thing more.
In the fact that, you should avoid override drawRectunless you really need it because it can cause bad performance. You can refer this https://yalantis.com/blog/mastering-uikit-performance/
If you only wanna change frame, position of buttons, labels, ... you can call setNeedLayout or layoutIfNeeded
You will call setNeedDisplay when you are changing the property on which your view custom drawing depends. It will explicitly call drawRect: method forcefully.
I have some views that have content, textures and shadows mostly, that need to be moved by sliding them to another part of the screen.
These textures are created in drawRect: and only need to be rendered once in the life of the view (with the possible exception of optionally recolouring them).
However, the slide is a little jerky, aI assume this is because the texture keeps being redrawn.
I was wondering if there is any value in rendering the texture by directly using the views CALayer. Will this avoid the texture being re rendered whenever the view moves?
I have never used CALayer before.
thanks
karl
If you override drawRect and use your view like that, it'll be redraw every time. What you can do is create a method that draw your view once and that is being called once (let's say after init) and don't override drawRect. That should fix your performance issue.
One more thing, every view are backed by a layer (CALayer), if you do animation and you want better performance you should move the layer instead of moving his view (UIView) during animation. To access the layer of a view, just do myView.layer.
Hope this was helpful.