Completion Blocks Objective-C - objective-c

Question

I'm trying to write a completion handler in IOS with a block and am not sure exactly why it's not working.
This is what I have so far
typedef void(^myCompletion)(BOOL);
-(void) showAnswer {
[self showAnswer:^(BOOL finished) {
if (finished) {
LnbScene *scene = (LnbScene *)gameScene.lnbScene;
//once the tiles position have been updated then move to the next riddle
[scene nextRiddle];
}
}];
}
// This method should update the position of tiles on the scene
-(void) showAnswer:(myCompletion) compblock{
LnbScene *scene = (LnbScene *)gameScene.lnbScene;
NSArray *tilesArray = [scene.tilesBoundary children];
for (Tile *tile in tilesArray) {
if (tile.positionInAnswer != 17) {
[tile removeFromParent];
[scene.spacesBoundary addChild:tile];
CGPoint targetPoint = CGPointMake(tile.targetPoint.x, tile.targetPoint.y + 6);
tile.position = targetPoint;
}
}
compblock(YES);
}
And I am calling the showAnswer method from a Scene as follows:
GameViewController *gVC = (GameViewController *)self.window.rootViewController;
[gVC showAnswer];
As I step through the code I don't encounter any errors and the sequence proceeds as expected ie. The tile positions are changed and then the completion handler triggers to the nextRiddle method. Only problem is that none of the interface is updated. Is there something I am missing?
I've tested that the tile repositioned code works by taking out the completion block and I view it in the interface and get the desired results. So I'm pretty sure the problem lies in how I'm implementing the blocks. I've never written a block before so I'm really in the dark.
-(void) showAnswer {
LnbScene *scene = (LnbScene *)gameScene.lnbScene;
NSArray *tilesArray = [scene.tilesBoundary children];
for (Tile *tile in tilesArray) {
if (tile.positionInAnswer != 17) {
[tile removeFromParent];
[scene.spacesBoundary addChild:tile];
CGPoint targetPoint = CGPointMake(tile.targetPoint.x, tile.targetPoint.y + 6);
tile.position = targetPoint;
}
}
}

Answer 1

I think the problem seems to be a conceptual mistake - that the "flow" of code
in the -(void) showAnswer:(myCompletion) comp block method truthfully represents the flow of UI elements in time in the presentation layer.
However in practice it is not that straightforward. The UI layer is rendered with 60 Hz framerate. That means you see 60 (frames) screens per second and contents of each of those screens needs to be calculated in advance, processed, flattened, rendered.
That means 1 frame (screen) cycle/pass lasts approx. 16 milliseconds. If I remember correctly you as a programmer have 11 milliseconds to provide all the relevant code so that it can be processed into visual info. This chunk of code is processed all at once which is the answer to our problem.
Imagine if you had a method that would say
{
//some other UI code...
self.view.backgroundColor = [UIColor greenColor];
self.view.backgroundColor = [UIColor yellowColor];
//some other UI code...
}
Now imagine we are inside 1 such 16 millisecond pass.
This piece of code would be processed in advance before any rendering happens and the resultative background colour will be yellow. Why? Because in the cycle preparation phase this code is processed and the framework interprets the green colour as pointless because the value is immediately overwritten by a yellow one.
Thus instructions for the renderer will contain only the yellow background information.
Now back to your actual code. I think all your code is processed fast enough to fit into that 11 milisec window, and the problem is that you do not really wait for the swapping of tiles because there is that block with YES parameter as part of the method and that one already slides to whatever is next.
So the rendered does get instructions to simply move on the the next thing. And it does not animate.
Try putting this "tile swapping" into the "animation" block
[UIview animateWithDuration:....completion] method..and put your block into the completion block. Yes you will have a block in a block but thats fine.
[UIView animateWithDuration:0.4f
animations:^{
//tile swapping code
}
completion:^(BOOL finished)
{
complBlock(YES);
}];
I am not completely sure if it will work but let's try.

Answer 2

The UI relate method should call on main thread. U can try this:
[self showAnswer:^(BOOL finished) {
if (finished) {
LnbScene *scene = (LnbScene *)gameScene.lnbScene;
dispatch_async(dispatch_get_main_queue(), ^{
[scene nextRiddle];
});
}
}];