Black Corners On Grouped UITableViewCells Only Aft

问题:

I am no graphics expert but I somehow managed to make some good looking custom grouped UITableViewCells by setting the background view to a backgroundView with some CG code. In all SDK's up to 3.1.3 (maybe 3.2... I haven't tested on the iPad) this was working great, but I think a more recent SDK has introduced a change in the way graphics are cached offscreen.

Upon first render, everything is great: The drawing is fine and the corners are transparent. If I push a couple of view controllers on the navigation stack and come back, there are now black corners that appear in the views:

BEFORE && AFTER

alt text http://new.tinygrab.com/c1f9e8196b4b92e6bb4d3130a7e08974.png     alt text http://new.tinygrab.com/531ee83e42cdeadc78d418d1c5e7c513.png

I have tons of code, most of which is written up here. I have tried tweaking to the best of my ability, looking at the docs for applicable changes, but after at least 8 hours in I still cannot find what might cause this. I have tried setting every view I can think of to be backgroundColor=clearColor and opaque=NO What else am I missing? Any debugging tips?

UPDATE:

I have some debug code in viewDidAppear that prints the backgroundColor and class description of all the subviews.

- (void)debugView:(UIView *)view {
    DebugLog(@"%@ - %@", view.backgroundColor, [[view class] description]);
    for (UIView* child in view.subviews) {
        [self debugView:child];
    }
}
- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    [DownloadController.networkQueue setSuspended:NO];
    for (TTTableViewCell *cell in [self.tableView visibleCells]) {
        [cell debugView:cell];
    }
}

With this code, I inspect the backgroundColor settings of the cell views on first load, when it is fine, and then again after coming back. There are some differences, but all the colors are still set to be clear. This leads me to believe the issue is underneath the UITableViewCell.

UPDATE 2:

I have created a simple sample application to highlight the problem.

回答1:

I have tested the sample application and can reproduce the black corners issue.

After some experiments, it seems that the black corners issue is related to the caching of the layers used to render the table view. The geometry of the cell's layer seems to be important:

• On the first paint, the cell is asked to be painted into a rect. Your code is painting a rounded path, but clipped out the corners. As the underlying tableview is already drawn, no problem occurs. The rect zone is cached, with its corners unpainted.
• When the controller is pushed, a cached image is stored, with rectangular placeholders for the cells.
• When the controller is popped, the cached image and the cells are drawn. But the place to draw cells is rectangular but the cell's cached image is not, leading to black corners.

In order to get rid of the black corners you can:

• Make sure that all the cell's rect is painted. This means using the same color to file the cell before drawing the edge as the tableview's background color. If your tableview use the default background color, you can use [UIColor groupTableViewBackgroundColor].CGColor as filling color; it is a pattern based color and follows device orientation (yeah); but the painting is not perfectly aligned with the background (damn).
• Use a CALayer mask on the cell's layer. This implies creating a mask CGImage, set it as the layer's content and assign the mask layer to the cell's layer. Not sure about the performance.

Hope it helps a bit.

Update

After some unsuccessful attempts, I dropped the mask idea because it was too clumsy.

I have re-read the code for the cell's layer and found out a way to remove the black corners in a simple way. The basic idea is that a CAGradientLayer is fully transparent only if its gradient colors are clear. By using the following display method, the black corners went away (both on the simulator and on the device):

- (void)display {
    if (_override) {
        self.colors =
        [NSArray arrayWithObjects:
         (id)[UIColor colorWithRed:colorComponents[0] green:colorComponents[1] blue:colorComponents[2] alpha:colorComponents[3]].CGColor,
         (id)[UIColor colorWithRed:colorComponents[4] green:colorComponents[5] blue:colorComponents[6] alpha:colorComponents[7]].CGColor,
         nil];
    } else {
        self.colors =
        [NSArray arrayWithObjects:
         (id)[UIColor clearColor].CGColor,
         (id)[UIColor clearColor].CGColor,
         nil];
    }
    [super display];
}

Of course, this could be optimized a bit:

• Create the color arrays once.
• Provide a custom setter for override property that change the layer's colors.
• Remove the display method as it is no needed anymore.

回答2:

I experimented with your sample code using OS3.2 on the simulator and it definitely shows the same symptom. I tried a few things and ended up with this fix:

self.cornerRadius = 12;

Add this into UAGradientCellBackgroundLayer, in init method.

回答3:

Throwing this out there (perhaps it will help), adding the following to cellForRowAtIndexPath has an interesting effect:

cell.backgroundView.alpha = 0.4;
cell.selectedBackgroundView.alpha = 0.4;

Not all of the cells have the black background. Changing the alpha seems to change the probability that any given cell will improperly render. alpha = 1.0 guarantees a black background, whereas any lower decreases the probability of this effect.

The black background also fades, so I'm sure you all knew, but yeah, it has to do with the UAGradientCellBackgroundView.

Good luck!