UITextView textContainer exclusion path fails if full width and positioned at top of textContainer

问题

In iOS 8, I'm trying to add a UIImageView as a subview of a UITextView, similar to what's shown here - but with the text below the image.

I want to do it using an exclusion path because on other devices, I might position the image differently depending on the screen size.

However there's a problem where if the CGRect used to create the exclusion path has a Y origin of 0, and takes up the full width of the textView, the exclusion fails and the text appears within exclusion path (so that the text is shown behind the imageView, as you can see in that screenshot).

To test this I built a simple app using the "single view" Xcode template, with the following:

- (void)viewDidLoad {
    [super viewDidLoad];

    // set up the textView
    CGRect frame = CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height);
    UITextView *textView = [[UITextView alloc] initWithFrame:frame];
    [textView setFont:[UIFont systemFontOfSize:36.0]];
    [self.view addSubview:textView];
    textView.text = @"I wish this text appeared below the exclusion rect and not within it.";


    // add the photo

    CGFloat textViewWidth = textView.frame.size.width;

    // uncomment if you want to see that the exclusion path DOES work when not taking up the full width:
    // textViewWidth = textViewWidth / 2.0;

    CGFloat originY = 0.0;

    // uncomment if you want to see that the exclusion path DOES work if the Y origin isn't 0
    // originY = 54.0;

    UIImageView *imageView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"photo_thumbnail"]];
    imageView.frame = CGRectMake(0, originY, textViewWidth, textViewWidth);
    imageView.alpha = 0.7; // just so you can see that the text is within the exclusion path (behind photo)
    [textView addSubview:imageView];


    // set the exclusion path (to the same rect as the imageView)
    CGRect exclusionRect = [textView convertRect:imageView.bounds fromView:imageView];
    UIBezierPath *exclusionPath = [UIBezierPath bezierPathWithRect:exclusionRect];
    textView.textContainer.exclusionPaths = @[exclusionPath];
}

I also tried subclassing NSTextContainer and overriding the -lineFragmentRectForProposedRect method, but adjusting the Y origin there doesn't seem to help either.

To use the custom NSTextContainer, I set up the UITextView stack like this in viewDidLoad():

// set up the textView stack
NSTextStorage *textStorage = [[NSTextStorage alloc] init];

NSLayoutManager *layoutManager = [[NSLayoutManager alloc] init];
[textStorage addLayoutManager:layoutManager];

CGSize containerSize = CGSizeMake(self.view.frame.size.width, CGFLOAT_MAX);
CustomTextContainer *textContainer = [[CustomTextContainer alloc] initWithSize:containerSize];
[layoutManager addTextContainer:textContainer];

CGRect frame = CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height);
UITextView *textView = [[UITextView alloc] initWithFrame:frame textContainer:textContainer];
[textView setFont:[UIFont systemFontOfSize:36.0]];
[self.view addSubview:textView];

textView.text = @"I wish this text appeared below the exclusion rect and not within it.";

Then I adjust the Y origin in the CustomTextContainer like this... but this fails just as spectacularly:

- (CGRect)lineFragmentRectForProposedRect:(CGRect)proposedRect atIndex:(NSUInteger)characterIndex writingDirection:(NSWritingDirection)baseWritingDirection remainingRect:(CGRect *)remainingRect {

    CGRect correctedRect = proposedRect;

    if (correctedRect.origin.y == 0) {
        correctedRect.origin.y += 414.0;
    }

    correctedRect = [super lineFragmentRectForProposedRect:correctedRect atIndex:characterIndex writingDirection:baseWritingDirection remainingRect:remainingRect];

    NSLog(@"origin.y: %f | characterIndex: %lu", correctedRect.origin.y, (unsigned long)characterIndex);

    return correctedRect;
}

I suppose this could be considered an Apple bug that I need to report (unless I'm missing something obvious), but if anybody has a workaround it would be much appreciated.

回答1:

This is an old bug.In the book Pushing the Limits iOS 7 Programming ,the author wrote this in page 377:

At the time of writing, Text Kit does not correctly handle some kinds of exclusion paths. In particular, if your exclusion paths would force some lines to be empty, the entire layout may fail. For example, if you attempt to lay out text in a circle this way, the top of the circle may be too small to include any text, and NSLayoutManager will silently fail. This limitation impacts all uses of NSTextContainer. Specifically, if lineFragmentRectForProposedRect:atIndex:writingDirection:remainingRect: ever returns an empty CGRect, the entire layout will fail.

Maybe you can override lineFragmentRectForProposedRect:atIndex:writingDirection:remainingRect: of your custom NSTextContainer to workaround.

回答2:

I came across this problem as well. If you only need to exclude full width space at the top or bottom of the textView, you can use the textContainerInset.

回答3:

Workaround suggestion:

Add a line break to your text.

textView.text = "\n" + textView.text

回答4:

A quick workaround:

CGRect exclusionRect = [textView convertRect:imageView.bounds fromView:imageView];
if (exclusionRect.origin.x <= 0 && exclusionRect.origin.y <= 0 && exclusionRect.size.width >= textView.bounds.size.width) {
  exclusionRect.origin.x = 1;
  exclusionRect.origin.y = 1;
  exclusionRect.size.width -= 2;
}

Your image will still draw the same and unless you're using a font with glyphs that are 1px wide (I'm not even sure that's possible given kerning, etc), your exclusionRect will be guaranteed to be smaller than the full width.

I would be interested to know what kind of results you see if you allow your rect to be moved around in real-time. Attach a UIPanGestureRecognizer and update your exclusionRect as you pan around the screen. At what point does the text jump into the image?

Edit: If you're seeing problems until it is able to fit at least one character, maybe try adjusting your text frame.

if (exclusionRect.origin.x <= 0 && exclusionRect.origin.y <= 0 && exclusionRect.size.width >= textView.bounds.size.width) {
  frame.origin.y += CGRectGetMaxY(exclusionRect);
  frame.size.height -= CGRectGetMaxY(exclusionRect);
  [textView setFrame:frame];
}

回答5:

I tried too hard toying with exclusion path.

My problem was that top level exclusion path never worked, it pushed content towards the top instead of center, while bottom content had double the exclusion path margin.

Here is what worked for me:

Override UITextView
Init it with your textcontainer by putting this inside init:

super.init(frame: frame, textContainer: textContainer)
As per Daniel's answer, put inside layoutSubviews:

self.textContainerInset = UIEdgeInsets.init(top: t, left: l, bottom: b, right: r)
Inside your UITextView subclass init() or in storyboard, disable scrolling.

self.isScrollEnabled = false

For more details, read this super-helpful thread.

来源：https://stackoverflow.com/questions/26632474/uitextview-textcontainer-exclusion-path-fails-if-full-width-and-positioned-at-to

标签

ios

uitextview

nstextcontainer