I have a CollectionView on a page that is displaying data from a List<> of items. I do sorting and filtering in a code behind on the page by changing the List with LINQ, and then setting the BindingContext of the view, like this:
MyView.BindingContext = FilteredData;
On IOS this works fine, every time, no problem. On Android, I can change the data a few times (it varies - normally three to five) and then the page doesn't display any data. In the debugger I can check and still see the ItemsSource has all of the items, but nothing is displayed. I then "switch" to my other view on the page, which is done by hiding one DataGrid row and showing another, then "switching" back. The data still isn't displayed, but after I do that I can run the code again that does the sorting and filtering and it starts working again - the data is displayed on the page. Not only that, but after I do that, it works every time from that point forward on Android.
I've spent hours trying every thing I can think of to try and find an event or property or anything that would indicate when this problem occurs so I can try and find a work around for it. I'm at a complete loss; wondering if there are suggestions for how to capture and/or fix this problem.
I did find a combination that worked to resolve this issue. I changed the page so that the CollectionView at issue is in the Grid row that has a non-zero (i.e. visible) height when the ContentPage loads. After doing that, the code works as expected. Previously, it was in a Grid.Row whose height was set to an absolute 0 when the page loaded, and then the height was changed to a non-zero value when a button was clicked.
FWIW, this is similar to other issues I've seen with MAUI on Android, where the visibility of certain controls as well as the order in which you modify them can interfere with rendering data binding results.
I solved sorting using a SortedList and made the list items inherit from IComparable, but I don't love the solution because a SortedList is not an ObservableCollection, and because implementing IComparable can be tricky and leaky.
I just discovered QField and I amazed by its features! Now I am trying to create a project that will fit my needs and have come quite far. I just have one questing on adding features in the app:
When adding a feature in the Qfield App in my project, everything is on one page and I have to scroll down in order to reach all input fields. This can be quite tedious while working in the field.
In the documentation and the demo project, that follows when downloading Qfield from Play store (offline_bees.qgs), the input is split up into sub-menus which you can reach by scrolling sideways (see image). This is much more effective than having everything on one page and having to scroll downwards. Unfortunately, I haven't found out how you can split your feature into such sub-menus as seen here. I have played a bit around with the available input widgets in the attribute form, to no avail.
I have also read the available documentation for Qfield 1.0 multiple times, but since I am neither a frequent user of QGIS nor a programmer, it is a bit challenging for me to understand the instructions (https://qfield.org/docs/project-management/vector-layers.html). Does it have something to do with the Relation Widget or Relation Reference?
I am thankful if somebody could point me in the right direction.
Looks like I didn't play around enough. I found out how to solve my problem. Here are the instructions
In QGis while preparing the project for QField:
Go to layer properties/ attribute forms
Set to "Drag and drop designer" (I failed to find this at first)
Create groups (these will be the sub menus) with the "+" button
Drag and drop your fields into the created groups
Some of the stakeholders on my project expressed doubt in easily recognizing hidden vertical content to scroll to in an MGWT ScrollPanel (without taking a swipe at it). As an attempt to address the concern I'd like to always show the pretty semi-transparent vertical scrollbar as both a hint of more content to scroll to and an indication of the ratio between the amount of displayed vs. all content in the panel.
Is it possible to keep the panel's scrollbars permanently displayed? At first I thought the setShowScrollBarY(boolean) would do the trick but quickly proved myself wrong.
P.S.: Shouldn't there be an m-gwt StackOverflow tag by now? AFAIKT MGWT has gained sufficient traction to have an MGWT-specific stream of questions.
I have had the same issues with clients that suggested that showing scrollbars would be a good idea. If you cut your content right its easy to see for the user that there is more content..
Anyhow I just added a setHideScrollBar(boolean) to the trunk. Download it from ( https://oss.sonatype.org/service/local/artifact/maven/redirect?r=snapshots&g=com.googlecode.mgwt&a=mgwt&e=jar&v=LATEST) and give it a try. Maybe we should also think of an option to flash the scrollbars once to indicate that there is more content.
By the way I would be supporting an mgwt specific tags as I do with the mailing list: https://groups.google.com/group/mgwt
I need to freeze the first column and first row of data in a CellTable, so that users can scroll through the data but still see the labels on the "axes." The first column should scroll when the user scrolls up and down, and the header row should scroll when the user scrolls left and right. Think "Freeze Panes" in Excel.
I'm using GWT 2.1 and am willing to write my own widget to do this if no solutions already exist. My question is a two-parter:
Do any widgets already have this behavior?
Any suggestions if I'm going to implement this myself?
Thanks!
I implemented a solution myself. Check out http://larkolicio.us/ScrollTable/ExperimentTables.html
It's a LayoutPanel with three AbsolutePanels inside it. The frozen columns are a CellTable, the main part is a CellTable, and the header is a Grid - I could find no way to set the width of a CellTable column! A ScrollHandler links the main part to the two frozen parts. There is a little bit of delay - I'd appreciate it if someone could find a way to get rid of the lag between the sections.
I got it working to a point that I could use it, and stopped. It is not a general-purpose widget. Please feel free to use it at your own risk.
This implementation is quite good. I have just tested it. It however needs some changes made to support asynchronous loading. GWT Issue 188 covering similar request for enhacement was created on Oct 2006?!
Thanks for sharing.
I'm implementing a document viewer with highlighting/annotation capabilities for a custom document format on iPad. The documents are kind of long (100 to 200 pages, if printed on paper) and I've had a hard time finding the right approach. Here are the requirments:
1) Basic rich-text styling: control of left/right margins. Control of font name, size, foreground/background color, and line spacing. Bold, italics, underline, etc.
2) Selection and highlighting of arbitrary text regions (not limited to paragraph boundaries, like in Safari/UIWebView).
3) Customization of the Cut/Copy/Paste popup (UIMenuController) This is one of the essential requirements of the app.
My first implementation was based on UIWebView. I just rendered the document as HTML with CSS for text styling. But I couldn't get the kind of text selection behavior I wanted (across paragraph boundaries) and the UIMenuController can't be customized from within UIWebView.
So I started working on a javascript approach, faking the device text-selection behavior using JQuery to trap touch events and dynamically modifying the DOM to change the background color of selected regions of text. I built a fake UIMenuController control as a hidden DIV, positioning it and unhiding it whenever there was an active selection region.
Not too shabby.
The main problem is that it's SLOOOOOOOW. Scrolling through the document is nice and quick, but dynamically changing the DOM is not very snappy. Plus, I couldn't figure out how to recreate the magnifier loupe, so my fake text-selection GUI doesn't look quite the same as the native implementation. Also, I haven't yet implemented the communication bridge between the javascript layer and the objective-c layer (where the rest of the app lives), but it was shaping up to be a huge hassle.
So I've been looking at CoreText, but there are precious few examples on the web. I spent a little time with this simple little demo:
http://github.com/jonasschnelli/I7CoreTextExample/
It shows how to use CoreText to draw an NSAttributedText string into a UIView. But it has its own problems: It doesn't implement text-selection behavior, and it doesn't present a UIMenuController, so I don't have any idea how to make that happen. And, more importantly, it tries to draw the entire document all at once, with significant performance degradations for long documents. My documents can have thousands of paragraphs, and less than 1% of the document is ever on screen at a time.
On the plus side, these documents already contain precise formatting information. I know the exact page-position of every line of text, so I don't need a layout engine.
Does anyone know how to implement this sort of view using CoreText? I understand that a full-fledged implementation is overkill for a question like this, but I'm looking for a good CoreText example with a few basic requirements:
1) Precise layout & formatting control (using the formatting metrics and text styles I've already calculated).
2) Arbitrary selection of text.
3) Customization of the UIMenuController.
4) Efficient recycling of resources for off-screen objects.
I'd be happy to implement my own recycling when text elements scroll off-screen, but wouldn't that require re-implementing UIScrollView?
I'm brand-new to iPhone development, and still getting used to Objective-C, but I've been working in other languages (Java, C#, flex/actionscript, etc) for more than ten years, so I feel confident in my ability to get the work done, if only I had a better feel for the iPhone SDK and the common coding patterns for stuff like this. Is it just me, or does the SDK documentation really suck?
Anyhow, thanks for your help!
Does your document have any semantic components other than each paragraph? If you already have some concept of sections or pages, I would recommend you render each one of those as an independent tablecell. It's pretty simple to create a tablecell that makes you forget you're actually looking at a UITableView. All you would need to do is override drawRect: and setSelected: and setHighlighted: and tah dah! No More cell dividers unless you want them. Furthermore you could do some nifty things by using a tableview as your base. If you defined sections in the UITableView then you could have a nifty header that scrolls along as you're paging through your document. Another thing you could do is add a "jump to section" bar / a bookmarks menu, and that way you don't have to provide selection across the boundaries of sections.
Massive copy paste blocks would be pretty painful on the system as well. Further, if you went through the trouble to provide this content you might not want to make it too easy for someone to copy it all at once... (Can't follow this line of thought more without more specifics on your project).
If you really do want to provide the copy paste options you could add buttons to each logical page or section that immediately selects and copies the whole section for the user's convenience. (Maybe with citation associated?)
I recommend you lookup the UITableViewCell UITableViewDelegate and UITableViewDataSource in the SDK docs as those pages will significantly help if you choose to use this suggestion.
Just two random observations:
Can you afford to create a paging interface? (As opposed to “endless scrolling”.) It looks like a paging interface would be a lot easier on system resources.
The UIActionBar is actually the UIMenuController class. The interface is a bit weird, as the menu is a singleton (wtf?), but I’m sure you’ll have no trouble figuring it out.
Hope that helps.
Here's a potential solution, but I don't know if it's crazy. Since I'm still so new to iPhone development, this might be a big no-no.
Anyhow, I had the idea to render each paragraph of the document (whose dimensions I've already precisely calculated) as a cell in a UITableView. Since UITableView already has mechanisms for cell recycling, I wouldn't have to implement that from scratch, and the document could be arbitrarily long without causing resource consumption problems.
Of course, I'd want to get rid of the line separators between cells, since I want the UI to look like a document instead of a table.
Or maybe I could render each page of the document (like a typical PDF, this is a paged-document format) as a table cell, and override the cell-separator graphic to look like a page boundary...
But would it be possible to get rid of the default touch behavior within the table, and instead implement text-selection on the table cell contents? Would it be completely impossible to implement text selection that crosses paragraph boundaries (between multiple table cells)?
The UIWebView is a good choise, but we need another application to pre render the pages percisely using each font and each style sheet and store the rendring information into a database table:
chapter_id int primary key,
startlocation int,
end location int,
fontsize int (or stylesheetname string)
Using JavaScript we can calculate how many words fit in a div with out scrolling.
UIWebView is good as it provide rich content and it has selection and highlighting behavior.
Hope this helps.