I've been stuck for two days trying to understand the layout principles of WPF.
I have read a bunch of articles and explored the wpf source code.
I know that measure/measureoverride method accepts a availablesize and then sets the DesiredSize property.
This makes sense. It recursively calls to the children and ask them to set their respective desired size.
There are two things (at least) I don't understand. Let us consider a WrapPanel.
Looking at the WPF source code, the MeasureOverride() method accepts an availablesize and then passes this to all the children. It then returns the largest width and largest height of the resulting desiredsize properties in the children. Shouldn't it divide the available space between the children? I would think that it would iterate over the children and then measure the first, then subtract the resulting desiredsize from the total availablesize so that the next child had less space to occupy. As I read the WPF, WrapPanel.MeasureOverride does not appear to set a desiredsize that it would need to fit all the children. It just gives the DesiredSize that any ONE of the children will fit in to.
Due to the nature of the wrappanel, I would expect that for a vertically oriented stackpanel a restriction in height would result in a wider DesiredSize (to fit more columns). Since a restriction in height affects the desired size of a wrap panel, doesn't this logic then belong in the MeasureOverride method? Why is the stacking then only reflected in the ArrangeOverride method?
I think I have some fundamental misunderstanding about the mechanics of these two method.
Can anybody give me a verbal description of DesiredSize and/or AvailableSize that makes this implementation make sense?
How to properly implement MeasureOverride and ArrangeOverride?
As I think this is the actual question you're asking, I will try to give you as much as I know about this topic.
Before we begin, you may want to start with reading Measuring and Arranging Children on MS Docs. It gives you a general idea of how the layout process works, although it doesn't really offer any information on how you should actually implement MeasureOverride and ArrangeOverride.
Note: For the sake of brevity, from here on out, whenever I say "control", I really mean "any class deriving from FrameworkElement".
1. What are the components that affect control's layout?
It is important to be aware that there are numerous parameters that affect the size and arrangement of a control:
Contents (i.e. child controls)
Explicit width and height
Margins
Horizontal and vertical alignment
Layout transform
Layout rounding
Something else I might have overlooked
Luckily, the only component we need to worry about when implementing custom layout, are child controls. This is because the other components are common to all controls, and are handled by the framework completely outside of MeasureOverride and ArrangeOverride. And by completely outside I mean that both input and output are adjusted to account for those components.
In fact, if you inspect the FrameworkElement API, you'll notice that measurement procedure is split into MeasureCore and MeasureOverride, the former taking care of all the required corrections, and that in fact you never call them directly on the child controls - you call Measure(Size) which does all the magic. Same goes to ArrangeCore and ArrangeOverride.
2. How to implement MeasureOverride?
The purpose of measure phase in layout pass is to provide feedback to the parent control on the size that our control would like to be. You may think of it as a hypothetical question:
Given that much available space, what is the minimal space you need to accommodate all your contents?
It goes without saying that this is (usually) required to determine the size of the parent control - after all, we (usually) measure our child controls to determine the size of our control, don't we?
Input
From docs:
The available size that this element can give to child elements. Infinity can
be specified as a value to indicate that the element will size to whatever content
is available.
The availableSize parameter tells us how much space do we have at our disposal. Be aware though that this might be an arbitrary value (including infinite width and/or height), and you should not expect to be given the exact same amount of space upon arrangement phase. After all, the parent control may call Measure(Size) on our control many times with whatever parameters, and then completely ignore it in arrangement phase.
As mentioned before, this parameter is already pre-corrected. For example:
If parent control calls Measure(100x100), and our control has margin set to 20 (on each side), the value of availableSize will be 60x60.
If parent control calls Measure(100x100), and our control has width set to 200, the value of availableSize will be 200x100 (hopefully it will become clear why as you continue reading).
Output
From docs:
The size that this element determines it needs during layout, based on its calculations
of child element sizes.
The resulting desired size should be minimal size required to accommodate all contents. This value must have finite width and height. It typically is, but is not required to be, smaller than availableSize in either dimension.
This value affects the value of DesiredSize property, and affects the value of finalSize parameter of subsequent ArrangeOverride call.
Again, the returned value is subsequently adjusted, so we should not pay attention to anything but child controls when determining this value.
Relation to DesiredSize property value
Size returned by MeasureOverride affects, but not necessarily becomes the value of DesiredSize. The key thing here is that this property is not really intended to be used by the control itself, but rather is a way of communicating the desired size to parent control. Note that Measure does not return any value - parent control needs to access DesiredSize to know the result of the call. Because of that, its value is actually tailored to be viewed by parent control. In particular, it is guaranteed not to exceed the original size passed as parameter of Measure, regardless of the result of child's MeasureOverride.
You may ask "Why do we need this property? Couldn't we simply make Measure return the size?". This I think was done for optimization reasons:
Often we need to access child's desired size in ArrangeOverride, so calling Measure(Size) again would trigger redundant measure pass on child control (and its descendants).
It is possible to invalidate arrange without invalidating measure, which triggers layout pass skipping the measure phase and going straight to the arrange phase. For example, if we reorder controls in a StackPanel, the total size of the child controls does not change, only their arrangement.
Summary
This is how measure phase looks like from perspective of our control:
Parent control calls Measure(Size) on the control.
MeasureCore pre-corrects the provided size to account for margins etc.
MeasureOverride is called with adjusted availableSize.
We do our custom logic to determine the desired size of our control.
Resulting desired size is cached. It is later used to adjust the finalSize parameter of ArrangeOverride. More on that later.
The returned desired size is clipped not to exceed the availableSize.
Clipped desired size is post-corrected to account for margins etc. (step 2. is reverted).
Value from step 7. is set as value of DesiredSize.
Possibly this value is clipped again not to exceed the original size passed as Measure(Size) parameter, but I think that should already be guaranteed by step 6.
3. How to implement ArrangeOverride?
The purpose of arrange phase in layout pass is to position all child controls in relation to the control itself.
Input
From docs:
The final area within the parent that this element should use to arrange itself
and its children.
The finalSize parameter tells us how much space do we have to arrange child controls. We should treat it as final constraint (hence the name), and do not violate it.
Its value is affected by the size of rectangle passed as parameter to Arrange(Rect) by the parent control, but also, as mentioned, by the desired size returned from MeasureOverride. Specifically, it is the maximum of both in either dimension, the rule being that this size is guaranteed not to be smaller than the desired size (let me re-emphasize this pertains to the value returned from MeasureOverride and not the value of DesiredSize). See this comment for reference.
In the light of that, if we use the same logic we used for measurement, we do not need any extra precautions to ensure we'll not violate the constraint.
You may wonder why there's this discrepancy between DesiredSize and finalSize. Well, that's what clipping mechanism benefits from. Consider this - if clipping was disabled (e.g. Canvas), how would the framework render the "overflowed" contents unless they were properly arranged?
To be honest, I'm not sure what will happen if you violate the constraint. Personally, I would consider it a bug if you report a desired size and then are not able to fit in it.
Output
From docs:
The actual size used.
This is the frontier of my ignorance, where knowledge ends and speculation begins.
I'm not really sure how this value affects the whole layout (and rendering) process. I know this affects the value of RenderSize property - it becomes the initial value, which is later modified to account for clipping, rounding, etc. But I have no idea what practical implications it might have.
My personal take on this is that we had our chance to be finicky in MeasureOverride; now it's time put our words into actions. If we're told to arrange the contents within given size, that's exactly what we should do - arrange child controls within finalSize, not less, not more. We don't have to tightly cover the whole area with child controls, and there may be gaps, but these gaps are accounted for, and are part of our control.
Having said that, my recommendation would be to simply return finalSize, as if saying "That's what you instructed me to be, so that's what I am" to the parent control. This approach seems to be notoriously practiced in stock WPF controls, such as:
Border
Canvas
Decorator
DockPanel
Grid
StackPanel
VirtualizingStackPanel
WrapPanel
Possibly others...
4. Epilogue
I guess that's all I know on the subject, or at least all I can think of. I bet you dollars to donuts there's more to it, but I believe this should be enough to get you going and enable you to create some non-trivial layout logic.
Disclaimer
Provided information is merely my understanding of the WPF layout process, and is not guaranteed to be correct. It is combined from experience gathered over the years, some poking around the WPF .NET Core source code, and playing around with code in a good old "throw spaghetti at the wall and see what sticks" fashion.
#grx70 answer is great and amazingly detailed. However, there is much more to know about the WPF layouting system and I wrote a whole article about it on CodeProject: Deep Dive into WPF Layouting and Rendering
Here is an overview how Properties and overwriting of MeasureOverride(), ArrangeOverride() and OnRender() work together to produce and use DesiredSize and RenderSize (which is by the way the exactly same value like ActualHeight and ActuelWidth).
For a detailed description see the article.
I have a list of "Group" objects, each of which contains a list of "Option" objects, that show up as checkboxes. I want these to be displayed in a condensed fashion automatically without me having to layout the UI manually with something such as a Grid (which is what I've done in the past, and takes a lot of effort).
My groups have varying numbers of options, so the size of the container for the group is not the same across groups. I'm using a WrapPanel, but it leads to a fairly ugly design because each item in the WrapPanel appears to be slotted into the same size container:
I know I've done this in HTML/CSS/JS, where I can have it automatically condense the unused space. Is there something like this for WPF? My list of options is manually created, but I can add/remove options fairly easily in my code, so I would rather not have to manually recalculate things in a grid view.
I've looked at Is there any way to occupy blank space in WrapPanel automatically?, which sounds similar, but the answer to that question does not have an example and I could not figure out what I was actually supposed to change/use in the answer (my attempts using it did not make any difference at all to layout).
I guess you use Horizontal Orientation of WrapPanel, so every row have height as the maximum its element. Your problem isn't free space in the ends of rows, so the solution that you mentioned doesn't work for you. You can try to use Vertical orientation of WrapPanel, your wrappanel' elements look like they have similar width, possible it would look better.
In HTML, for a table (at least), one can style the element so that margins are merged. I.e. two adjacent rows both have top and bottom margins of 10, so the gap between these two rows will be 20. When their margins are merged, the gap is only 10.
Is there any way to achieve this in WPF?
There is no easy way to do this, and when you look at how WPF handles layout you'll see why, but also see that there is a hard way if you're up to it.
As you can see from the WPF source code for FrameworkElement, the MeasureCore sealed override method (which prevents us from overriding it further) adds the element's margins before returning with its desired size. Annoyingly, they seem to have a BypassLayoutPolicies option which would prevent this, but for reasons possibly ranging from short-sighted to sadistic, they made this internal so it's not an option. Thus the fully margined size of the element is what always winds up being assigned to DesiredSize, which is basically what all layout panels (StackPanel) etc. naively use to arrange the items during ArrangeOverride.
But therein is the solution, if you're willing to subclass all the panels and override their MeasureOverride and ArrangeOverride. Knowing the margin values of two adjacent children you could collapse their respective right/left or bottom/top margins together during the measure and arrange passes, by just subtracting the duplicative portion of the margin from their `DesiredSize's.
But if you're gonna subclass anyway, another option, which I would consider cleaner and a better practice overall, is to add a Spacing property to your subclassed Panels, which exists in the WinUI version of StackPanel. This would require you to either ensure that your child items all have zero margins, or to subtract their actual margin values out from their DesiredSize's. You would then add this spacing instead between the items during the arrange pass.
StackPanel is easy enough to extend this way as both passes are very simple. Grid, unfortunately, is a lot more complicated. VirtualizingStackPanel is probably out of the question... So this is by no means a silver bullet, but given how common StackPanel is, even just extending that control would cover a whole lot of ground and lead to much cleaner layouts IMO.
If I had more reputation I'd mark this as a duplicate of Is it possible to emulate border-collapse (ala CSS) in a WPF ItemsControl? . It looks as if this is kludgeable (for a ListBox at least) using a DataTrigger to check the value of preceding entries and set borders accordingly for null values.
I have a XamDataGrid in one of my user controls, inside of a stackpanel. I want the grid to maintain the same height regardless of how many rows are present in the grid. To do that, I set the grid's Height property to an explicit value.
Is that how things are done in WPF? Every time I do explicit sizing I feel like I am doing WinForms and not using WPF properly. Is setting the Height directly the only/correct solution?
There's nothing wrong with setting an explicit Height in situations where you want an element to always stay the same height. Where it's less appropriate is in situations where sizing is better handled by the parent layout Panel or the element's child content which can use the available space dynamically.
WPF uses a relative measurement system which at first glance is not intuitive. I have never found an example when I was forced to use explicit sizes ( once when I paint something on Canvas). I use styles in 90% cases where I define Padding, Margin, Aligment etc. Sometimes I use MinHeight and MinWidth for simple things.
About that Grid you can put it in the ScrollViewer or ViewBox to have dynamic sizing, yet If it won't be trouble set the explicit Height.
I always use a Canvas when I'm laying out my visuals usually because I will need adjust the RenderTransform.TranslateTransform to animate in some way. A colleague recently told me that unless I explicitly need to animate I should always use the A Stackpanel because it is faster than a RenderTransform.TranslateTransform when laying out objects to the visual.
Is this true?
Anyone have any data either way?
I don't have any data on this, but if we're just talking about stacking then you using a TranslateTransform to achieve the exact positioning of each item seems extremely fragile since the item could theoretically be of different heights/widths which could also theoretically change dynamically at runtime not to mention if the designer changes them by hand they have to redo the translate transform for N other UI elements. Using StackPanel means the Measure/Arrange phases will occur and no matter what size the items are they will be laid out precisely.