What is the difference between NSLayoutAttributeLeft and NSLayoutAttributeLeading in iOS autolayouts?
"Leading" does not always mean "Left".
For RTL-written languages (locales) leading edge of the object’s alignment rectangle will be located at the right side of the object.
Quote from Auto Layout Guide:
The attributes leading and trailing are the same as left and right for left-to-right languages such as English, but in a right-to-left environment such as Hebrew or Arabic, leading and trailing are the same as right and left. When you create constraints, leading and trailing are the default values. You should usually use leading and trailing to make sure your interface is laid out appropriately in all languages unless you’re making constraints that should remain the same regardless of language (such as the order of master and detail panes in a split view).
Related
I can't find a source that says what the definition of trailing and leading edges mean in SwiftUI
The same as they mean in UIKit. They are a locale-dependent way of specifying horizontal edges. In a left-to-right locale, such as English, the leading edge is the left and the trailing edge is the right. In a right-to-left locale (e.g. Hebrew), the leading edge is on the right and the trailing edge is on the left.
By specifying spacing in terms of leading/trailing rather than left/right you can ensure that your layout is consistent regardless of the user's locale.
Trailing edge - like it's right side of view. When you want to view should have left padding 20 from left side of view controller its means you should set padding for leading edge. this is for right side same. It's just sides of view.
There great examples for clearly understand link.
I started iOS development a week ago and I'm currently learning AutoLayout. and sadly I can't wrap my head around the Leading & Trailing constraints and when to use them, I don't even understand them at all. I have done some research and I'm just actually more confused now. Can someone give some examples?
This definition is form the Apple website:
The values increase as you move towards the trailing edge. For a
left-to-right layout directions, the values increase as you move to
the right. For a right-to-left layout direction, the values increase
as you move left.
https://developer.apple.com/library/content/documentation/UserExperience/Conceptual/AutolayoutPG/AnatomyofaConstraint.html
Thanks
TL;DR
They couldn't just name it left and right because it had to make sense in both layout orientations: left-to-right and right-to-left. Hence Trailing and Leading.
Longer Answer
I think the question you're asking yourself is why the heck Apple is not just naming it left and right. What's with the trailing and leading.
The reason behind this is that there might be 2 different layouts. Starting with iOS 9, the UI layout for left-to-right languages (like English) is.. well left-to-right. But in case of Arabic for example, it's right-to-left.
However Autolayout is smart enough that you don't need to setup your layout twice for these 2 types of layouts. You just set it up once and the system auto-inverts it in case your app supports right-to-left languages.
For this answer, ignore languages like Arabic and Hebrew that flow right to left. I'm going to answer for the majority of languages that flow left to right.
For those languages, the "leading" constraint determines the space on the left of an object, and the "trailing" constraint determines the space on the right side. (A good mnemonic for this is that left and leading both start with "L".)
For languages that flow the opposite way, the sides are switched, but the "leading" still pins an object to the leading edge (The side where you start reading.)
Leading means the starting point of the view and trailing means the ending point of the view.
For example, if a view has a frame (x: 0, y: 0, width: 100, height: 100) its leading will me 0 and trailing will be 100.
So if you move right from the leading the distance between the leading and the point moved will increase and similarly this happens in case of trailing.
This was covered (briefly) in one of the WWDC 2015 videos on the Mysteries of Autolayout (I think it was part 1 but both are worth watching).
Left and Right constraints are absolute, they will always refer to the left/right of the screen or the control. Leading and trailing constraints are affected by the device locale; In locales where the reading direction is left to right (English, French, Spanish and so on) leading & left (and trailing & right) can be used interchangeably. In locales where the reading direction is right to left (e.g Hebrew) then 'leading' will be the right side and 'trailing' will be the left side.
The advice in the video was you should almost always use leading/trailing rather than left/right unless you have a specific requirement for absolute left/right.
Consider the typical 'form' of a label and a text field. If you use leading/trailing constraints then you will get the label on the left and text field on the right for an English locale and label on the right, text field on the left for a Hebrew locale.
If you made a children's app where you were teaching left from right and you always wanted the 'left' button on the left of the screen then left/right constraints would be appropriate.
Check the original answer here: Difference between leftAnchor and leadingAnchor?
I have an iOS Application that supports different languages, which uses LTR Storyboard and RTL Storyboard, and reads from Plists. When I run the app on my Device with English iOS Version, everything looks perfect when switching between the languages. But when I change the iPhone OS Language to RTL languages. The UI is messed up.
This happens especially when I enable Localizations to change the app name on home screen.
Any idea why does that happen?
The whole menu is messed up for example, it's to the left side while it should be to the right side, and the menu items icons must be to the left and the labels to the right.
RTL Language on LTR iOS Language Device i.e English
RTL Language on RTL iOS Language Device
This is because you are use leading and trailing constraint.
In a LTR environment leading means left while in an RTL environment, leading means right.
This is Apple way of helpings us to deal with RTL/LTR easily :[ in your case, not so much...
To disable this behaviour, change the semantic attribute of your views.
Semantic Content
If you lay out your views using leading and trailing constraints, the
views automatically flip positions when switching between
left-to-right languages (like English) and right-to-left languages
(like Arabic). However, some interface elements should not change
their position based on the reading direction. For example, buttons
that are based on physical directions (up, down, left, and right)
should always stay in the same relative orientation.
The view’s semanticContentAttribute property determines whether the
view’s content should flip when switching between left-to-right and
right-to-left languages.
In interface builder, set the Semantic option in the Attribute
inspector. If the value is Unspecified, the view’s content flips with
the reading direction. If it is set to Spatial, Playback, or Force
Left-to-Right, the content is always laid out with the leading edges
to the left and trailing edges to the right. Force Right-to-Left
always lays out the content with the leading edges to the right and
the trailing edges to the left.
Assuming that you are already using Auto Layout,
Make sure you have done the following
Remove fixed width constraints. Allow views that display localized text to resize. If you use fixed width constraints, localized text may appear cropped in some languages.
Use intrinsic content sizes. The default behavior for text fields and labels is to resize automatically. If a view is not adjusting to the size of localized text, select the view and choose Editor > Size To Fit Content.
Use leading and trailing attributes. When adding constraints, use the attributes leading and trailing for horizontal constraints. For left-to-right languages, such as English, the attributes leading and trailing are equivalent to left and right. For right-to-left language, such as Hebrew or Arabic, leading and trailing are equivalent to right and left. The leading and trailing attributes are the default values for horizontal constraints.
Pin views to adjacent views. Pinning causes a domino effect. When one view resizes to fit localized text, other views do the same. Otherwise, views may overlap in some languages.
Constantly test your layout changes. Test your app using different language settings, as described in Testing Your Internationalized App.
Don’t set the minimum size or maximum size of a window. Let the window and its content view adjust to the size of the containing views, which may change when the language changes.
See more
Probably a matters of constraints. In the storyboard, you can use the preview editor to get a .. well.. preview, of your UI in diffrent device size in different language.
I use Auto Layout programmatically a lot but there are still two things that I do not understand.
What is the difference between leading and left attribute/anchor and between trailing and right attribute/anchor?
To me there seem to be the same but according to the docs:
However, Auto Layout does not allow constraints that mix leading and trailing attributes with left or right attributes. As a result, this constraint crashes at runtime.
The order of the the items inside the NSLayoutConstraint does not matter for the Auto Layout system at all, am I right? (At least for .Equal relation.)
Do not get confused about the attributes for each individual item, I'm aware of that.
Left and right are always left and right.
Leading and trailing are not. From Working with Constraints in Interface Builder:
If you lay out your views using leading and trailing constraints, the views automatically flip positions when switching between left-to-right languages (like English) and right-to-left languages (like Arabic). However, some interface elements should not change their position based on the reading direction. For example, buttons that are based on physical directions (up, down, left, and right) should always stay in the same relative orientation.
The order of items for an equal relation is irrelevant.
The difference between leading and left is important when taking into account RTL languages, such as Hebrew. In that case, the UI is inverted to complement text direction, and so are the UI elements with constraints. In this case, leading means the side where text starts. left is always left, for text direction agnostic UIs, such as media playback controls.
The order constraints are placed in does not matter. If there are conflicting constraints, the behavior is undefined.
I have been working with constraints in Swift for iOS, and most of the naming conventions/documentation make things quite self explanatory, but one thing that has recently confused me is two pairs of the NSLayoutXAxisAnchor class:
leadingAnchor, leftAnchor
along with
trailingAnchor, rightAnchor
When I have set constraints programmatically in my iOS app, I have found that I can use these totally interchangeably. If I use leadingAnchor, run the app and observe the behavior, and then use leftAnchor, run the app and observe the behavior, it functions exactly the same.
I checked out the documentation from developer.apple.com, and the results are even exactly the same (ignoring the difference in names). For example comparing the leadingAnchor and leftAnchor:
"Use this anchor to create constraints with the view’s leading edge.
You can only combine this anchor with a subset of the
NSLayoutXAxisAnchor anchors. You can combine a leadingAnchor with
another leadingAnchor, a trailingAnchor, or a centerXAnchor. For more
information, see NSLayoutAnchor Class Reference."
compare to:
"Use this anchor to create constraints with the view’s left edge. You
can only combine this anchor with a subset of the NSLayoutXAxisAnchor
anchors. You can combine a leftAnchor with another leftAnchor, a
rightAnchor, or a centerXAnchor. For more information, see
NSLayoutAnchor Class Reference."
Does anyone know if there is any intended difference between these properties?
This was covered (briefly) in one of the WWDC 2015 videos on the Mysteries of Autolayout (I think it was part 1 but both are worth watching).
Left and Right constraints are absolute, they will always refer to the left/right of the screen or the control. Leading and trailing constraints are affected by the device locale; In locales where the reading direction is left to right (English, French, Spanish and so on) leading & left (and trailing & right) can be used interchangeably. In locales where the reading direction is right to left (e.g Hebrew, Arabic) then 'leading' will be the right side and 'trailing' will be the left side.
The advice in the video was you should almost always use leading/trailing rather than left/right unless you have a specific requirement for absolute left/right.
Consider the typical 'form' of a label and a text field. If you use leading/trailing constraints then you will get the label on the left and text field on the right for an English locale and label on the right, text field on the left for a Hebrew locale.
If you made a children's app where you were teaching left from right and you always wanted the 'left' button on the left of the screen then left/right constraints would be appropriate