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.
Related
I have the following components:
What I would like to do is have the same sizes for the leading and trailing constraints of my view on the right, whatever the size of the screen is.
I though about adding two views as placeholders. The first one starting on the left edge of the main view and finishing on the middle of the big orange square. And for the second one, starting on the middle of the big orange square and finishing on the right edge of the main view. Then both small orange squares would be centrally align inside.
But I was wondering if there was a better way whether it's by code or with the Interface Builder.
I think you can reach this by setting the priority to the constraint between the big orange and the view on left to Heigh or low
And required priority to leading and trading to the view on the right.
Maybe I'm missing something, but why not simply:
Pin Blue Leading to BigOrange Trailing
Pin Blue Trailing to SuperView
Center LittleOrange inside Blue...
And turn Blue transparent at runtime.
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'm just getting started with iOS development, and I'm following a series for making a game.
I want to align the circle (see screenshot) but there is no constraint for left
How can I make sure it aligns left for example on the iPhone 4 (see screenshot).
Xcode version (7.3.1 (7D1014))
Option 1: Hold control button, drag to the left and select `Leading spaces to container. The object that you are defining the space from should be highlighted. You should get the following menu:
Option 2:
1. Select the object
2. Press the pin button
3. Set the distance from the frame
4. Pressed the red button
5. Select Add constraints
Leading is left, and Trailing is right
First step, the most important one :
Remove that circle, and make sure your view has no constraint exception (look at the debugger) and that everything is where it should, on all phones. Ff that is correct, move on to the next steps.
Technically you should just be using Left or Leading, they're almost identical. (Leading is actually different, it'll align to the right in Arabic localized phones for example).
So what you wanna do is make sure your view has all its constraints set correctly, obviously, and for what I'm looking at you just need Top, Bottom Left and Right (or Leading and Trailing).
Just drag & drop from the circle to the view that is supposed to be the "anchor". For example, I'm guessing your top left circle will have its Left constraint going against the border of the view, and Top/Right/Bottom will be against the black bars. Easy.
For the left, drag & drop between the circle and the superview (the uppermost view in the hierarchy) and select Left/Leading. Then select the constraint and look at the properties.
Look at the image I have as an example. If you see the constraint properties on the right, I'm saying :
First item = "The leading side of my message box"
Relation = Should be equal (in distance)
Second Item = To the leading side of the Header label
Its plain english, the left side should align to the left side.
Your case is similar, you want the left side of your Image to be aligned with the left side of the View !
Now the 3 others constraints are sliiiiightly different, because you don't want Right-on-Right, Top-on-Top and Bottom-on-Bottom.
You actually need the Bottom of your circle to be aligned with the Top of the view under it, the Right of your circle to be aligned with the Left of the view on its right, and the top of the circle to be aligned with the bottom of the top bar.
Following?
Now you can adjust these constraints, you have a "Constant". This is the distance that the "Equal" refers to. If you say "the distance between this and that should be equal to zero", the two views will be sticking together. So usually you can add some margin.
Important note ! If you use the drop down for each Item (first and Second), you will see you can tick/untick margins ! This is important, it automatically adds Apple-advised margins, which is a value of 8, that is counted on top of the Constant value. So if you HAVE a margin and a constant of 0, you actually have a spacing of 8. This is to make your life easier because you'll usually have many margins and its easier to count from zero.
Any question?
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.
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).