Each happily does its own thing. However, life is not always normal. When we face these circumstances, SwiftUI provides us with some great tools. Unfortunately, its documentation is brief at best.
This three part article tries to put a remedy to that. We are going to learn about the PreferenceKey protocol and its associated modifiers:. These attributes are called Preferencesand these may be passed up the view hierarchy easily. Have you ever wondered how does NavigationView get the title from. Note that. So how does it do it! Well you probably guessed it already. It uses preferences. It is just a 20 second mention, and easily missed. These are very useful to retrieve all kinds of geometry information from our child views.
We will cover this anchored preferences in the next part of this article. In this case, each view knows what to do by itself.
We are going to create a view that displays month names. When a month label is touched, a border gently appears around it removing the one from the previous selection. The code is pretty straight forward. Whenever a month label is tapped, we change a State variable that keeps track of the month last tapped. We also made the border color of each month view dependant on this variable. If the view is selected, the border color is set to red. Otherwise, the border is made transparent.
This case is simple, because each view draws its own border. Now, instead of fading, we want our border to move from month to month. I would like you to pause for a second, and think about how you would approach this problem.
Unlike the previous example where you had 12 borders one for each viewwe now have a single border, that needs to change size and position using an animation. In this new example, the border is no longer part of the month view. Now, we need to create a separate single border, but it needs to move and resize accordingly. That means there must be a way of keeping track of the size and location of each month view. If you read my previous article GeometryReader to the Rescueyou already have one of the tools required to solve this problem.
If you do not know how GeometryReader works, please stop here and go check that first. One way to solve this problem, is by making each month view to use GeometryReader in order to obtain its own size and location. Each view will in turn update an array of rectangles shared with their parent through a Binding.For a new project, we need to draw tree diagrams in SwiftUI. As a first step, we can draw the nodes of the tree recursively: for each tree, we create a VStack containing the value and its children.
The children themselves are drawn using an HStack. We require that each element is identifiable so that we can use them with a ForEach. Since Tree is generic over the node values, we also need to provide a function that turns a node value into a view:. We are almost ready to draw our tree. There is one problem we still have to solve: the integers in our example binary tree do not conform to the Identifiable protocol.
This will be useful when we want to modify our tree later on; by being able to uniquely identify elements we can have great animations. To add some styling to the nodes, we create a view modifier that wraps each element view in a frame, adds a white circle with a black stroke as background, and some padding around everything:. To draw these, we need to hook into the layout system. Preferences are the mechanism used to communicate values up the view tree, from children to their ancestors.
Any child in the view tree can define a preference, and any ancestor can read that preference. In our implementation the default value is an empty dictionary, and the reduce method merges multiple dictionaries into one.
With this preference key in place, we can now use. Now we can use backgroundPreferenceValue to read out all the node centers for our current tree. Line is a custom Shape that has absolute from and to coordinates. We also add both points to the animatableDataso that SwiftUI knows how to animate lines to be able to use CGPoint as animatble data, we have to conform it to the VectorArithmetic protocol.
This conformance is ommitted here for brevity :. Given all of the machinary above, we finally can use the Diagram view and draw a nice tree with edges:. Because we wrapped each element in a Unique object, we can animate between different states. For example, when we insert a new number into the tree, SwiftUI can animate that insertion for us:. We have also used this technique to draw different kinds of diagrams. Our latest public episode recreates the iOS Stopwatch app, starting with custom buttons.Today we will continue mastering view preferences in SwiftUI that we touched a few weeks ago.
Anchor preferences are another type of view preferences provided by SwiftUI.
How to Layout in SwiftUI?
The main goal of anchor preferences is to pass layout data like bounds, center coordinates, etc. First of all, I want to ask you to check the post about view preferences if you are not familiar with these API.
Anchor preferences use a very similar API. The only difference is that it is tuned to pass layout-specific data. The parent view will draw an overlaying rectangle in that position. As you can see in the example above, we still use the PreferenceKey protocol to create an anchor preference key.
It has two requirements: default value and reduce function. Reduce function allows us to merge multiple values that appear from different views. We can replace the current value with the new one for now.
We will see more advanced usage of reduce function later in the post. Anchor preferences use opaque Anchor type. You have to use it in pair with GeometryProxy provided by GeometryReader. We use the anchorPreference modifier to define the type of PreferenceKey and the value we want to gather.
It can be bounds, center, leading, trailing, top, bottom. We also pass a closure that transforms provided anchor value. In the end, we use overlayPreferenceValue on ancestor view to access gathered preference values and return overlay view. As I mentioned before, we need a GeometryProxy to resolve an anchor.
Now we can move to more advanced usage of anchor preferences. As an example, we will build a grid view. We will need to gather the size of every view inside the grid to calculate its positions. As you can see here, we will store the dictionary that represents an item and its size.
In the reduce function, we merge old and new dictionaries by overriding new values. Now we can define our grid view. We use ZStack with top leading alignment. It allows us to position items inside in an effortless way.
We also resolve our anchors here, because we already have access to the instance of GeometryProxy. As the last step, we calculate bounds for every item using onPreferenceChange modifier which provides us the access to gathered sizes.
SwiftUI provides us so many great tools that we can use to build impressive views. Anchor preferences feature is one of the powerful hidden gems of SwiftUI. I hope you enjoy the post.I'm fiddling with a view layout - a graph - where I'm seeing how far I can get within an all-SwiftUI layout.
Each of the component pieces are fine, but assembling the whole isn't working quite as I'd like. What I've found is that I can easily constrain sizing along a single stack axis - but not two axis at once: both vertical and horizontal.
I started to reach for AlignmentGuides, as I noticed you can align non-siblings with a custom guide. That will help my goal, but it doesn't solve the sizing part, which is the heart of this question:. Is there a way to tell CellFour which isn't in the same HStack as cell's 1 and 2 that I want it to constrain itself and align to the width of cell CellTwo?
The areas that roughly map to cell 1, cell 2, and cell 4. I want the heights of Cell 1 and Cell 2 to be the same accomplished easily with the current HStackand the widths of Cell 2 and Cell 4 to be the same - that's where I'm struggling. I've also inquired on StackOverflow. Since your cells 2 and 4 are not part of the same view hierarchy, you would have to use PreferenceKeys to push information from one of them up the chain and then down the chain to the other one.
Here is my quick attempt to use the PreferenceKey protocol to push the cell frame info up the view hierarchy. All data is attached automatically, and it only takes a line of code to setup. Start your free trial now and get 3 months off exclusively for the Hacking with Swift Community. Start your free trial!
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Start Here. About Hacking with Swift. Is there a way to constrain or inform a view size? Jul ' That will help my goal, but it doesn't solve the sizing part, which is the heart of this question: Is there a way to constrain a view's size based on another, non-sibling, view?
Inspecting the View Tree – Part 2: AnchorPreferences
PS: If someone have a better idea regarding the components' names it'll be much appreciated :. Not logged in Log in. Link copied to your pasteboard.In the first part of this article, we introduced the use of preferences.
These are very useful to communicate information upwards from children to ancestors. By defining the associated type of our PreferenceKey, we now know that we can put anything we want in there. In this second part, the moment for Anchor Preferences to make their appearance has arrived. At the time of writing, I could not find a single document, blog post or article, that would explain how to use these elusive tools. So please join me in exploring this uncharted territory. To make things simple, we are going to address the same problem we solve in the first part.
It is good you are already familiar with the challenge, so you can concentrate on all these exciting new features. Unlike the previous solution, we will no longer need to use space coordinates, and we will be replacing.
So, here it is again: we want to create a border that moves from one month name to another, using an animation:. Because it is an opaque value, we cannot use it by itself. Well now you know what it is for. And as a plus, you get it already translated to the coordinate space of the GeometryReader view. We start by modifying the data handled by our PreferenceKey. The MonthView is much more simple now.
A guide to the SwiftUI layout system - Part 2
Instead of using. Unlike the other method, here we can specify a value in this case. This way, we no longer need to use GeometryReader inside a. Finally, we update our ContentView. There are a couple of changes here. For starters, we no longer use. Instead, we call. This is a modifier similar to. This way, we get all the bounds of all the month views, and we can use them to calculate where the border needs to be drawn.
That makes it a problem, if you want to use. That function, as you can probably imagine, requires the preference key value to be Equatable. Fortunately for us, in this example we are not using. Since the conformance was never deprecated and silently removed, I am hopeful it will eventually return in the future, before the GM is released. I submitted a bug report FB I encourage you to do the same. There is still one place where we need to use GeometryReader.
Notice that we no longer need to worry about coordinate spaces, GeometryReader takes care of it. It does the same thing, but instead of drawing behind, it does so in front of the modified view. There may be a case where we need to get more than one of these values. However, as we will learn, it is not as easy as just calling. One for the topLeading and the other for the bottomTrailing of the month view rect.Every iOS app consists of many objects that need to communicate with each other to get the job done.
In this article, we will study all the available options, and look at some best practices regarding when you should use which mechanism. The most common way of transferring data in SwiftUI is from a parent view to its direct child. The parent just instantiates the child and passes data to its initializer. Similarly, we can pass a TodoItem from the list to an individual row:.
Often we have dependencies that are required by some views within a hierarchy, but not all of them. Imagine that we need to pass an image cache from the app composition root aka SceneDelegate to TodoItemDetail :. If we pass an image cache via initializer, we will create 3 unnecessary levels of indirection and couple all intermediate views to the image cache.
This sounds bad. Luckily, SwiftUI offers a solution out-of-the-box. Meet Environmentwhich is essentially a dictionary with app-wide preferences.
SwiftUI passes it automatically from a parent view to its children.
A deeper understanding of SwiftUI
Environment allows us to insert arbitrary values into a view hierarchy and read them only when necessary. We can read a value from an environment using the Environment property wrapper :. Note that the default image cache will be created when we access it for the first time via Environment. There are two ways of passing data from a child to its direct parent — using bindings and callbacks. Here are some rational to make the choice:.
An obvious use case for the callback mechanism is buttons. Say, we want to add an info button to a todo list row:. Binding allows us to declare a property that is owned by a parent but can be changed both by the parent and the child, effectively passing that change back and forth.
For an API that uses binding, we are going to look at the sheet item:content: method that presents a modal sheet. Here is how we can use it to display a todo item details:. SwiftUI has a preference system that enables us to pass key-value pairs up the view hierarchy. The process is next:. As an example, consider how we can use SwiftUI view preferences to show an alert.
The PreferenceKey protocol has two requirements. We must provide the default value for a preference and a reduce method that combines all child values into a single one visible to their parent . Therefore, in the reduce method, we store the latest provided value. Conformance to the Equatable protocol is important since it allows SwiftUI to determine diffs and call the parent only when preferences have changed.
With AlertPreferenceKey in place, we can now use preference key:value: to pass an alert up the view tree:. Now we can use onPreferenceChange to read an alert from the current view tree. To pass data between siblings, we need to lift the state one level up and use their parent view as a middleman. As an example, we will consider a Toggle and Button with a synchronized state:.
The number of mechanisms available to pass data between SwiftUI views can first seem overwhelming. But once we put them in a systematic way, they all have unique requirements and usage cases. If you enjoyed this post, be sure to follow me on Twitter to not miss any new content. Thanks for reading! Yet Another Swift Blog.Arguably the biggest difference between a website and an app is their approach to user data. On one hand, websites do their best to invade privacy by tracking cookies, serving up remarketing adverts, and watching every move we make, so very few users want to trust them with more data.
The first is called UserDefaultsand it allows us to store small amount of user data directly attached to our app. To give you at least an ideayou should aim to store no more than KB in there. How much is that? UserDefaults is perfect for storing user settings and other important data — you might track when the user last launched the app, which news story they last read, or other passively collected information.
However, there is a catch: it is stringly typed. As this is clearly A Very Important App, we want to save the number of taps that the user made, so when they come back to the app in the future they can pick up where they left off. Well, making that happen only takes two changes. First, we need to write the tap count to UserDefaults whenever it changes, so add this after the line self. Speaking of reading the data back, rather than start with tapCount set to 0 we should instead make it read the value back from UserDefaults like this:.
Sometimes having a default value like 0 is helpful, but other times it can be confusing. All data is attached automatically, and it only takes a line of code to setup. Start your free trial now and get 3 months off exclusively for the Hacking with Swift Community. Start your free trial! Sponsor Hacking with Swift and reach the world's largest Swift community! Start Here. About Hacking with Swift.
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