### 计算机代写|app代写安卓代写,Android代写|Modifying the UI

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• Foundations of Data Science 数据科学基础

## 计算机代写|app代写安卓代写,Android代写|Modifying the UI

In this section, we will see how to make changes to a View-based UI. Let’s start by looking at the enableOrDisableButton () function, which is invoked in onCreate (). Its name gives you a clue regarding its purpose – enabling or disabling a button. But why do we need this? Hello View is a reimplementation of the Hello app from Chapter 1, Building Your First Compose App, but it has one additional feature. As long as the user has not entered at least one non-blank character, Done can’t be clicked:
private fun enableOrDisableButton() {
binding. done . isknabled = binding. name.text. isNotBlank ()
}

binding. done refers to the button during runtime. It can be clicked only if isenabled is true. The text input field is denoted by binding. name. Its text property reflects what the user has already entered. isNotBlank () tells us if at least one non-whitespace character is present.
In the code I have shown you so far, enableOrDisableButton () is called only at the end of onCreate (). But we also need to invoke the function whenever the user has input something. Let’s see how to do this (please note that the following code snippets belong inside oncreate () so that they are executed when the activity is created):
Text input fields can modify certain aspects of the onscreen keyboard. For example, to have it show a Done key instead of the usual Enter, we add an android:imeOptions= ” actionDone” attribute to the layout file. To react to clicks on this key, we need to register code by invoking setOnEditorActionListener (). Then, binding. done performClick() simulates clicks on the Done button. You will see shortly why I do this.

The lambda function we pass to doAftertextChanged() is invoked every time the user enters or deletes something in the text input field. When this happens, enableOrDisableButton () is called, which makes the button clickable if the text currently present in the input field is not blank.
Finally, visibility = VISIBLE occurs inside binding. name run {, so it makes the text input field visible. This is the desired state when the activity is created.
Now, let’s turn to code related to the Done button:
binding. done run {
setOnclickListener {
val name $=$ binding . name $\cdot$ text
if (name. isNotBlank()) {

## 计算机代写|app代写安卓代写,Android代写|Moving from components to composable

So far, I explained the word component by saying that it refers to UI elements. In fact, the term is used in quite a few other areas. Generally speaking, components structure systems by separating distinct portions or parts of them. The inner workings of a component are typically hidden from the outside (known as the black box principle).
Tip
Components communicate with other parts of the system by sending and receiving messages. The appearance or behavior of a component is controlled through a set of attributes, or properties.

Consider TextView. We set text by modifying the text property and we control its visibility through visibility. What about sending and receiving messages? Let’s look at Button. We can react to clicks (receive a message) by registering (sending a message) an OnclickListener instance. The same principle applies to EditText. We configure its appearance through setting properties (text), send a message by invoking setonEditorActionListener (), and receive one through the lambda expression we passed as a parameter.

Message-based communication and configuration via properties make components very tool-friendly. In fact, most component-based UI frameworks work well with drawing board-like editors. The developer defines a UI using drag and drop. Components are configured using property sheets. Figure $2.1$ shows the Layout Editor in Android Studio. You can switch between a Design view, browse Code (an XML file), or a combination of both (Split):We now have a more precise understanding of how the component term is used in the context of UIs. Building on this foundation, we will now look at component hierarchies.

## 计算机代写|app代写安卓代写,Android代写|Component hierarchies

If you compare the XML attributes of ConstraintLayout, TextView, and EditText, you will find unique attributes per tag, one example being android: inputType. On the other hand, android: layout_width and android: layout_height are present in all three tags, defining the size of the corresponding element. Size and position are relevant for all components.
Yet, specific attributes influence visual appearance or behavior; this is not relevant for all kinds of UI elements, only a subset. Here’s an example: text fields and buttons will want to show or receive text. A FrameLayout UI element won’t. Think of it this way: the more specialized an attribute is, the less likely is its reuse in another component.
However, general ones (such as width, height, location, or color) will be needed in most UI elements.

Based on its attributes, each component has a level of specialization. For example, EditText is more specific than TextView because it can handle text input. Button is a general-purpose button; clicking on it triggers some action. On the other hand, a CheckBox component can be either checked or unchecked. This type of button can represent two states. A switch component has two states, too. It’s a toggle switch widget that can select between two options.
The degree of specialization can be modeled easily in object-oriented programming languages through inheritance. A more specialized UI element (class) extends a general element. Therefore, many often-used UI frameworks have been implemented in Java, $\mathrm{C}++$, or $\mathrm{C}$ # (object-oriented languages). It is important to note, though, that componentlike concepts can be achieved with other types of programming languages too. So, object orientation may be considered a benefit, but it’s not a necessity.

At this point, you may be thinking, Didn’t he mix two different things? How are tags and attributes of Android layout files related to classes? Allow me to explain. Earlier, I said that an XML file is inflated to a component tree. To be more precise – it becomes an object tree. The tags in the XML file represent class names and its attributes correspond to members of that class. inflate () creates a tree of objects based on this information.
So, Android layout files describe component trees outside of Java or Kotlin files using a different syntax (an XML syntax). But they are not declarative in the same way Jetpack Compose is because layout files define a UI regardless of the current state. For example, they do not take into account that a button should be disabled because a text field is empty. A Compose UI, on the other hand, is declared based on that.
The remaining part of this section will look closer at some of Android’s UI components and how they are related. Before that, let’s recap what we have learned so far:

• All Android views are classes.
• Tags in layout files represent classes and attributes are their members.
• inflate () creates an object tree.
• Changes to the UI are achieved by modifying this tree.
Some of Android’s UI elements are quite specific. RatingBar, for example, allows the user to rate something by selecting a certain number of stars. Others are way more general; for example, ImageView just displays image resources, and FrameLayout blocks out an area on the screen to display a stack of children.

## 计算机代写|app代写安卓代写,Android代写|Modifying the UI

private fun enableOrDisableButton() {
binding. 完毕 。isknabled = 绑定。名称.文本。isNotBlank ()
}

setOnclickListener {
val 名称=捆绑 。姓名⋅文本
if (name.isNotBlank()) {

## 计算机代写|app代写安卓代写,Android代写|Component hierarchies

• 所有 Android 视图都是类。
• 布局文件中的标签代表类，属性是它们的成员。
• inflate() 创建一个对象树。
• 对 UI 的更改是通过修改此树来实现的。
Android 的一些 UI 元素非常具体。例如，RatingBar 允许用户通过选择一定数量的星来对某物进行评分。其他的则更为笼统。例如，ImageView 只是显示图像资源，而 FrameLayout 在屏幕上屏蔽了一个区域以显示一堆孩子。

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