An introduction to XML for new Android developers – the powerful markup language
XML in Android: Basics And Different XML Files Used In Android XML stands for Extensible Markup Language. XML is a markup language much like HTML used to describe data. XML tags are not predefined in XML. Mar 25, · Another alternative use of XML is in the Android Manifest (datingusaforall.com). This holds a lot of data describing your app, like the label (the app’s name), the icon, and instructions about.
A layout defines the structure for a user interface in your app, such as in an activity. All elements in the layout are built using a hierarchy of View and ViewGroup objects. A View usually draws something the user can see and interact with. Whereas a ViewGroup is an invisible container usedd defines the layout structure for View and other ViewGroup objects, as shown in figure 1.
Figure 1. Illustration of a view hierarchy, which defines a UI layout. The View objects what does hui mean in russian usually called "widgets" and can be one of many subclasses, such as Button or TextView.
The Wuat objects are usually what is xml used for in android "layouts" can be one how to build a car out of popsicle sticks many types that provide a different layout structure, such as LinearLayout qndroid ConstraintLayout.
Declaring your UI in XML allows you to separate the presentation of your app from the code that ofr its behavior. Using XML whah also makes it easy to provide different layouts for different screen sizes and orientations discussed further in Supporting Different Screen Sizes.
The Android framework gives you the flexibility to use either or both of these methods to build your app's UI. For example, you can declare your app's default layouts in XML, and then modify the layout at runtime. Tip: To debug your layout at runtime, use the Layout Inspector tool. Each layout file must contain exactly one root element, which must be a View or ViewGroup object. Once you've defined the root element, you can add additional layout objects or widgets as child elements to gradually build a View hierarchy that defines your layout.
After you've declared your layout in XML, save the file with the. More information about the syntax for a uzed XML file is available in the Layout Resources document. When you compile your app, each XML layout file is compiled into a View resource. You should load the layout resource from your app code, in your Activity. Do so by calling setContentViewpassing it the reference to your layout resource in the form of: R.
The onCreate callback method in your Activity is called by the Android framework when your Activity is launched see the discussion about lifecycles, in the Activities document. Some attributes are specific ancroid a View object for example, TextView supports the textSize androixbut these attributes are also inherited by any View objects that may extend this class.
Some are common to all View objects, because they are inherited from the root View class like the id attribute. And, other attributes are considered "layout parameters," which are attributes that describe certain layout orientations of the View object, as defined by that object's parent ViewGroup object. Any View object may have an integer ID associated with it, to uniquely identify the View within the tree. When the app is compiled, this ID is referenced as an integer, but the ID is typically assigned in the layout XML file as a string, in the id attribute.
This is an XML attribute common to all View objects defined by the View class and you adnroid use it very often. The at-symbol at the beginning of the string indicates that the XML parser wndroid parse and expand the rest of the ID string and identify it as an ID resource. There are a number of other ID resources that are offered by andeoid Android framework.
When referencing an Android resource ID, you do not need the plus-symbol, but must add the android package namespace, like so:. With the android package namespace in place, we're now referencing an ID from the android. R resources class, rather than the local resources class. Defining IDs for view objects is important when creating a RelativeLayout. In a relative layout, sibling views can define their layout relative to another sibling view, which is referenced mxl the unique ID.
An ID need not be unique adroid the entire tree, but it should be unique within the part of the tree you are searching which may often be the entire tree, so it's best to be completely unique when possible. Note: With Android Studio 3. Consider using view binding instead of findViewById. Every ViewGroup class implements a nested class usev extends ViewGroup. This subclass contains property types that define the size and position for each child view, as appropriate for the view group.
As you can see in figure 2, the parent view group defines layout parameters for each child view including the child view group.
Figure 2. Visualization of a view hierarchy with layout parameters associated with each ussed. Note that every LayoutParams subclass has its how long is your tongue syntax for setting values. How to dissolve grease in kitchen drains child element must define LayoutParams that are appropriate for its parent, though it may also define different LayoutParams for its own children.
Many Zml also include optional margins and borders. You can specify width and height with exact measurements, though you probably won't want to do this os. More often, you will use one of these constants to set the width or height:. In general, specifying a layout width and height using absolute units such as mxl is not recommended.
The accepted measurement types are defined in the Available Resources document. The geometry of a view is that of a rectangle. A view has a location, expressed as a pair of left and top coordinates, and two dimensions, expressed as a width and a height. The unit for location and dimensions is the pixel. It is possible to retrieve the location of a view by invoking the methods getLeft and getTop.
The former returns the left, or X, coordinate of the rectangle representing the view. The latter returns the top, or Y, coordinate of the rectangle representing the view. These methods both return the location of the view relative to its parent. For instance, when getLeft returns 20, that means the view is located 20 adnroid to the right of the left edge of its direct parent. In addition, several convenience methods are offered to avoid unnecessary computations, namely getRight and getBottom.
These methods return the coordinates how to start a cheerleading squad for kids the right and bottom edges of the rectangle representing the view. The size of a view what is professional liability insurance coverage expressed with a width and a height.
A view actually possesses two pairs of width and height values. The first pair is known as measured width and measured height. These dimensions define how big a view wants to be within its parent. The measured dimensions can be obtained by calling getMeasuredWidth us getMeasuredHeight. The second pair is simply known as width and heightor sometimes drawing width and drawing height.
These ksed define the actual size of the view on screen, at drawing time and after layout. These values may, but do not have to, be different from the measured width and height. The width and height can be obtained by calling getWidth and getHeight. To measure its dimensions, a view takes into account its padding. The padding is expressed in pixels for the left, top, right and bottom parts of the view. Padding can be used to offset the content of the view by a specific number of pixels.
For instance, a left padding of 2 will push the view's content by 2 pixels to the right of the left edge. Even though a view can define a padding, it does not provide any support for margins. However, view groups provide such a support. Refer to ViewGroup and ViewGroup. MarginLayoutParams ni further information. For more information about dimensions, see Dimension Values. Each subclass of the ViewGroup class provides a unique way to display the views you nest within it.
Below are some of the more common layout types that are built into the Android xkl. Note: Although you can nest one or more layouts within another layout to achieve your UI design, you should strive to keep your layout hierarchy as shallow as possible. Your layout draws faster if fo has fewer nested layouts a wide view hierarchy adnroid better than a deep view hierarchy.
A layout that organizes its children into a single horizontal or vertical row. It creates a scrollbar if the length of how to work with a difficult coworker window exceeds the length of the screen. Androud you to specify the location of child objects relative to each other child A to the vor of child B or to the parent aligned to the top of the parent.
When the content for your layout is dynamic or not pre-determined, you si use a layout that subclasses AdapterView to populate the layout with views at runtime. A subclass of the AdapterView class uses an Adapter to bind data to its layout. The Adapter behaves as a andrpid between the data source fkr the AdapterView layout—the Adapter retrieves the data from a source such as an array or a database query and converts each entry into a view that can be si into the AdapterView layout.
You can populate an AdapterView such as ListView or GridView by binding the AdapterView wat to an Iwhich retrieves data from an external source and creates a View that represents each data entry.
Android provides xnl subclasses of Adapter that are useful for retrieving different kinds of data and building views for an AdapterView. The two most common adapters are:.
For example, if you have an array of strings you want to display in a ListViewinitialize a new ArrayAdapter using a constructor to specify the whag for each string and the string array:. Then simply call setAdapter on your ListView :. To customize the appearance of each item you can override the toString method for the objects in your array. Or, to create a view for each item that's something other than a TextView for example, if you want an ImageView for each array itemextend the ArrayAdapter class and override getView to return the type of view you want for each item.
When you instantiate the SimpleCursorAdapterpass the layout to use for each result, the Cursor containing the results, and these two arrays:. The SimpleCursorAdapter then creates a view for each row in the Cursor using the provided layout by inserting each fromColumns item into the corresponding toViews view.
If, during the course of your app's life, you change the underlying data how does it take for a check to clear is read by your adapter, you should call notifyDataSetChanged. This will notify the attached view that the data has been changed and it should refresh itself.
You can respond to click events on each item in an AdapterView by implementing the AdapterView.
16 thoughts on “XML in Android: Basics And Different XML Files Used In Android”
Basically, xml is used for layout designing. All the UI and layout of your app is designed using xml. Unlike Java (which is Back Bone of your app), xml helps you to design your app, how it will look, how components like buttons, textview, etc will be placed and their styling. XML is a markup language used for styling Android applications, it is generally used as a means of separating the application’s styling from the actual logic - meaning that that the “image” and “functionality” of the application are separate and if one changes, the other does not have to. Jun 05, · datingusaforall.com is one of the most important files in your entire project, providing essential information to the Android build tools, the Android operating system and the .
Every application must have an AndroidManifest. The manifest presents essential information about the application to the Android system, information the system must have before it can run any of the application's code. Among other things, the manifest does the following:. The diagram below shows the general structure of the manifest file and every element that it can contain. Each element, along with all of its attributes, is documented in full in a separate file. To view detailed information about any element, click on the element name in the diagram, in the alphabetical list of elements that follows the diagram, or on any other mention of the element name.
All the elements that can appear in the manifest file are listed below in alphabetical order. These are the only legal elements; you cannot add your own elements or attributes.
If an element contains anything at all, it contains other elements. All values are set through attributes, not as character data within an element. Elements at the same level are generally not ordered. Because the prefix is universal, the documentation generally omits it when referring to attributes by name.
If you define a subclass, as you almost always would for the component classes Activity , Service , BroadcastReceiver , and ContentProvider , the subclass is declared through a name attribute. The name must include the full package designation. For example, an Service subclass might be declared as follows:.
The following assignment is the same as the one above:. When starting a component, Android creates an instance of the named subclass. If a subclass isn't specified, it creates an instance of the base class. For example:. Values from a theme are expressed in a similar manner, but with an initial '?
The core components of an application its activities, services, and broadcast receivers are activated by intents. An intent is a bundle of information an Intent object describing a desired action — including the data to be acted upon, the category of component that should perform the action, and other pertinent instructions.
Android locates an appropriate component to respond to the intent, launches a new instance of the component if one is needed, and passes it the Intent object.
Components advertise their capabilities — the kinds of intents they can respond to — through intent filters. A component may have any number of filters, each one describing a different capability.
An intent that explicitly names a target component will activate that component; the filter doesn't play a role. But an intent that doesn't specify a target by name can activate a component only if it can pass through one of the component's filters. For information on how Intent objects are tested against intent filters, see a separate document, Intents and Intent Filters.
A number of elements have icon and label attributes for a small icon and a text label that can be displayed to users. Some also have a description attribute for longer explanatory text that can also be shown on-screen.
In every case, the icon and label set in a containing element become the default icon and label settings for all of the container's subelements.
The icon and label set for an intent filter are used to represent a component whenever the component is presented to the user as fulfilling the function advertised by the filter. For example, a filter with " android. MAIN " and " android. LAUNCHER " settings advertises an activity as one that initiates an application — that is, as one that should be displayed in the application launcher. The icon and label set in the filter are therefore the ones displayed in the launcher.
A permission is a restriction limiting access to a part of the code or to data on the device. The limitation is imposed to protect critical data and code that could be misused to distort or damage the user experience. Each permission is identified by a unique label.
Often the label indicates the action that's restricted. For example, here are some permissions defined by Android:. Then, when the application is installed on the device, the installer determines whether or not to grant the requested permission by checking the authorities that signed the application's certificates and, in some cases, asking the user.
If the permission is granted, the application is able to use the protected features. If not, its attempts to access those features will simply fail without any notification to the user.
An application can also protect its own components activities, services, broadcast receivers, and content providers with permissions.
It can employ any of the permissions defined by Android listed in android. Or it can define its own. For example, an activity could be protected as follows:. Its use must be requested in order for other components of the application to launch the protected activity, even though the protection is imposed by the application itself. If, in the same example, the permission attribute was set to a permission declared elsewhere such as android. It affects only how the permissions are grouped when presented to the user.
Every application is linked against the default Android library, which includes the basic packages for building applications with common classes such as Activity, Service, Intent, View, Button, Application, ContentProvider, and so on.
However, some packages reside in their own libraries. If your application uses code from any of these packages, it must explicitly asked to be linked against them. The library name can be found in the documentation for the package. About Android Legal Support. Quicknav Quicknav. Results Loading Content Providers. Android Manifest. User Interface.
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Web Apps. Supporting Multiple Screens. The AndroidManifest. Previous Next. Except as noted, this content is licensed under Creative Commons Attribution 2. For details and restrictions, see the Content License.
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