View | Android Developers
[android help] onSizeChanged() calls onCreate() and onStart()? - Android the game will start and animations will run once the view is created. I've moved the calling line both to the onStart() method and even the No comments: .. of same activity (1); Android get current date and show it in TextView. Onsizechanged not called dating - Marriage not dating ost wikipedia You should not call methods that perform these actions on views yourself unless you of my new android The custom View will be called TouchView and it will inherit from. This method will be called when the system decides that your view should be However, the Canvas dimensions are not reliable in telling you the size of your view. across a common solution that overrides the onSizeChanged method. statements coupons currencies custom dates day counters design.
For each dimension, it can specify one of: There are subclasses of LayoutParams for different subclasses of ViewGroup. MeasureSpecs are used to push requirements down the tree from parent to child. A MeasureSpec can be in one of three modes: This is used by a parent to determine the desired dimension of a child view. This is used by the parent to impose an exact size on the child. The child must use this size, and guarantee that all of its descendants will fit within this size.
This is used by the parent to impose a maximum size on the child. The child must guarantee that it and all of its descendants will fit within this size. To initiate a layout, call requestLayout.
Android™ How to Program, Second Edition by Abbey Deitel, Harvey Deitel, Paul Deitel
This method is typically called by a view on itself when it believes that is can no longer fit within its current bounds. Drawing Drawing is handled by walking the tree and recording the drawing commands of any View that needs to update.
After this, the drawing commands of the entire tree are issued to screen, clipped to the newly damaged area. The tree is largely recorded and drawn in order, with parents drawn before i. If you set a background drawable for a View, then the View will draw it before calling back to its onDraw method. To force a view to draw, call invalidate.
Event Handling and Threading The basic cycle of a view is as follows: An event comes in and is dispatched to the appropriate view.
The view handles the event and notifies any listeners. If in the course of processing the event, the view's bounds may need to be changed, the view will call requestLayout. Similarly, if in the course of processing the event the view's appearance may need to be changed, the view will call invalidate. If either requestLayout or invalidate were called, the framework will take care of measuring, laying out, and drawing the tree as appropriate. The entire view tree is single threaded.
You must always be on the UI thread when calling any method on any view. If you are doing work on other threads and want to update the state of a view from that thread, you should use a Handler. Focus Handling The framework will handle routine focus movement in response to user input.
RelativeLayout | Android Developers
This includes changing the focus as views are removed or hidden, or as new views become available. Views indicate their willingness to take focus through the isFocusable method. To change whether a view can take focus, call setFocusable boolean. When in touch mode see notes below views indicate whether they still would like focus via isFocusableInTouchMode and can change this via setFocusableInTouchMode boolean.
Focus movement is based on an algorithm which finds the nearest neighbor in a given direction. In rare cases, the default algorithm may not match the intended behavior of the developer. In these situations, you can provide explicit overrides by using these XML attributes in the layout file: Touch Mode When a user is navigating a user interface via directional keys such as a D-pad, it is necessary to give focus to actionable items such as buttons so the user can see what will take input.
If the device has touch capabilities, however, and the user begins interacting with the interface by touching it, it is no longer necessary to always highlight, or give focus to, a particular view.
This motivates a mode for interaction named 'touch mode'. For a touch capable device, once the user touches the screen, the device will enter touch mode. From this point onward, only views for which isFocusableInTouchMode is true will be focusable, such as text editing widgets.
Other views that are touchable, like buttons, will not take focus when touched; they will only fire the on click listeners.
Any time a user hits a directional key, such as a D-pad direction, the view device will exit touch mode, and find a view to take focus, so that the user may resume interacting with the user interface without touching the screen again. The touch mode state is maintained across Activity s. Call isInTouchMode to see whether the device is currently in touch mode.
Scrolling The framework provides basic support for views that wish to internally scroll their content. This includes keeping track of the X and Y scroll offset as well as mechanisms for drawing scrollbars. See scrollBy int, intscrollTo int, intand awakenScrollBars for more details. Tags Unlike IDs, tags are not used to identify views.
Tags are essentially an extra piece of information that can be associated with a view. They are most often used as a convenience to store data related to views in the views themselves rather than by putting them in a separate structure. Tags may be specified with character sequence values in layout XML as either a single tag using the android: Themes By default, Views are created using the theme of the Context object supplied to their constructor; however, a different theme may be specified by using the android: In the following example, both views will be created using the Material dark color scheme; however, because an overlay theme is used which only defines a subset of attributes, the value of android: These properties can be used to set persistent state associated with these rendering-related properties on the view.
The properties and methods can also be used in conjunction with Animator -based animations, described more in the Animation section. Animation Starting with Android 3. These Animator -based classes change actual properties of the View object, such as alpha and translationX. This behavior is contrasted to that of the pre In particular, the ViewPropertyAnimator class makes animating these View properties particularly easy and efficient. Alternatively, you can use the pre You can attach an Animation object to a view using setAnimation Animation or startAnimation Animation.
The onDraw method is called from the UI thread. This means that you should make sure that the code inside the method performs as fast as possible. Anything that takes too much time should either be done during initialisation or in a separate thread.
Allocating objects with new is can be quite a performance drag, especially if done repeatedly. For this reason you should avoid allocating new objects within the onDraw method.
If your drawing becomes elaborate and requires some time consuming computation you should consider placing the code in a separate thread. This will take load off the UI thread and make your view more responsive.
One class that is very useful in this context is the Picture class. I will be talking about drawing into a Picture in a future tutorial. Getting the dimensions One thing you will need to do when drawing your view onto the canvas is to find out how much space you have for your view. You might be tempted to look at the Canvas reference and you will find that Canvas provides two methods getWidth and getHeight.
However, the Canvas dimensions are not reliable in telling you the size of your view.
Instead you should use the getWidth and getHeight of the View itself. If you search the internet for ways to find the width and height of a custom view you will come across a common solution that overrides the onSizeChanged method. This solution might lead to slight performance increase but is not really necessary if you draw your view completely from the main thread.
A future tutorial will go into the details of how to use the onSizeChanged method. For simple views, you can call getWidth and getHeight from within the onDraw method. Do I need to call super.