From the developer guide it says that
though a view can define a padding, it does not provide any support for margins. However, view groups provide such a support.
but why i can set layout_margin attributes in ImageView,EditView and so on,they exist and work just like padding,
I can't understand what the guide says, Can someone help me to understand it?
Basically that means that margins are defined in xml for child views, but are used by their parent.
Technically, paddings are fields of the View class. Paddings are being used in the View.draw() method by a View itself. See:
http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/5.0.2_r1/android/view/View.java#15156
Margins are fields of the MarginLayoutParams class. Margins are used by a ViewGroup to layout its children. See:
http://developer.android.com/reference/android/view/ViewGroup.MarginLayoutParams.html
EDIT:
Margins are loaded to MarginLayoutParams and then used in the layouting phase.
Method which uses these xml attributes to create MarginLayoutParams in FrameLayout:
http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/5.0.2_r1/android/widget/FrameLayout.java#678
Loading margins: http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/5.0.2_r1/android/view/ViewGroup.java#6619
Layouting: http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/5.0.2_r1/android/widget/LinearLayout.java#1539
View class does not contain margins. It contains the padding because padding offset the contents of the view. Margin is meant to offset the view itself from the parent, ViewGroup. As such, the margins are contained in the ViewGroup class.
However remember the principle of inheritance. A view is a child of the ViewGroup class. As a result, it inherits the properties of the ViewGroup, including the margins. So when you apply the margin on the view, it responds because it already has the margins properties by virtue of inheritance.
Related
I'm new to Android development so this might be an obvious question, but I've looked at multiple sources and can't find the answer. If you put widget elements inside a layout element then the parent of those widget elements is the layout element, right? So what is the parent element of the layout? Because in the default generated file, activity_main.xml (for the layout), there is the attribute android:layout_width="match_parent" for the topmost constraint layout element. So, what is the parent of this constraint layout element?
Your ConstraintLayout is the root layout of your Activity but it is just part of a layout created by the system.
You can for example get the reference of the parent layout of your ConstraitLayout:
ViewGroup layoutRoot = findViewById(android.R.id.content);
When you set match_parent, your View (a Layout is a ViewGroup which is just a View) will try to fill the space available in its parent layout. So, in a certain way, it just tell to how much your view can grow in the X/Y-axis. In your case, your ConstraintLayout will grow in order to match that android.R.id.content
Of course, there's one View on the top of the stack (probably the decorView of your Window) which is created by the System and is assuming whole area available on the Window created by the system.
Basically we use parent layout as Linear Layout/Relative Layou/ConstraintLayout/Frame Layout etc. So all are classes which extends ViewGroup. Please refer to Google developer documentation for better understanding.
Read this -
https://developer.android.com/reference/android/view/ViewGroup.html
match_parent is a fixed value for views to stretch according to their parents.
For example let's say you have a Textview inside a RelativeLayout. If the Textview is set to match parent then it would stretch according to its parent/rootview which is the Relative layout.
So basically it would have as the same height and width as the parent layout.
I would like to know which is more profficient way of placing children in RelativeLayout. There are two approaches of doing this:
1) Place the main view with absolute position (like layout_centerInParent or set margins/paddings correspondent to parent view) After that you add other views and set them attributes like android:layout_above="#id/relative_view_id" and place them below relative view. It is not good way because your views hierarchy in xml does not match to what you see in preview.
2) You assign to children of RelativeLayout attributes with absolute id android:layout_above="#+id/relative_view_id" (+ appeared). It provides the correct views order in xml. BUT when you looking for declaration of view with relative_view_id from java code (by pressing cmd+B) Android Studio suggests all the views where you declared #+id. In our case View with attribute android:layout_above="#+id/relative_view_id" will also appear in search results.
What is your way of placing Views in RelativeLayout?
android:layout_above
Positions the bottom edge of this view above the given anchor view ID.
Accommodates bottom margin of this view and top margin of anchor view.
For your question I would prefer No 1 way .
android:layout_above="#id/relative_view_id"
Its refer the already generated id (relative_view_id) .
I am looking for a way to define custom attributes for views that are used by their children. For example layout_centerInParent for children of RelativeLayouts or layout_span for children of TableLayouts.
Just in case someone else gets here hoping for a direction like I did:
The layout parameters that are defined in the children are used in the parents. For example, layout_centerInParent is used by the RelativeLayout parent to position the child according to its value of this attribute.
In a custom ViewGroup you can create an inner class that extends ViewGroup.LayoutParams. Then you can use XML attributes within the children and retrieve them in the custom layout using the View#getLayoutParams() method.
You can look at this question and its answer.
I've been trying to start programming with the Android OS. Using the tutorials on the website I see that such views as LinearLayout, GridLayout use layout_width and layout_height xml attributes but the documentation never shows the inheritance of these attributes for these layouts. I have read where they're required but don't see how they can use something they never inherit.
Hopefully a simple question that has been bugging me.
Check http://developer.android.com/reference/android/view/ViewGroup.LayoutParams.html and http://developer.android.com/guide/topics/ui/declaring-layout.html
XML layout attributes named layout_something define layout parameters for the View that are appropriate for the ViewGroup in which it resides.
Every ViewGroup class implements a nested class that extends ViewGroup.LayoutParams. 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 1, the parent view group defines layout parameters for each child view (including the child view group).
Note that every LayoutParams subclass has its own syntax for setting values. Each child element must define LayoutParams that are appropriate for its parent, though it may also define different LayoutParams for its own children.
All view groups include a width and height (layout_width and layout_height), and each view is required to define them.
These attributes are inherited by LayoutParams from ViewGroup.LayoutParams: LinearLayout.LayoutParams, GridLayout.LayoutParams, etc.
I came through many examples in internet.I found that neither Relative Layout nor Linear Layout supports android:layout_gravity.By it I mean the views inside these layouts does not support android:layout_gravity attribute. So any one having idea which layout supports android:layout_gravity and how to use it in better way?
Children (that is, direct descendants in the View hierarchy) of LinearLayout do use layout_gravity (see LinearLayout.LayoutParams), but only on the "secondary" axis. So, in a vertical LinearLayout, center_horiztonal will work, but center_vertical will do nothing.
Children of FrameLayout also support layout_gravity (see FrameLayout.LayoutParams).
Keep in mind that the layout_* parameters set values in a LayoutParams object provided by the view parent. So a layout_* parameter will only have an effect if the parent view supports the parameter.
Actually if you use RelativeLayout you don't need to use layout_gravity.Better way to position your layout's elements are android. Here you can get a good explanation how to use RelativeLayout.