RelativeLayout/ 相对布局简介、使用、说明

 android  RelativeLayout/ 相对布局简介、使用、说明已关闭评论
7月 012021
 

网上找到的一篇对relativeLayout使用非常好的文章,分享下, 原文地址见文件末尾


在上一节中我们对LinearLayout进行了详细的解析,LinearLayout也是我们 用的比较多的一个布局,我们更多的时候更钟情于他的weight(权重)属性,等比例划分,对屏幕适配还是 帮助蛮大的;但是使用LinearLayout的时候也有一个问题,就是当界面比较复杂的时候,需要嵌套多层的 LinearLayout,这样就会降低UI Render的效率(渲染速度),而且如果是listview或者GridView上的 item,效率会更低,另外太多层LinearLayout嵌套会占用更多的系统资源,还有可能引发stackoverflow; 但是如果我们使用RelativeLayout的话,可能仅仅需要一层就可以完成了,以父容器或者兄弟组件参考+margin +padding就可以设置组件的显示位置,是比较方便的!当然,也不是绝对的,具体问题具体分析吧! 总结就是:尽量使用RelativeLayout + LinearLayout的weight属性搭配使用吧!


1.核心属性图


2.父容器定位属性示意图


3.根据兄弟组件定位

恩,先说下什么是兄弟组件吧,所谓的兄弟组件就是处于同一层次容器的组件,如图

图中的组件1,2就是兄弟组件了,而组件3与组件1或组件2并不是兄弟组件,所以组件3不能通过 组件1或2来进行定位,比如layout_toleftof = “组件1″这样是会报错的!切记! 关于这个兄弟组件定位的最经典例子就是”梅花布局”了,下面代码实现下:

运行效果图:

实现代码:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"    
    xmlns:tools="http://schemas.android.com/tools"    
    android:id="@+id/RelativeLayout1"    
    android:layout_width="match_parent"    
    android:layout_height="match_parent" >    
    
    <!-- 这个是在容器中央的 -->    
        
    <ImageView    
        android:id="@+id/img1"     
        android:layout_width="80dp"    
        android:layout_height="80dp"    
        android:layout_centerInParent="true"    
        android:src="@drawable/pic1"/>    
        
    <!-- 在中间图片的左边 -->    
    <ImageView    
        android:id="@+id/img2"     
        android:layout_width="80dp"    
        android:layout_height="80dp"    
        android:layout_toLeftOf="@id/img1"    
        android:layout_centerVertical="true"    
        android:src="@drawable/pic2"/>    
        
    <!-- 在中间图片的右边 -->    
    <ImageView    
        android:id="@+id/img3"     
        android:layout_width="80dp"    
        android:layout_height="80dp"    
        android:layout_toRightOf="@id/img1"    
        android:layout_centerVertical="true"    
        android:src="@drawable/pic3"/>    
        
    <!-- 在中间图片的上面-->    
    <ImageView    
        android:id="@+id/img4"     
        android:layout_width="80dp"    
        android:layout_height="80dp"    
        android:layout_above="@id/img1"    
        android:layout_centerHorizontal="true"    
        android:src="@drawable/pic4"/>    
        
    <!-- 在中间图片的下面 -->    
    <ImageView    
        android:id="@+id/img5"     
        android:layout_width="80dp"    
        android:layout_height="80dp"    
        android:layout_below="@id/img1"    
        android:layout_centerHorizontal="true"    
        android:src="@drawable/pic5"/>    
    
</RelativeLayout>

4.margin与padding的区别

初学者对于这两个属性可能会有一点混淆,这里区分下: 首先margin代表的是偏移,比如marginleft = “5dp”表示组件离容器左边缘偏移5dp; 而padding代表的则是填充,而填充的对象针对的是组件中的元素,比如TextView中的文字 比如为TextView设置paddingleft = “5dp”,则是在组件里的元素的左边填充5dp的空间! margin针对的是容器中的组件,而padding针对的是组件中的元素,要区分开来! 下面通过简单的代码演示两者的区别:

比较示例代码如下:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"    
    xmlns:tools="http://schemas.android.com/tools"    
    android:layout_width="match_parent"    
    android:layout_height="match_parent"    
    android:paddingBottom="@dimen/activity_vertical_margin"    
    android:paddingLeft="@dimen/activity_horizontal_margin"    
    android:paddingRight="@dimen/activity_horizontal_margin"    
    android:paddingTop="@dimen/activity_vertical_margin"    
    tools:context=".MainActivity" >    
    
    <Button    
        android:id="@+id/btn1"     
        android:layout_height="wrap_content"    
        android:layout_width="wrap_content"    
        android:text="Button"/>    
    <Button    
        android:paddingLeft="100dp"     
        android:layout_height="wrap_content"    
        android:layout_width="wrap_content"    
        android:text="Button"    
        android:layout_toRightOf="@id/btn1"/>    
        
    <Button    
        android:id="@+id/btn2"     
        android:layout_height="wrap_content"    
        android:layout_width="wrap_content"    
        android:text="Button"    
        android:layout_alignParentBottom="true"/>    
    <Button    
        android:layout_marginLeft="100dp"     
        android:layout_height="wrap_content"    
        android:layout_width="wrap_content"    
        android:text="Button"    
        android:layout_toRightOf="@id/btn2"     
        android:layout_alignParentBottom="true"/>    
        
</RelativeLayout>

运行效果图比较:


5.很常用的一点:margin可以设置为负数

相信很多朋友都不知道一点吧,平时我们设置margin的时候都习惯了是正数的, 其实是可以用负数的,下面写个简单的程序演示下吧,模拟进入软件后,弹出广告 页面的,右上角的cancle按钮的margin则是使用负数的!

贴出的广告Activity的布局代码吧,当然,如果你对这个有兴趣的话可以下下demo, 因为仅仅是实现效果,所以代码会有些粗糙!

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"  
    xmlns:tools="http://schemas.android.com/tools"  
    android:layout_width="match_parent"  
    android:layout_height="match_parent"  
    tools:context="com.jay.example.relativelayoutdemo.MainActivity"   
    android:background="#00CCCCFF">  
  
    <ImageView  
        android:id="@+id/imgBack"  
        android:layout_width="200dp"  
        android:layout_height="200dp"  
        android:layout_centerInParent="true"  
        android:background="@drawable/myicon" />  
  
    <ImageView  
        android:id="@+id/imgCancle"  
        android:layout_width="28dp"  
        android:layout_height="28dp"  
        android:layout_alignRight="@id/imgBack"  
        android:layout_alignTop="@id/imgBack"  
        android:background="@drawable/cancel"  
        android:layout_marginTop="-15dp"  
        android:layout_marginRight="-10dp" />  
  
</RelativeLayout>

来自:https://www.runoob.com/w3cnote/android-tutorial-relativelayout.html

css中 border, margin, padding,Background,Font,属性简写速记说明

 css  css中 border, margin, padding,Background,Font,属性简写速记说明已关闭评论
9月 262016
 

以下文字来自msn官网,内容简明扼要,分享下:

Definition

Shorthand properties are CSS properties that let you set the values of several other CSS properties simultaneously. Using a shorthand property, a Web developer can write more concise and often more readable style sheets, saving time and energy.

The CSS specification defines shorthand properties to group the definition of common properties acting on the same theme. E. g. the CSSbackground property is a shorthand property that’s able to define the value of background-colorbackground-imagebackground-repeat, and background-position. Similarly, the most common font-related properties can be defined using the shorthand font, and the different margins around a box can be defined using the margin shorthand.

Tricky edge cases

Even if they are very convenient to use, there are a few edge cases to keep in mind when using them:

  1. A value which is not specified is set to its initial value. That sounds anecdotal, but it really means that it overrides previously set values. Therefore:

    background-color: red; background: url(images/bg.gif) no-repeat top right;

    will not set the color of the background to red but to background-color‘s default, transparent, as the second rule has precedence.

  2. Only the individual properties values can inherit. As missing values are replaced by their initial value, it is impossible to allow inheritance of individual properties by omitting them. The keyword inherit can be applied to a property, but only as a whole, not as a keyword for one value or another. That means that the only way to make some specific value to be inherited is to use the longhand property with the keywordinherit.
  3. Shorthand properties try not to force a specific order for the values of the properties they replace. This works well when these properties use values of different types, as the order has no importance, but this does not work as easily when several properties can have identical values. Handling of these cases are grouped in several categories:

    1. Shorthands handling properties related to edges of a box, like border-stylemargin or padding, always use a consistent 1-to-4-value syntax representing those edges:

      border1.png The 1-value syntax: border-width: 1em — The unique value represents all edges
      border2.png The 2-value syntax: border-width: 1em 2em — The first value represents the vertical, that is top and bottom, edges, the second the horizontal ones, that is the left and right ones.
      border3.png The 3-value syntax: border-width: 1em 2em 3em — The first value represents the top edge, the second, the horizontal, that is left and right, ones, and the third value the bottom edge
      border4.png

      The 4-value syntax: border-width: 1em 2em 3em 4em — The four values represent the top, right, bottom and left edges respectively, always in that order, that is clock-wise starting at the top (The initial letter of Top-Right-Bottom-Left matches the order of the consonant of the word trouble: TRBL)

    2. Similarly, shorthands handling properties related to corners of a box, like border-radius, always use a consistent 1-to-4-value syntax representing those corners:

      corner1.png The 1-value syntax: border-radius: 1em — The unique value represents all corners
      corner2.png The 2-value syntax: border-radius: 1em 2em — The first value represents the top left and bottom right corner, the second the top right and bottom left ones.
      corner3.png The 3-value syntax: border-radius: 1em 2em 3em — The first value represents the top left corner, the second the top right and bottom left ones, and the third value the bottom right corner
      corner4.png

      The 4-value syntax: border-radius: 1em 2em 3em 4em — The four values represent the top left, top right, bottom right and bottom left corners respectively, always in that order, that is clock-wise starting at the top left.

Background properties

A background with the following properties:

background-color: #000; background-image: url(images/bg.gif); background-repeat: no-repeat; background-position: top right;

Can be shortened to just one declaration:

background: #000 url(images/bg.gif) no-repeat top right;

(The shorthand form is actually the equivalent of the longhand properties above plus background-attachment: scroll and, in CSS3, some additional properties.)

Font properties

The following declarations:

font-style: italic; font-weight: bold; font-size: .8em; line-height: 1.2; font-family: Arial, sans-serif;

can be shortened to the following:

font: italic bold .8em/1.2 Arial, sans-serif;

This shorthand declaration is actually equivalent to the longhand declarations above plus font-variant: normal and font-size-adjust: none (CSS2.0 / CSS3), font-stretch: normal (CSS3).

Border properties

With borders, the width, color, and style can be simplified into one declaration. For example,

border-width: 1px; border-style: solid; border-color: #000;

Can be written as

border: 1px solid #000;

Margin and Padding properties

Shorthand versions of margin and padding values work the same way. The following CSS declarations:

margin-top: 10px; margin-right: 5px; margin-bottom: 10px; margin-left: 5px;

are the same as the following declaration (note that the values are in clockwise order from top: top, right, bottom, then left (TRBL, the consonants in “trouble”))

margin: 10px 5px 10px 5px;