Full introduction to flexboxlayout (Google official flexible implementation of flow layout control)

One 、FlexboxLayout What is it?

FlexboxLayout yes Google A powerful open source control , Direct inheritance ViewGroup, The effect is similar to the enhanced version LinearLayout, But with LinearLayout There is no connection .

The official original words are ：

FlexboxLayout is a library project which brings the similar capabilities of CSS Flexible Box Layout Module to Android.
intend ：FlexBoxLayout Is for Android Brought with CSS Flexible Box Layout（CSS Elastic box ） Libraries with similar functions .

Github Address ：https://github.com/google/flexbox-layout

Two 、 How to use FlexboxLayout

FlexBoxLayout There are many properties , Next, verify the functions of each attribute one by one

Citation depends on

dependencies {
    implementation 'com.google.android:flexbox:1.1.0'
}

If you haven't used it yet AndroidX：

dependencies {
    implementation 'com.google.android:flexbox:1.0.0'
}

Simple use in layout

<com.google.android.flexbox.FlexboxLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="@color/colorPrimary"
        app:flexWrap="wrap">

        <TextView
            style="@style/TextStyle"
            android:text=" Push and instant messaging "/>

        <TextView
            style="@style/TextStyle"
            android:text=" bluetooth " />

        <TextView
            style="@style/TextStyle"
            android:text=" The programmer "/>

        <TextView
            style="@style/TextStyle"
            android:text=" Movie paradise " />

        <TextView
            style="@style/TextStyle"
            android:text=" Guo Degang " />

        <TextView
            style="@style/TextStyle"
            android:text=" travel —— On the road " />

        <TextView
            style="@style/TextStyle"
            android:text=" The avengers 4" />

    </com.google.android.flexbox.FlexboxLayout>

TextStyle as follows :

 <style name="TextStyle">
        <item name="android:layout_margin">5dp</item>
        <item name="android:layout_width">wrap_content</item>
        <item name="android:layout_height">wrap_content</item>
        <item name="android:background">@drawable/shape_pink_border</item>
        <item name="android:ellipsize">end</item>
        <item name="android:maxLines">1</item>
        <item name="android:padding">8dp</item>
        <item name="android:textColor">@android:color/white</item>
    </style>

shape_pink_border file ：

<shape xmlns:android="http://schemas.android.com/apk/res/android">
    <corners android:radius="8dp"/>
    <stroke android:width="1dp" android:color="@android:color/holo_red_light"/>
</shape>

Run to see the effect ：

You can see that the content has come out , But they are all in the same line , This is obviously not the effect I want .

Then the next step is FlexboxLayout The properties have come out to save the field ：

3、 ... and 、FlexboxLayout Property introduction

FlexboxLayout Direct attributes

1. app:flexWrap=“nowrap”

Simply speaking , This attribute indicates whether the line breaks and the direction of the line breaks .

Visible from above ,flexWrap Property has three enumerated values , Namely nowrap、wrap and wrap_reverse

• nowrap: Single line display , Don't wrap , The default is this property .
  
• wrap： When the content exceeds one line , Word wrap . effect ：
  
• wrap_reverse ： Reverse wrap （ The next line is above the current line ）
  

2. app:flexDirection=“row”

This attribute indicates the direction of the spindle , The arrangement of sub elements is added according to the axis direction .

Take a look at the source code , so flexDirection The enumeration values of are row and row_reverse,column and column_reverse

• row : The spindle direction shall be horizontal （ That's ok ） Direction layout , The default is this property . effect ：
  

• row_reverse : The spindle direction shall be horizontal （ That's ok ） The direction is reversed . effect ：
  
  As can be seen from the renderings ,row Is drawn from left to right ,row_reverse yes row The reverse of , That is, draw from right to left . Empathy column Is the same .

• column : The spindle direction shall be vertical （ Column ） Direction layout . effect ：
  

• column_reverse : The spindle direction shall be vertical （ Column ） The direction is reversed （ From the bottom up ）.
  

3. app:justifyContent=“flex_start”

About this property , There is an official statement ：

<!-- Omitting flex-flow property since it's reflected in the parent flex container. Set the flexDirection and/or flexWrap to the parent flex container explicitly if you want to use the flex-flow similar way to the web. -->

The function is to control the alignment of elements on the spindle , Need to cooperate with flexDirection or flexWrap Attribute to use .

Look at the source code , so app:justifyContent Attributes are flex_start、flex_end、center、space_between、space_around and space_evenly6 Enumeration values .

Next, change the above two attributes to ：

		app:flexWrap="wrap"
        app:flexDirection="row"

Look at the effect .

• flex_start: Align left , The default value is .
  

• flex_end : Right alignment .
  

• center： Align center .
  

• space_between: full-justified .
  

• space_around : Spread alignment .
  

• space_evenly: The child elements are evenly distributed in a row .
  

4. app:alignItems=“stretch”

This attribute indicates that the element is in Each axis Alignment on .

Be careful ： If set alignContent, And it's not worth it stretch, Then the attribute is invalid .

app:alignItems Attributes are flex_start、flex_end、center、baseline and stretch

The following FlexBoxLayout Specify a height , And for TextView Add different padding Look at the effect .

• stretch: The default value is , If the child element has no height set , The height of the parent layout .
  
• flex_start : Top alignment .
  
• flex_end : Bottom alignment .
  
• center : Align center .
  
• baseline : Align according to the baseline of the first element .
  

The following picture can directly express the function of this attribute . picture source

5. app:alignContent=“stretch”

The attribute represents Each axis Alignment throughout the layout .

app:alignContent Attributes are flex_start、flex_end、center、space_between、space_around and stretch6 Enumeration values .

• stretch: The default value is , The axis occupies the entire parent layout .
  

• flex_start: Align all grid lines at the top .
  

• flex_end: Align all grid lines at the bottom .
  

• center: Center all grid lines .
  

• space_between: Align all grid lines at both ends .
  

• space_around: Align all grid lines dispersedly .
  
  A word in , The axis direction can be determined by flexDirection Property to specify , Pay attention to attribute collocation , The lines .

6. dividerDrawable （reference）

Horizontal and vertical split lines .

7. dividerDrawableHorizontal / dividerDrawableVertical （reference）

level / Vertical split line .

8. showDivider

Displays the horizontal and vertical split line method .
Enumeration values are ：

• none
  No display .

• beginning
  

• middle
  

• end
  

9. showDividerHorizontal / showDividerVertical

Display level / Vertical split line method .

FlexboxLayout Child element attribute

1. app:layout_order=“1”

Specify the child element sorting priority , The smaller the value, the higher the ranking , The default value is 1. Set the value type as float.

Pictured , bluetooth TextView The elements are in xml Is ranked as the second , But for it layout_order Property specified as 2, bluetooth Ranked last .

2. app:layout_flexGrow=“0”

Assign the weight of the remaining controls on the same axis , The default value is 0, Means not participating in the distribution . The usage is similar to LinearLayout Of weight, however weight The entire parent layout control is assigned , and layout_flexGrow The same row is allocated / The remaining space of the column .

for instance ： take bluetooth Of layout_flexGrow Set to 1, Program Of layout_flexGrow Property set to 2

Pictured , bluetooth And Program Are involved in the allocation of remaining space , because Program The weight set is 2, So than bluetooth Double the space allocated .

3. app:layout_flexShrink=“0”

Sub element scaling , If line feed is set （flexWrap=“wrap or wrap_reverse”） The property is invalid .
Set the value type as float,0 Means no zoom , The greater the numerical , Larger scale , The default is 1, Invalid negative value .

Example ： Push and instant messaging Set to 0( Don't zoom ), travel —— On the road Set to 2（ Double zoom ）

4. app:layout_flexBasisPercent="-1"

Official explanation ：

	<!--
            The initial length in a percentage format relative to its parent. This is similar to the
            flex-basis property in the original CSS specification.
            (https://www.w3.org/TR/css-flexbox-1/#flex-basis-property)
            But unlike the flex-basis property, this attribute only accepts a value in fraction
            (percentage), whereas flex-basis property accepts width values such as 1em, 10px and
            the 'content' string.
            But specifying initial fixed width values can be done by specifying width values in
            layout_width (or layout_height, varies depending on the flexDirection). Also the same
            effect can be done by specifying "wrap_content" in layout_width (or layout_height) if
            developers want to achieve the same effect as 'content'.
            Thus, this attribute only accepts fraction values, which can't be done through
            layout_width (or layout_height) for simplicity.
        -->

Indicates that the child element length is a percentage of its parent layout length , Sets the of the element layout_flexBasisPercent The original length of the child element will be overwritten , The default value is -1. It should be noted that , Only when the length of the parent layout is specifically set can it take effect .
The set value is percentage , for example 50%.

Example ： Set the width of the first element to that of the parent layout 50%（app:layout_flexBasisPercent="50%"）.

5. app:layout_alignSelf=“auto”

Used and alignItems Same property , The difference is alignItems All elements are set , and layout_alignSelf Act on a single element .

One thing to note is that , If the parent layout is set alignContent, And the enumeration value is not stretch, Then the modified attribute is invalid .

layout_alignSelf The enumeration values of are auto,flex_start,flex_end,center,baseline and stretch, The functions and alignItems Same attribute .

Example ： take Push and instant messaging Set to flex_start, Bluetooth layout_alignSelf Set to stretch. effect ：

6.app:layout_wrapBefore=“false”

Forced line wrap , The default is false.
If... Is set for child element layout_wrapBefore The attribute is false, Then this child element will start a new line .

example ： by bluetooth Set up layout_wrapBefore The attribute is true

7. layout_minWidth / layout_minHeight

Limit FlexboxLayout Child elements （ Width or height ） Not less than the minimum , No matter what layout_flexShrink What are the attributes , Child elements will not be reduced to less than the minimum value set .

8. layout_maxWidth / layout_maxHeight

Limit FlexboxLayout Child elements （ Width or height ） Not greater than minimum , No matter what layout_flexGrow What are the attributes , Child elements will not be magnified to greater than the minimum value set .

Summary

FlexBoxLayout There are so many common attributes of , The combination of these attributes can achieve a very flexible effect .

Four 、 And RecyclerView In combination with

The official not only provides FlexboxLayout Layout , It also provides FlexboxLayoutManager Come and RecyclerView In combination with .

The second one is FlexboxLayoutManager that can be used within RecyclerView.

although RecyclerView Before that StaggeredGridLayoutManager To provide a waterfall effect , But still specify the row / Number of columns 、 by comparison ,FlexboxLayoutManager More flexible , And the application scenarios are different .

Next RecyclerView Of layoutManager Set to FlexboxLayoutManager
Let's take a look at the effect :

The white background is set to FlexboxLayoutManager Of RecyclerView, The green background is FlexboxLayout, so FlexboxLayoutManagery And FlexboxLayout Can achieve the same effect .

Main code ：

		val flexboxLayoutManager = FlexboxLayoutManager(this)
        flexboxLayoutManager.flexWrap = FlexWrap.WRAP
        flexboxLayoutManager.flexDirection = FlexDirection.ROW
        flexboxLayoutManager.justifyContent = JustifyContent.FLEX_START
        flexboxLayoutManager.alignItems = AlignItems.FLEX_START
		//flexboxLayoutManager.alignContent = AlignContent.FLEX_START

        val adapter = FlexAdapter()
        initData(adapter)
        rv_flex.layoutManager = flexboxLayoutManager
        rv_flex.adapter = adapter

Be careful ：flexboxLayoutManager.alignContent,FlexboxLayoutManager I won't support it alignContent attribute

to glance at setAlignContent Source code ：

	@Override
    public void setAlignContent(@AlignContent int alignContent) {
        throw new UnsupportedOperationException("Setting the alignContent in the "
                + "FlexboxLayoutManager is not supported. Use FlexboxLayout "
                + "if you need to use this attribute.");
    }

so ,FlexboxLayoutManager I won't support it alignContent attribute , If the setting is forced, the following exceptions will be reported .

Caused by: java.lang.UnsupportedOperationException: Setting the alignContent in the FlexboxLayoutManager is not supported. Use FlexboxLayout if you need to use this attribute.

Demo Source code

that FlexboxLayoutManager and FlexboxLayout What other uses are different ？ The official table gives a good explanation .

Attribute / Feature	FlexboxLayout	FlexboxLayoutManager (RecyclerView)
flexDirection
flexWrap		（except wrap_reverse）
justifyContent
alignItems
alignContent		-
layout_order		-
layout_flexGrow
layout_flexShrink
layout_alignSelf
layout_flexBasisPercent
layout_(min/max)Width
layout_(min/max)Height
layout_wrapBefore
Divider
View recycling	-
Scrolling	*1

5、 ... and 、 practice

Such a drain layout , If you encounter , It seems that you can only customize View, Or seek the library written by others .
But there is. FlexboxLayout Then it can be solved perfectly .

The above examples can be referred to ：https://gitee.com/zhengyz_0430/CloudReaderKotlin

6、 ... and 、 summary

Empty talk misses the country , Work hard to prosper the country

If you really want to understand FlexboxLayout Properties of , It is essential to practice and read the source code yourself , Don't talk much , Let's practice more .

FlexboxLayout And FlexboxLayoutManager Flexible streaming layout can be perfectly realized , And the library has been added to androidX 了 ,Google Dada really broke her heart for our development .

Reference resources

https://blog.csdn.net/tabolt/article/details/51799226
https://www.jianshu.com/p/3c471953e36d
https://www.oschina.net/news/73442/google-flexbox-layout