Android流式布局的实现原理

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我是靠谱客的博主健康黑裤，最近开发中收集的这篇文章主要介绍Android流式布局的实现原理，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

1.流式布局的特点以及应用场景
特点：当上面一行的空间不够容纳新的TextView时候，
才开辟下一行的空间

　　原理图：

　　这里写图片描述

场景：主要用于关键词搜索或者热门标签等场景

2.自定义ViewGroup,重点重写下面两个方法

1、onMeasure:测量子view的宽高，设置自己的宽和高

2、onLayout:设置子view的位置

onMeasure:根据子view的布局文件中属性，来为子view设置测量模式和测量值
测量=测量模式+测量值；

测量模式有3种：
EXACTLY：表示设置了精确的值，一般当childView设置其宽、高为精确值、match_parent时，ViewGroup会将其设置为EXACTLY；
AT_MOST：表示子布局被限制在一个最大值内，一般当childView设置其宽、高为wrap_content时，ViewGroup会将其设置为AT_MOST；
UNSPECIFIED：表示子布局想要多大就多大，一般出现在AadapterView的item的heightMode中、ScrollView的childView的heightMode中；此种模式比较少见。

3.LayoutParams
ViewGroup LayoutParams :每个 ViewGroup 对应一个 LayoutParams; 即 ViewGroup -> LayoutParams
getLayoutParams 不知道转为哪个对应的LayoutParams ,其实很简单，就是如下：
子View.getLayoutParams 得到的LayoutParams对应的就是子View所在的父控件的LayoutParams;
例如，LinearLayout 里面的子view.getLayoutParams ->LinearLayout.LayoutParams
所以咱们的FlowLayout 也需要一个LayoutParams，由于上面的效果图是子View的 margin，
所以应该使用MarginLayoutParams。即FlowLayout->MarginLayoutParams

自定义热门标签的ViewGroup实现

根据上面的技术分析，自定义类继承于ViewGroup，并重写 onMeasure和onLayout等方法。具体实现代码如下：

package com.czm.flowlayout;  

import java.util.ArrayList;  
import java.util.List;  

import android.content.Context;  
import android.util.AttributeSet;  
import android.view.View;  
import android.view.ViewGroup;  
/**  
 *   
 * @author caizhiming  
 * @created on 2015-4-13  
 */  
public class XCFlowLayout extends ViewGroup{  

    //存储所有子View  
    private List<List<View>> mAllChildViews = new ArrayList<>();  
    //每一行的高度  
    private List<Integer> mLineHeight = new ArrayList<>();  

    public XCFlowLayout(Context context) {  
        this(context, null);  
        // TODO Auto-generated constructor stub  
    }  
    public XCFlowLayout(Context context, AttributeSet attrs) {  
        this(context, attrs, 0);  
        // TODO Auto-generated constructor stub  
    }  
    public XCFlowLayout(Context context, AttributeSet attrs, int defStyle) {  
        super(context, attrs, defStyle);  
        // TODO Auto-generated constructor stub  
    }  
    @Override  
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {  
        // TODO Auto-generated method stub  

        //父控件传进来的宽度和高度以及对应的测量模式  
        int sizeWidth = MeasureSpec.getSize(widthMeasureSpec);  
        int modeWidth = MeasureSpec.getMode(widthMeasureSpec);  
        int sizeHeight = MeasureSpec.getSize(heightMeasureSpec);  
        int modeHeight = MeasureSpec.getMode(heightMeasureSpec);  

        //如果当前ViewGroup的宽高为wrap_content的情况  
        int width = 0;//自己测量的 宽度  
        int height = 0;//自己测量的高度  
        //记录每一行的宽度和高度  
        int lineWidth = 0;  
        int lineHeight = 0;  

        //获取子view的个数  
        int childCount = getChildCount();  
        for(int i = 0;i < childCount; i ++){  
            View child = getChildAt(i);  
            //测量子View的宽和高  
            measureChild(child, widthMeasureSpec, heightMeasureSpec);  
            //得到LayoutParams  
            MarginLayoutParams lp = (MarginLayoutParams) getLayoutParams();  
            //子View占据的宽度  
            int childWidth = child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin;  
            //子View占据的高度  
            int childHeight = child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin;  
            //换行时候  
            if(lineWidth + childWidth > sizeWidth){  
                //对比得到最大的宽度  
                width = Math.max(width, lineWidth);  
                //重置lineWidth  
                lineWidth = childWidth;  
                //记录行高  
                height += lineHeight;  
                lineHeight = childHeight;  
            }else{//不换行情况  
                //叠加行宽  
                lineWidth += childWidth;  
                //得到最大行高  
                lineHeight = Math.max(lineHeight, childHeight);  
            }  
            //处理最后一个子View的情况  
            if(i == childCount -1){  
                width = Math.max(width, lineWidth);  
                height += lineHeight;  
            }  
        }  
        //wrap_content  
        setMeasuredDimension(modeWidth == MeasureSpec.EXACTLY ? sizeWidth : width,  
                modeHeight == MeasureSpec.EXACTLY ? sizeHeight : height);  
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);  
    }  

    @Override  
    protected void onLayout(boolean changed, int l, int t, int r, int b) {  
        // TODO Auto-generated method stub  
        mAllChildViews.clear();  
        mLineHeight.clear();  
        //获取当前ViewGroup的宽度  
        int width = getWidth();  

        int lineWidth = 0;  
        int lineHeight = 0;  
        //记录当前行的view  
        List<View> lineViews = new ArrayList<View>();  
        int childCount = getChildCount();  
        for(int i = 0;i < childCount; i ++){  
            View child = getChildAt(i);  
            MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();  
            int childWidth = child.getMeasuredWidth();  
            int childHeight = child.getMeasuredHeight();  

            //如果需要换行  
            if(childWidth + lineWidth + lp.leftMargin + lp.rightMargin > width){  
                //记录LineHeight  
                mLineHeight.add(lineHeight);  
                //记录当前行的Views  
                mAllChildViews.add(lineViews);  
                //重置行的宽高  
                lineWidth = 0;  
                lineHeight = childHeight + lp.topMargin + lp.bottomMargin;  
                //重置view的集合  
                lineViews = new ArrayList();  
            }  
            lineWidth += childWidth + lp.leftMargin + lp.rightMargin;  
            lineHeight = Math.max(lineHeight, childHeight + lp.topMargin + lp.bottomMargin);  
            lineViews.add(child);  
        }  
        //处理最后一行  
        mLineHeight.add(lineHeight);  
        mAllChildViews.add(lineViews);  

        //设置子View的位置  
        int left = 0;  
        int top = 0;  
        //获取行数  
        int lineCount = mAllChildViews.size();  
        for(int i = 0; i < lineCount; i ++){  
            //当前行的views和高度  
            lineViews = mAllChildViews.get(i);  
            lineHeight = mLineHeight.get(i);  
            for(int j = 0; j < lineViews.size(); j ++){  
                View child = lineViews.get(j);  
                //判断是否显示  
                if(child.getVisibility() == View.GONE){  
                    continue;  
                }  
                MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();  
                int cLeft = left + lp.leftMargin;  
                int cTop = top + lp.topMargin;  
                int cRight = cLeft + child.getMeasuredWidth();  
                int cBottom = cTop + child.getMeasuredHeight();  
                //进行子View进行布局  
                child.layout(cLeft, cTop, cRight, cBottom);  
                left += child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin;  
            }  
            left = 0;  
            top += lineHeight;  
        }  

    }  
    /**  
     * 与当前ViewGroup对应的LayoutParams  
     */  
    @Override  
    public LayoutParams generateLayoutParams(AttributeSet attrs) {  
        // TODO Auto-generated method stub  

        return new MarginLayoutParams(getContext(), attrs);  
    }  
}

2.相关的布局文件：

引用自定义控件：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"  
    xmlns:tools="http://schemas.android.com/tools"  
    android:id="@+id/container"  
    android:layout_width="match_parent"  
    android:layout_height="match_parent" >  

    <com.czm.flowlayout.XCFlowLayout  
        android:id="@+id/flowlayout"  
        android:layout_width="match_parent"  
        android:layout_height="match_parent" >  

    </com.czm.flowlayout.XCFlowLayout>  

</RelativeLayout>

TextView的样式文件：

[plain] view plaincopy
<?xml version="1.0" encoding="utf-8"?>  
<shape xmlns:android="http://schemas.android.com/apk/res/android" >  
    <solid android:color="#666666" />  
    <corners android:radius="10dp" />  
    <padding   
        android:left="5dp"  
        android:right="5dp"  
        android:top="5dp"  
        android:bottom="5dp"   
        />  

</shape>

三、使用该自定义布局控件类

最后，如何使用该自定义的热门标签控件类呢？很简单，请看下面实例代码：

[plain] view plaincopy
package com.czm.flowlayout;  

import android.app.Activity;  
import android.graphics.Color;  
import android.os.Bundle;  
import android.view.ViewGroup.LayoutParams;  
import android.view.ViewGroup.MarginLayoutParams;  
import android.widget.TextView;  
/**  
 *   
 * @author caizhiming  
 * @created on 2015-4-13  
 */  
public class MainActivity extends Activity {  

    private String mNames[] = {  
            "welcome","android","TextView",  
            "apple","jamy","kobe bryant",  
            "jordan","layout","viewgroup",  
            "margin","padding","text",  
            "name","type","search","logcat"  
    };  
    private XCFlowLayout mFlowLayout;  
    @Override  
    protected void onCreate(Bundle savedInstanceState) {  
        super.onCreate(savedInstanceState);  
        setContentView(R.layout.activity_main);  

        initChildViews();  

    }  
    private void initChildViews() {  
        // TODO Auto-generated method stub  
        mFlowLayout = (XCFlowLayout) findViewById(R.id.flowlayout);  
        MarginLayoutParams lp = new MarginLayoutParams(  
                LayoutParams.WRAP_CONTENT,LayoutParams.WRAP_CONTENT);  
        lp.leftMargin = 5;  
        lp.rightMargin = 5;  
        lp.topMargin = 5;  
        lp.bottomMargin = 5;  
        for(int i = 0; i < mNames.length; i ++){  
            TextView view = new TextView(this);  
            view.setText(mNames[i]);  
            view.setTextColor(Color.WHITE);  
            view.setBackgroundDrawable(getResources().getDrawable(R.drawable.textview_bg));  
            mFlowLayout.addView(view,lp);  
        }  
    }  

}

public class FlowLayout extends ViewGroup {
    private float mVerticalSpacing; //每个item纵向间距
    private float mHorizontalSpacing; //每个item横向间距

    public FlowLayout(Context context) {
        super(context);
    }
    public FlowLayout(Context context, AttributeSet attrs) {
        super(context, attrs);
    }
    public void setHorizontalSpacing(float pixelSize) {
        mHorizontalSpacing = pixelSize;
    }
    public void setVerticalSpacing(float pixelSize) {
        mVerticalSpacing = pixelSize;
    }
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int selfWidth = resolveSize(0, widthMeasureSpec);

        int paddingLeft = getPaddingLeft();
        int paddingTop = getPaddingTop();
        int paddingRight = getPaddingRight();
        int paddingBottom = getPaddingBottom();

        int childLeft = paddingLeft;
        int childTop = paddingTop;
        int lineHeight = 0;

        //通过计算每一个子控件的高度，得到自己的高度
        for (int i = 0, childCount = getChildCount(); i < childCount; ++i) {
            View childView = getChildAt(i);
            LayoutParams childLayoutParams = childView.getLayoutParams();
            childView.measure(
                    getChildMeasureSpec(widthMeasureSpec, paddingLeft + paddingRight,
                            childLayoutParams.width),
                    getChildMeasureSpec(heightMeasureSpec, paddingTop + paddingBottom,
                            childLayoutParams.height));
            int childWidth = childView.getMeasuredWidth();
            int childHeight = childView.getMeasuredHeight();

            lineHeight = Math.max(childHeight, lineHeight);

            if (childLeft + childWidth + paddingRight > selfWidth) {
                childLeft = paddingLeft;
                childTop += mVerticalSpacing + lineHeight;
                lineHeight = childHeight;
            } else {
                childLeft += childWidth + mHorizontalSpacing;
            }
        }

        int wantedHeight = childTop + lineHeight + paddingBottom;
        setMeasuredDimension(selfWidth, resolveSize(wantedHeight, heightMeasureSpec));
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        int myWidth = r - l;

        int paddingLeft = getPaddingLeft();
        int paddingTop = getPaddingTop();
        int paddingRight = getPaddingRight();

        int childLeft = paddingLeft;
        int childTop = paddingTop;

        int lineHeight = 0;

        //根据子控件的宽高，计算子控件应该出现的位置。
        for (int i = 0, childCount = getChildCount(); i < childCount; ++i) {
            View childView = getChildAt(i);

            if (childView.getVisibility() == View.GONE) {
                continue;
            }

            int childWidth = childView.getMeasuredWidth();
            int childHeight = childView.getMeasuredHeight();

            lineHeight = Math.max(childHeight, lineHeight);

            if (childLeft + childWidth + paddingRight > myWidth) {
                childLeft = paddingLeft;
                childTop += mVerticalSpacing + lineHeight;
                lineHeight = childHeight;
            }
            childView.layout(childLeft, childTop, childLeft + childWidth, childTop + childHeight);
            childLeft += childWidth + mHorizontalSpacing;
        }
    }
}

从控件创建过程说起
当这个流式布局在被加载如内存并显示在屏幕上这一过程中，首先会调用view.measure(w,h)这个方法，表示测量view的宽度与高度，其中参数w与h分别表示这个控件的父控件的宽高。
在view.measure()方法的调用过程中又会调用view本身的一个回调方法,onMeasure()，这个是view自身的一个回调方法，用于让开发者在自定义View的时候重新计算自身的大小。一般会在这个方法中循环遍历，计算出这个控件的全部子孙控件的宽高。
在View的宽高计算完成以后，考虑将这个控件显示到屏幕的指定位置上，此时view的onLayout()方法会被调用。一般同时会在这个方法中计算出全部子孙控件在这个控件中的位置。
可能基本流程有些枯燥，接下来结合代码看看。
流布局的实现
看到onMeasure()方法中的这段： //通过计算每一个子控件的高度，得到自己的高度

for (int i = 0, childCount = getChildCount(); i < childCount; ++i) {
        View childView = getChildAt(i);
        LayoutParams childLayoutParams = childView.getLayoutParams();
        childView.measure(
                getChildMeasureSpec(widthMeasureSpec, paddingLeft + paddingRight,
                        childLayoutParams.width),
                getChildMeasureSpec(heightMeasureSpec, paddingTop + paddingBottom,
                        childLayoutParams.height));
        int childWidth = childView.getMeasuredWidth();
        int childHeight = childView.getMeasuredHeight();

        lineHeight = Math.max(childHeight, lineHeight);

        if (childLeft + childWidth + paddingRight > selfWidth) {
            childLeft = paddingLeft;
            childTop += mVerticalSpacing + lineHeight;
            lineHeight = childHeight;
        } else {
            childLeft += childWidth + mHorizontalSpacing;
        }
    }

首先通过循环，遍历这个控件的所有子控件，同时调用子控件的measure()方法，这时measure方法的两个参数是控件能给这个子控件的最大宽高(我们都知道的，子控件再大，显示的大小也不能比父控件还大)。这里getChildMeasureSpec()方法的作用是用来计算一个合适子视图的尺寸大小（宽度或者高度)，结合我们从子视图的LayoutParams所给出的MeasureSpec信息来获取最合适的结果。比如，如果这个View知道自己的大小尺寸（因为它本身的MeasureSpec的model为Exactly,）并且子视图的大小恰好跟父窗口一样大，父窗口必须用给定的大小去layout子视图
参数含义：spec 父窗口传递给子视图的大小和模式
padding 父窗口的边距，也就是xml中的android:padding
childDimension 子视图想要绘制的准确大小，但最终不一定绘制此值
当得到了每一个子控件的大小以后，再要计算自己的宽高就简单了。
int wantedHeight = childTop + lineHeight + paddingBottom;
同理，在onLayout中的这一句

for (int i = 0, childCount = getChildCount(); i < childCount; ++i) {
        View childView = getChildAt(i);

        if (childView.getVisibility() == View.GONE) {
            continue;
        }

        int childWidth = childView.getMeasuredWidth();
        int childHeight = childView.getMeasuredHeight();

        lineHeight = Math.max(childHeight, lineHeight);

        if (childLeft + childWidth + paddingRight > myWidth) {
            childLeft = paddingLeft;
            childTop += mVerticalSpacing + lineHeight;
            lineHeight = childHeight;
        }
        childView.layout(childLeft, childTop, childLeft + childWidth, childTop + childHeight);
        childLeft += childWidth + mHorizontalSpacing;
    }

首先通过循环遍历，控制每个item子控件的显示位置，如果当前行还能放得下一个item，就放到当前行，如果放不下就放到下一行的最左边。
最终，遍历完成，也就相当于把自己的位置显示完成了。