Android流式布局的实现原理

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：

　　

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

1. 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。