LayoutInflater 参数详解

1.inflate 的 参数详解

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot)

resource : 资源文件
root ：父节点
attachToRoot ： 表示是否将resource指定的布局添加到root中

2.root 为 null 或 非null 时区别

root 为 null 时，resource的顶层结点宽高和间距都不生效
root 不为 null 时，resource 便根据 root 来决定
**让我们跟进源码来解析 **

2.1 root 为null时

        View view = LayoutInflater.from(MainActivity.this).inflate(R.layout.activity_test,null);
        ll.addView(view)//Linearlayout

此时一直跟进到

//LayoutInflater.java
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
    //上面代码忽略
      // Temp is the root view that was found in the xml
      final View temp = createViewFromTag(root, name, inflaterContext, attrs);//此时 temp 没有 mLayoutParams
       ViewGroup.LayoutParams params = null;

    if (root != null) {
        if (DEBUG) {
            System.out.println("Creating params from root: " +
                               root);
        }
        // Create layout params that match root, if supplied
        params = root.generateLayoutParams(attrs);
        if (!attachToRoot) {
            // Set the layout params for temp if we are not
            // attaching. (If we are, we use addView, below)
            temp.setLayoutParams(params);
        }
    }
    //下面代码忽略
}

由上可知，当 root 为空时，temp 的 mLayoutParams 未赋值，故 temp 没有布局参数

    注意：temp没有布局参数，但是temp的child都有 mLayoutParams

接下来分析 ll.addView(view)

//ViewGroup.java
 public void addView(View child, int index) {
        if (child == null) {
            throw new IllegalArgumentException("Cannot add a null child view to a ViewGroup");
        }
        LayoutParams params = child.getLayoutParams();
        if (params == null) {
            params = generateDefaultLayoutParams();
            if (params == null) {
                throw new IllegalArgumentException("generateDefaultLayoutParams() cannot return null");
            }
        }
        addView(child, index, params);
    }
//LinearLayout.java
    @Override
    protected LayoutParams generateDefaultLayoutParams() {
        if (mOrientation == HORIZONTAL) {
            return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
        } else if (mOrientation == VERTICAL) {
            return new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.WRAP_CONTENT);
        }
        return null;
    }

可以看到当 view 的 mLayoutParams 为空时，会给 view 设置一个 LayoutParams

2.2 root 不为 null 时

由上面代码可知，当root不为null时，会走到

 if (root != null) {
        if (DEBUG) {
            System.out.println("Creating params from root: " +
                               root);
        }
        // Create layout params that match root, if supplied
        params = root.generateLayoutParams(attrs);
        if (!attachToRoot) {
            // Set the layout params for temp if we are not
            // attaching. (If we are, we use addView, below)
            temp.setLayoutParams(params);
        }
    }

当attachToRoot为false时，此时会执行 temp.setLayoutParams(params);，那么view就有宽高间距等属性，那么 ll.addView(view)就直接用view的布局了

3. attachToRoot 为false或true的区别

attachToRoot 为 false 时，会给根据layout生成的view赋值

 if (!attachToRoot) {
 // Set the layout params for temp if we are not
 // attaching. (If we are, we use addView, below)
     temp.setLayoutParams(params);
 }

attachToRoot 为 true 时，自动将布局加入到 root 中

 if (root != null && attachToRoot) {
     root.addView(temp, params);
 }

4.总结

LayoutInflater.get(context).inflate(1,2,3) 核心方法就是

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {}

当 root 为 null 时，生成的 View mLayoutParmas=null
当 root 不为 null 时，生成的 View 有 mLayoutParams 字段

当 attachToRoot 为 true 时，会执行 root.addView(view,params) 方法 ，所以不用手动加