inflate方法两个参数和三个参数的区别

前言

今天在lint项目的代码时，又遇到了inflate方法的参数问题，之前看过相关的文章，又没有记录下来，导致时间长了就忘记了。今天再次遇到这个问题，便老老实实记录一下LayouInflater中inflate方法两个参数和三个参数的区别。

用法

LayoutInflater.from(RecylerActivity.this).inflate(R.layout.my_text_view,viewGroup,false);View.inflate(RecylerActivity.this, R.layout.my_text_view, null);

通过看源码得知View.inflate（）方法实际上调用的也是LayoutInflater.from（int resource, ViewGroup root, boolean attachToRoo）；

区别

我们最常用的便是LayoutInflater的inflate方法，这个方法重载了四种调用方式，分别为：

1. public View inflate(int resource, ViewGroup root)2. public View inflate(int resource, ViewGroup root, boolean attachToRoot)3. public View inflate(XmlPullParser parser, ViewGroup root)4. public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot)

这四种使用方式中，我们最常用的是第一种方式，inflate方法的主要作用就是将xml转换成一个View对象，用于动态的创建布局。虽然重载了四个方法，但是这四种方法最终调用的，还是第四种方式。第四种方式也很好理解，内部实现原理就是利用Pull解析器，对Xml文件进行解析，然后返回View对象。

inflate方法有三个参数，分别是
1.resource 布局的资源id

2.root 填充的根视图

3.attachToRoot 是否将载入的视图绑定到根视图中

我们经常使用的第一种形式为例，你在重写BaseAdapter的getView方法的时候是否这样做过

public View getView(int position, View convertView, ViewGroup parent) {    if (convertView == null) {        convertView = inflate(R.layout.item_row, null);    }    return convertView;}

我们将root参数设为空，功能确实实现了，但是这里还隐藏着一个隐患，这种方式并不是inflate正确的使用姿势，下面我们通过一个Demo，来说一下这样使用造成的弊端。
对应的布局文件如下

<!--?xml version="1.0" encoding="utf-8"?-->    <linearlayout xmlns:android="http://schemas.android.com/apk/res/android"        android:layout_width="match_parent"        android:layout_height="60dp"        android:background="@android:color/holo_orange_light"        android:gravity="center"        android:orientation="vertical">        <textview            android:id="@+id/tv"            android:layout_width="wrap_content"            android:layout_height="wrap_content"            android:text="11"            android:textcolor="@android:color/black"            android:textsize="22sp">        </textview>    </linearlayout>

OneActivity的代码如下

public class OneActivity extends Activity {    private ListView list1;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_one);        list1 = (ListView) findViewById(R.id.list1);        list1.setAdapter(new MyAdapter(this));    }    private class MyAdapter extends BaseAdapter {        private LayoutInflater inflater;        MyAdapter(Context context) {            inflater = LayoutInflater.from(context);        }        @Override        public int getCount() {            return 20;        }        @Override        public Object getItem(int position) {            return position;        }        @Override        public long getItemId(int position) {            return position;        }        @Override        public View getView(int position, View convertView, ViewGroup parent) {            if (convertView == null) {                convertView = inflater.inflate(R.layout.item_list, null);            }            TextView tv = (TextView) convertView.findViewById(R.id.tv);            tv.setText(position+"");            return convertView;        }    }}

TwoActivity的代码如下

public class TwoActivity extends Activity {    private ListView list2;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_two);        list2 = (ListView) findViewById(R.id.list2);        list2.setAdapter(new MyAdapter(this));    }    private class MyAdapter extends BaseAdapter {        private LayoutInflater inflater;        MyAdapter(Context context) {            inflater = LayoutInflater.from(context);        }        @Override        public int getCount() {            return 20;        }        @Override        public Object getItem(int position) {            return position;        }        @Override        public long getItemId(int position) {            return position;        }        @Override        public View getView(int position, View convertView, ViewGroup parent) {            if (convertView == null) {                convertView = inflater.inflate(R.layout.item_list, parent,false);            }            TextView tv = (TextView) convertView.findViewById(R.id.tv);            tv.setText(position + "");            return convertView;        }    }}

两个文件最关键的区别就一句话，
在getView方法中，OneActivity是
convertView = inflater.inflate(R.layout.item_list, null);
在getView方法中，TwoActivity是
convertView = inflater.inflate(R.layout.item_list, parent,false);

我们先看一下显示效果，再说两者的区别
OneActivity效果

TwoActivity的显示效果

我们可以很明显的看出来，使用第一种方式，根布局的高度设置60dp没有起作用，系统还是按照包裹内容的方式加载的，为什么会产生这种效果呢？我们从需要inflate方法的源代码中找一下答案。

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {        return inflate(resource, root, root != null);    }

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {        final Resources res = getContext().getResources();        if (DEBUG) {            Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("                    + Integer.toHexString(resource) + ")");        }        final XmlResourceParser parser = res.getLayout(resource);        try {            return inflate(parser, root, attachToRoot);        } finally {            parser.close();        }    }

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {        synchronized (mConstructorArgs) {            Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");            final Context inflaterContext = mContext;            final AttributeSet attrs = Xml.asAttributeSet(parser);            Context lastContext = (Context) mConstructorArgs[0];            mConstructorArgs[0] = inflaterContext;            View result = root;            try {                // Look for the root node.                int type;                while ((type = parser.next()) != XmlPullParser.START_TAG &&                        type != XmlPullParser.END_DOCUMENT) {                    // Empty                }                if (type != XmlPullParser.START_TAG) {                    throw new InflateException(parser.getPositionDescription()                            + ": No start tag found!");                }                final String name = parser.getName();                if (DEBUG) {                    System.out.println("**************************");                    System.out.println("Creating root view: "                            + name);                    System.out.println("**************************");                }                if (TAG_MERGE.equals(name)) {                    if (root == null || !attachToRoot) {                        throw new InflateException("<merge /> can be used only with a valid "                                + "ViewGroup root and attachToRoot=true");                    }                    rInflate(parser, root, inflaterContext, attrs, false);                } else {                    // Temp is the root view that was found in the xml                    final View temp = createViewFromTag(root, name, inflaterContext, attrs);                    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);                        }                    }                    if (DEBUG) {                        System.out.println("-----> start inflating children");                    }                    // Inflate all children under temp against its context.                    rInflateChildren(parser, temp, attrs, true);                    if (DEBUG) {                        System.out.println("-----> done inflating children");                    }                    // We are supposed to attach all the views we found (int temp)                    // to root. Do that now.                    if (root != null && attachToRoot) {                        root.addView(temp, params);                    }                    // Decide whether to return the root that was passed in or the                    // top view found in xml.                    if (root == null || !attachToRoot) {                        result = temp;                    }                }            } catch (XmlPullParserException e) {                final InflateException ie = new InflateException(e.getMessage(), e);                ie.setStackTrace(EMPTY_STACK_TRACE);                throw ie;            } catch (Exception e) {                final InflateException ie = new InflateException(parser.getPositionDescription()                        + ": " + e.getMessage(), e);                ie.setStackTrace(EMPTY_STACK_TRACE);                throw ie;            } finally {                // Don't retain static reference on context.                mConstructorArgs[0] = lastContext;                mConstructorArgs[1] = null;                Trace.traceEnd(Trace.TRACE_TAG_VIEW);            }            return result;        }    }

重点关注下面的

        final View temp = createViewFromTag(root, name, inflaterContext, attrs);                    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);                        }                    }

这些代码的意思就是，当我们传进来的root参数不是空的时候，并且attachToRoot是false的时候，也就是上面的TwoActivity的实现方式的时候，会给temp设置一个LayoutParams参数。

// We are supposed to attach all the views we found (int temp)                    // to root. Do that now.                    if (root != null && attachToRoot) {                        root.addView(temp, params);                    }                    // Decide whether to return the root that was passed in or the                    // top view found in xml.                    if (root == null || !attachToRoot) {                        result = temp;                    }

当我们传进来的root不是null，并且第三个参数是false的时候，这个temp就被加入到了root中，并且把root当作最终的返回值返回了。而当我们设置root为空的时候，没有设置LayoutParams参数的temp对象，作为返回值返回了。

因此，我们可以得出下面的结论：
1.若我们采用convertView = inflater.inflate(R.layout.item_list, null);方式填充视图，item布局中的根视图的layout_XX属性会被忽略掉，然后设置成默认的包裹内容方式
2.如果我们想保证item的视图中的参数不被改变，我们需要使用convertView = inflater.inflate(R.layout.item_list, parent,false);这种方式进行视图的填充
3.除了使用这种方式，我们还可以设置item布局的根视图为包裹内容，然后设置内部控件的高度等属性，这样就不会修改显示方式了。

总之一句话，推荐用下边这种方式：

inflater.inflate(R.layout.item, parent, false);