InflaterLayout 原理

LayoutInflater 的基本用法

LayoutInflater 是用来加载布局的。一般在 Activity 里会调用 setContentView 来设置布局。其实 setcontentView 内部也是调用LayoutInflater 来加载布局的。只不过这部分源码是内部的没有公开。
首先要获取到 LayoutInflater 的实例，有2种方法。

1.LayoutInflater layoutInflater = LayoutInflater.from(context);

2.LayoutInflater layoutInflater = (LayoutInflater)context.getSystemService.LAYOUT_INFLATER_SERVICE;

其实第一种写法就是第二种，只是 Android 给我们做了一下封装。得到实例后就可以调用他的 inflate 方法来加载布局了。
layoutInflater.inflate(resourceId,root)

inflate 接收2个参数，第一个参数就是要加载的布局 id，第二个参数是指给该布局的外部再嵌套一层父布局，如果不需要就直接传 null。

举个小例子

通过一个小例子，更加直观的看一下LayoutInflater的用法。比如说当前有一个项目，其中MainActivity对应的布局文件叫做activity_main.xml,代码如下所示：

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

</LinearLayout>    

接下来我们再定义一个布局文件，给它取名为button_layout.xml

<Button xmlns:android="http://schemas.android.com/apk/res/android"  
android:layout_width="wrap_content"  
android:layout_height="wrap_content"  
android:text="Button" >  

</Button>  

在 MainActivity 添加 代码

    linearLayout = (LinearLayout) findViewById(R.id.linearLayout);
    LayoutInflater layoutInflater = LayoutInflater.from(this);
    View inflate = layoutInflater.inflate(R.layout.button_layout, null);
    linearLayout.addView(inflate);

LayoutInflater 技术广泛用于需要动态添加 View 的时候， 比如在 ScrollView 和 ListView 中， 经常都可以看 到 LayoutInflater 的身影。

剖析源码

看一下 LayoutInflater 到底是如何工作的。

 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) {
            InflateException ex = new InflateException(e.getMessage());
            ex.initCause(e);
            throw ex;
        } catch (Exception e) {
            InflateException ex = new InflateException(
                    parser.getPositionDescription()
                            + ": " + e.getMessage());
            ex.initCause(e);
            throw ex;
        } finally {
            // Don't retain static reference on context.
            mConstructorArgs[0] = lastContext;
            mConstructorArgs[1] = null;
        }

        Trace.traceEnd(Trace.TRACE_TAG_VIEW);

        return result;
    }
}