LayoutInflater简单理解

在LayoutInflater的学习中有以下体会：

先看几个例子

@Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        LinearLayout mainLayout = (LinearLayout) findViewById(R.id.main_layout);        // Example1 出错        //View trueLayout = LayoutInflater.from(this).inflate(R.layout.button_layout, mainLayout, true);        //mainLayout.addView(trueLayout);        // Example2 Button的Params无效        //View trueLayout = LayoutInflater.from(this).inflate(R.layout.button_layout,null);        //mainLayout.addView(trueLayout);        // Example3        LayoutInflater.from(this).inflate(R.layout.button_layout, mainLayout, true);        // Example4        View falseLayout = LayoutInflater.from(this).inflate(R.layout.button_layout, mainLayout, false);        mainLayout.addView(falseLayout);    }

布局文件如下：
activity_main.xml

    <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"        android:orientation="horizontal">    </LinearLayout>

button_layout.xml

    <Button xmlns:android="http://schemas.android.com/apk/res/android"        android:layout_width="100dp"        android:layout_height="wrap_content"        android:text="Button">    </Button>

Example1:
会抛出异常
Example2:
Button的宽度没有100dp只是WRAP_CONTNET
Example3:
正常
Example4:正常

原因：
首先进入LayoutInflater#inflate()源码看看：

    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();        }    }

然后进入inflate(parser, root, attachToRoot);

    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                }                //找到xml文档的Start位置                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 {                    //重点1：Temp节点是要解析的xml文档的root节点                    // Temp is the root view that was found in the xml                    final View temp = createViewFromTag(root, name, inflaterContext, attrs);                    //重点2：parms到底是temp节点还是root的LayoutParams                    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                        // 创建params                        params = root.generateLayoutParams(attrs);                        if (!attachToRoot) {                            // Set the layout params for temp if we are not                            // attaching. (If we are, we use addView, below)                            // 重点3：当attachToRoot == false 时,调用下面函数                            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) {                        // 重点4：当attachToRoot == true时,调用下面函数                        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;        }    }

• 解析xml文档采用的PULL方式
• 区分root与temp：root为函数参数ViewGroup,temp为xml文档的根节点
• 当attachToRoot == true会调用root.addView(temp, params);，所以在一开始的Example1就会出错（不能重复添加啊），Example3就对了。当attachToRoot == fasle调用temp.setLayoutParams(params);所以在Example4要调用mainLayout.addView(falseLayout);不然就看不到Button了。
• 对于params变量（我一开始想了好久），params = root.generateLayoutParams(attrs);这句代码

    /**     * Returns a new set of layout parameters based on the supplied attributes set.     *     * @param attrs the attributes to build the layout parameters from     *     * @return an instance of {@link android.view.ViewGroup.LayoutParams} or one     *         of its descendants     */    public LayoutParams generateLayoutParams(AttributeSet attrs) {        return new LayoutParams(getContext(), attrs);    }

返回一个LayoutParams基于给的AttributeSet参数，那么AttributeSet从哪里来？

final AttributeSet attrs = Xml.asAttributeSet(parser);

parser为xml文档的parser，所以params变量为temp的LayoutParams（xml文档里面设置啦）。
- 当attachToRoot == true，inflate()函数返回root；当attachToRoot == true，inflate()函数返回temp。

最后说一点：

1. layout_height与layout_width不是View的真实高度，它是根据Parent View与当前View决定的。
2. Example2，里面由于root为空，默认attachToRoot为true，params = root.generateLayoutParams(attrs);就不会调用，params就为空。还有一点就是：root.addView(temp, params);不会调用。