Android通过xml生成创建View的过程解析

Android的布局方式有两种，一种是通过xml布局，一种是通过java代码布局，两种布局方式各有各的好处，当然也可以相互混合使用。很多人都习惯用xml布局，那xml布局是如何转换成view的呢？本文从源码的角度来简单分析下整个过程。

首先，创建一个新的项目，默认生成一个activity，其中xml布局很简单，就一个RelativeLayout套了一个ImageView，代码及效果如下：

public class MainActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
 }

 

界面1

其中关键之处在于调用了父类Activity的setContentView方法：

/**
 * Set the activity content from a layout resource.  The resource     will be
 * inflated, adding all top-level views to the activity.
 *
 * @param layoutResID Resource ID to be inflated.
 */
public void setContentView(int layoutResID) {
    getWindow().setContentView(layoutResID);
}

getWindow返回的是PhoneWindow实例，那我们直接来看PhoneWindow中的setContentView方法：

@Override
public void setContentView(int layoutResID) {
    if (mContentParent == null) {
        installDecor();
    } else {
        mContentParent.removeAllViews();
    }
    mLayoutInflater.inflate(layoutResID, mContentParent);
    final Callback cb = getCallback();
    if (cb != null) {
        cb.onContentChanged();
    }
}

我们知道每个activity实际都对应一个PhoneWindow，拥有一个顶层的DecorView，DecorView继承自FrameLayout，作为根View，其中包含了一个标题区域和内容区域，这里的mContentParent就是其内容区域。关于PhoneWindow和DecorView的具体内容，读者可自行查阅。这段代码的意思很简单，如果DecorView的内容区域为null，就先初始化，否则就先把内容区域的子View全部移除，最后再引入layout布局，所以，关键在于mLayoutInflater.inflate(layoutResID, mContentParent); 代码继续往下看：

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

public View inflate(int resource, ViewGroup root, boolean attachToRoot) {
    if (DEBUG) System.out.println("INFLATING from resource: " + resource);
    XmlResourceParser parser = getContext().getResources().getLayout(resource);
    try {
        return inflate(parser, root, attachToRoot);
    } finally {
        parser.close();
    }
}

这里首先根据layout布局文件的Id生成xml资源解析器，然后再调用inflate(parser, root, attachToRoot)生成具体的view。XmlResourceParser是继承自XmlPullParser和AttributeSet的接口，这里的parser其实是XmlBlock的内部类Parser的实例。

public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
    synchronized (mConstructorArgs) {
        final AttributeSet attrs = Xml.asAttributeSet(parser);
        Context lastContext = (Context)mConstructorArgs[0];
        mConstructorArgs[0] = mContext;
        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, attrs);
                } else {
                // Temp is the root view that was found in the xml
                View temp = createViewFromTag(name, 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
                rInflate(parser, temp, attrs);
                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 (IOException 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;
        }  

        return result;
    }
}

第21行，获取xml根节点名：

final String name = parser.getName();

第39行根据节点名创建临时View（temp），这个临时view（temp）也是xml布局的根view：

View temp = createViewFromTag(name, attrs);

第61行，在临时view（temp）的节点下创建所有子View，显然这个方法里是通过遍历xml所有子view节点，调用createViewFromTag方法生成子view并加载到根view中：

rInflate(parser, temp, attrs);

第68到76行，则是判断，如果inflate方法有父view，则把临时view（temp）加载到父view中再返回，如果没有，则直接返回临时view（temp），我们这里调用inflate方法的时候显然有父view，即mContentParent，也就是最顶层view DecorView的内容区域。这里最关键有两个方法，一个是createViewFromTag，另一个是rInflate，现在来逐一分析：createViewFromTag实际最终调用的是createView方法：

public final View createView(String name, String prefix, AttributeSet attrs)
        throws ClassNotFoundException, InflateException {
    Constructor constructor = sConstructorMap.get(name);
    Class clazz = null;  

    try {
        if (constructor == null) {
            // Class not found in the cache, see if it's real, and try to add it
            clazz = mContext.getClassLoader().loadClass(
                    prefix != null ? (prefix + name) : name);  

            if (mFilter != null && clazz != null) {
                boolean allowed = mFilter.onLoadClass(clazz);
                if (!allowed) {
                    failNotAllowed(name, prefix, attrs);
                }
            }
            constructor = clazz.getConstructor(mConstructorSignature);
            sConstructorMap.put(name, constructor);
        } else {
            // If we have a filter, apply it to cached constructor
            if (mFilter != null) {
                // Have we seen this name before?
                Boolean allowedState = mFilterMap.get(name);
                if (allowedState == null) {
                    // New class -- remember whether it is allowed
                    clazz = mContext.getClassLoader().loadClass(
                            prefix != null ? (prefix + name) : name);  

                    boolean allowed = clazz != null && mFilter.onLoadClass(clazz);
                    mFilterMap.put(name, allowed);
                    if (!allowed) {
                        failNotAllowed(name, prefix, attrs);
                    }
                } else if (allowedState.equals(Boolean.FALSE)) {
                    failNotAllowed(name, prefix, attrs);
                }
            }
        }  

        Object[] args = mConstructorArgs;
        args[1] = attrs;
        return (View) constructor.newInstance(args);  

    } catch (NoSuchMethodException e) {
        InflateException ie = new InflateException(attrs.getPositionDescription()
                + ": Error inflating class "
                + (prefix != null ? (prefix + name) : name));
        ie.initCause(e);
        throw ie;  

    } catch (ClassNotFoundException e) {
        // If loadClass fails, we should propagate the exception.
        throw e;
    } catch (Exception e) {
        InflateException ie = new InflateException(attrs.getPositionDescription()
                + ": Error inflating class "
                + (clazz == null ? "<unknown>" : clazz.getName()));
        ie.initCause(e);
        throw ie;
    }
}

其实这个方法很简单，就是通过xml节点名，通过反射获取view的实例再返回，其中先去map中查询构造函数是否存在，如果存在则直接根据构造函数创建实例，这样做的好处是不用每次都通过class去获取构造函数再创建实例，我们看第18行通过类实例获取构造函数：

constructor = clazz.getConstructor(mConstructorSignature);

其中mConstructorSignature定义如下：

private static final Class[] mConstructorSignature = new Class[] {
    Context.class, AttributeSet.class};

很显然，这里用的是带有Context和AttributeSet两个参数的构造函数，这也就是为什么，自定义view一定要重载这个构造函数的原因。最后就是rInflate方法：

private void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs)
        throws XmlPullParserException, IOException {  

    final int depth = parser.getDepth();
    int type;  

    while (((type = parser.next()) != XmlPullParser.END_TAG ||
            parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {  

        if (type != XmlPullParser.START_TAG) {
            continue;
        }  

        final String name = parser.getName();  

        if (TAG_REQUEST_FOCUS.equals(name)) {
            parseRequestFocus(parser, parent);
        } else if (TAG_INCLUDE.equals(name)) {
            if (parser.getDepth() == 0) {
                throw new InflateException("<include /> cannot be the root element");
            }
            parseInclude(parser, parent, attrs);
        } else if (TAG_MERGE.equals(name)) {
            throw new InflateException("<merge /> must be the root element");
        } else {
            final View view = createViewFromTag(name, attrs);
            final ViewGroup viewGroup = (ViewGroup) parent;
            final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
            rInflate(parser, view, attrs);
            viewGroup.addView(view, params);
        }
    }  

    parent.onFinishInflate();
}

实这个方法也很简单，就是通过parser解析xml节点再生成对应View的过程。
XML转换成View的过程就是这样了，如有错误之处，还望指正，回到本文开头，其实我们还可以这样写：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    View content = LayoutInflater.from(this).inflate(R.layout.activity_main, null);
    setContentView(content);
}

 

界面2

大家发现问题没，相较于本文开头的写法，后面的灰色布局变成全屏了，我们来看看xml代码：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="300dip"
    android:layout_height="300dip"
    android:background="#888888"
    tools:context=".MainActivity" >  

    <ImageView
        android:layout_width="200dip"
        android:layout_height="200dip"
        android:background="#238712"
        android:contentDescription="@null" />  

</RelativeLayout>

我明明设置了RelativeLayout的宽度和高度分别为300dip，但为什么全屏了？这是因为layout_width和layout_height是相对于父布局而言的，我们这里inflate的时候设置的父布局为null，所以这个属性设置也就无效了，指定一个父布局就可以了，例如：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    RelativeLayout rootView = new RelativeLayout(this);
    View content = LayoutInflater.from(this).inflate(R.layout.activity_main, rootView);
    setContentView(content);
}

现在，界面显示效果就和“界面1”相同了。

转载自： https://www.jianshu.com/p/b081e3fbe4ee