LinearLayout详解二:从其父类View说起

这个View类说来就话长了，但我们又不得不说，要说呢，就得说的彻底，要让大家看得一清二楚，明明白白。所以我们就从源代码角度来看一个view是如何被加载的吧。

如果大家不知道怎么下载android的源代码，或者说懒得去下载（因为源代码确实比较大，大概有10G）的话，教大家几个取巧的办法：

1.直接在google中输入“android view.java”即可。这种方法成功率非常高，一般android的比较重要的类都能搜到。

2.给大家提供一个人家用于放源码的的git：[email protected]:rowboat/frameworks-base.git 大家自己去clone一下。（什么！你Y的居然连git都不会用！服了，好吧，请继续往下看）。

3登录到这里。这是2里面提供的那个git的网页地址，在这里搜索相应要查找的类就可以查找android的源代码了。

从头说起吧，一般android启动的是AndroidManifest.xml中带有

<intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>

这样的activity。然后去调用该activity中的

	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.frame_content);

	}

方法。其中setContentView是把相应的Layout对应的xml解析成view并显示在屏幕上的方法，也就是以上代码中的R.layout.frame_content。

由于activity.java中的代码量太长，我这边就贴个链接，大家看一下：Activity.java

可以发现在1650行的地方有这么一句话：

/**
     * 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);
    }

然后我们跳转到738行

/**
     * Retrieve the current {@link android.view.Window} for the activity.
     * This can be used to directly access parts of the Window API that
     * are not available through Activity/Screen.
     *
     * @return Window The current window, or null if the activity is not
     *         visual.
     */
    public Window getWindow() {
        return mWindow;
    }

发现这个setContentView其实调用的是mWindow的setContentView，mWindow很容易理解，就是 member window嘛，下面我们继续往window里找。

有关window.java的源代码大家可以在这里找到。

很伤心的是，window类是个抽象类

/**
 * Abstract base class for a top-level window look and behavior policy.  An
 * instance of this class should be used as the top-level view added to the
 * window manager. It provides standard UI policies such as a background, title
 * area, default key processing, etc.
 *
 * <p>The only existing implementation of this abstract class is
 * android.policy.PhoneWindow, which you should instantiate when needing a
 * Window.  Eventually that class will be refactored and a factory method
 * added for creating Window instances without knowing about a particular
 * implementation.
 */
public abstract class Window {

，他只是定义了这些方法

/**
     * Convenience for
     * {@link #setContentView(View, android.view.ViewGroup.LayoutParams)}
     * to set the screen content from a layout resource.  The resource will be
     * inflated, adding all top-level views to the screen.
     *
     * @param layoutResID Resource ID to be inflated.
     * @see #setContentView(View, android.view.ViewGroup.LayoutParams)
     */
    public abstract void setContentView(int layoutResID);

    /**
     * Convenience for
     * {@link #setContentView(View, android.view.ViewGroup.LayoutParams)}
     * set the screen content to an explicit view.  This view is placed
     * directly into the screen‘s view hierarchy.  It can itself be a complex
     * view hierarhcy.
     *
     * @param view The desired content to display.
     * @see #setContentView(View, android.view.ViewGroup.LayoutParams)
     */
    public abstract void setContentView(View view);

    /**
     * Set the screen content to an explicit view.  This view is placed
     * directly into the screen‘s view hierarchy.  It can itself be a complex
     * view hierarchy.
     *
     * <p>Note that calling this function "locks in" various characteristics
     * of the window that can not, from this point forward, be changed: the
     * features that have been requested with {@link #requestFeature(int)},
     * and certain window flags as described in {@link #setFlags(int, int)}.
     *
     * @param view The desired content to display.
     * @param params Layout parameters for the view.
     */
    public abstract void setContentView(View view, ViewGroup.LayoutParams params);

而没有实现它,那真正实现的地方在哪里呢，请猛戳这里PhoneWindow.java 。可爱的你肯定在疑惑，我砸知道的呢，难道我有“第三只眼”！靠，其实这个说出来你就会沮丧了，四个字：官方文档。详见：

好，下面，我们开始看PhoneWindow.java 的源码，直接看函数setContentView：

 @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();
        }
    }

可以发现，其实它又调用了mLayoutInflater的inflate方法。我，草！！！妈的，搞这么一层又一层干甚。不写了，且听下回分解。

LinearLayout详解二:从其父类View说起