【Android 个人理解（三）】从源码剖析如何实现实现全屏效果

实现全屏的代码：

// 全屏显示
requestWindowFeature(Window.FEATURE_NO_TITLE);	
//turning off the title at the top of the screen. 
getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,WindowManager.LayoutParams.FLAG_FULLSCREEN);
//the status bar will be hidden when an app window with this flag set is on the top layer.

很明显，第一句是关闭标题栏，第二句是关闭状态栏。

但是，这个不够深入，我们从源码挨个分析。

一、requestWindowFeature()的源码：

/**
     * Enable extended window features.  This is a convenience for calling
     * {@link android.view.Window#requestFeature getWindow().requestFeature()}.
     * 
     * @param featureId The desired feature as defined in
     *                  {@link android.view.Window}.
     * @return Returns true if the requested feature is supported and now
     *         enabled.
     * 
     * @see android.view.Window#requestFeature
     */
    public final boolean requestWindowFeature(int featureId) {
        return getWindow().requestFeature(featureId);
    }

由上我们知道，这个方法的作用直接等价于 getWindow().requestFeature()，是后者的方便调用的方式。其中的参数是Window类下的标记窗口特点的int标记，返回布尔值，如果为true时，说明参数传递的特点是支持的，被实现的。

由此我们来看Window类下定义的窗口特点的种类：

/**
 * 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 {
    /** Flag for the "options panel" feature.  This is enabled by default. */
    public static final int FEATURE_OPTIONS_PANEL = 0;
    /** Flag for the "no title" feature, turning off the title at the top
     *  of the screen. */
    public static final int FEATURE_NO_TITLE = 1;
    /** Flag for the progress indicator feature */
    public static final int FEATURE_PROGRESS = 2;
    /** Flag for having an icon on the left side of the title bar */
    public static final int FEATURE_LEFT_ICON = 3;
    /** Flag for having an icon on the right side of the title bar */
    public static final int FEATURE_RIGHT_ICON = 4;
    /** Flag for indeterminate progress */
    public static final int FEATURE_INDETERMINATE_PROGRESS = 5;
    /** Flag for the context menu.  This is enabled by default. */
    public static final int FEATURE_CONTEXT_MENU = 6;
    /** Flag for custom title. You cannot combine this feature with other title features. */
    public static final int FEATURE_CUSTOM_TITLE = 7;
    /**
     * Flag for enabling the Action Bar.
     * This is enabled by default for some devices. The Action Bar
     * replaces the title bar and provides an alternate location
     * for an on-screen menu button on some devices.
     */
    public static final int FEATURE_ACTION_BAR = 8;
    /**
     * Flag for requesting an Action Bar that overlays window content.
     * Normally an Action Bar will sit in the space above window content, but if this
     * feature is requested along with {@link #FEATURE_ACTION_BAR} it will be layered over
     * the window content itself. This is useful if you would like your app to have more control
     * over how the Action Bar is displayed, such as letting application content scroll beneath
     * an Action Bar with a transparent background or otherwise displaying a transparent/translucent
     * Action Bar over application content.
     */
    public static final int FEATURE_ACTION_BAR_OVERLAY = 9;
    /**
     * Flag for specifying the behavior of action modes when an Action Bar is not present.
     * If overlay is enabled, the action mode UI will be allowed to cover existing window content.
     */
    public static final int FEATURE_ACTION_MODE_OVERLAY = 10;
    /** Flag for setting the progress bar's visibility to VISIBLE */
    public static final int PROGRESS_VISIBILITY_ON = -1;
    /** Flag for setting the progress bar's visibility to GONE */
    public static final int PROGRESS_VISIBILITY_OFF = -2;
    /** Flag for setting the progress bar's indeterminate mode on */
    public static final int PROGRESS_INDETERMINATE_ON = -3;
    /** Flag for setting the progress bar's indeterminate mode off */
    public static final int PROGRESS_INDETERMINATE_OFF = -4;
    /** Starting value for the (primary) progress */
    public static final int PROGRESS_START = 0;
    /** Ending value for the (primary) progress */
    public static final int PROGRESS_END = 10000;
    /** Lowest possible value for the secondary progress */
    public static final int PROGRESS_SECONDARY_START = 20000;
    /** Highest possible value for the secondary progress */
    public static final int PROGRESS_SECONDARY_END = 30000;
    
    /** The default features enabled */
    @SuppressWarnings({"PointlessBitwiseExpression"})
    protected static final int DEFAULT_FEATURES = (1 << FEATURE_OPTIONS_PANEL) |
            (1 << FEATURE_CONTEXT_MENU);

    /**
     * The ID that the main layout in the XML layout file should have.
     */
    public static final int ID_ANDROID_CONTENT = com.android.internal.R.id.content;

以上是Window类所有的公开的成员变量。

先说明Window类的概括：顶级窗口的外观和行为的抽象基类，这个类的实例常常作为顶层视图添加到窗口管理器。它提供了标准UI方法，比如背景，标题区域等。

注意：唯一实现这个抽象类的类是android.policy.PhoneWindow，而且这个类也会被重写和通过工厂模式来创造新的实例。一般我们直接调用getWindow获得实例。

关于Window的实例，也属于View对象，只是这个对象在最底层，超越了我们应用的界限，我们可以通过工具看在内存中某个应用的视图树进一步理解。事实上，整个Android系统也可以看作由许多应用组成。这里不再对Window进行深究。

然后说明和全屏显示相关的标记：

/** Flag for the "no title" feature, turning off the title at the top
     *  of the screen. */
    public static final int FEATURE_NO_TITLE = 1;

（PS：本来想介绍大部分的的，实在有点多！）

然后是requestFeature()，从字面意思上就知道获得顶级窗口的外观的特点。当然requestWindowFeature()同前。

二、setFlags(int flags, int mask)的源码：

/**
     * Set the flags of the window, as per the
     * {@link WindowManager.LayoutParams WindowManager.LayoutParams}
     * flags.
     * 
     * <p>Note that some flags must be set before the window decoration is
     * created (by the first call to
     * {@link #setContentView(View, android.view.ViewGroup.LayoutParams)} or
     * {@link #getDecorView()}:
     * {@link WindowManager.LayoutParams#FLAG_LAYOUT_IN_SCREEN} and
     * {@link WindowManager.LayoutParams#FLAG_LAYOUT_INSET_DECOR}.  These
     * will be set for you based on the {@link android.R.attr#windowIsFloating}
     * attribute.
     *
     * @param flags The new window flags (see WindowManager.LayoutParams).
     * @param mask Which of the window flag bits to modify.
     */
    public void setFlags(int flags, int mask) {
        final WindowManager.LayoutParams attrs = getAttributes();
        attrs.flags = (attrs.flags&~mask) | (flags&mask);
        mForcedWindowFlags |= mask;
        if (mCallback != null) {
            mCallback.onWindowAttributesChanged(attrs);
        }
    }

也是flag即标志，但为什么不是和上一句相同，因为换了对象了。前面是Window类的，后者是WindowManager.LayoutParams的。WindowManager好理解，就是窗口管理器，管Window对象的，那么LayoutParams：

LayoutParams are used by views to tell their parents how they want to be laid out.

就是说，这个被用来告诉他们的父对象如何排布他们。比如告诉linnerLayout View如何在自己上面放置gallery对象，这里告诉窗口管理器如何排布窗口是全屏还是可以溢出的等等。

注意：这个方法因为设置的是顶级窗口的属性，所以需要在一些其他设置布局的方法前面调用，比如setContentView()。

两个参数的意思：第一个是设置成什么样，第二个是设置哪里（byte型）。

而flags具体与哪些，有分别代表什么样的窗口布置特点。在WindowManager类下的LayoutParams里，这里拿出和FLAG_FULLSCREEN有关的的源码：

/** Window flag: allow window to extend outside of the screen. */
        public static final int FLAG_LAYOUT_NO_LIMITS   = 0x00000200;
        
        /** Window flag: Hide all screen decorations (e.g. status bar) while
         * this window is displayed.  This allows the window to use the entire
         * display space for itself -- the status bar will be hidden when
         * an app window with this flag set is on the top layer. */
        public static final int FLAG_FULLSCREEN      = 0x00000400;
        
        /** Window flag: Override {@link #FLAG_FULLSCREEN and force the
         *  screen decorations (such as status bar) to be shown. */
        public static final int FLAG_FORCE_NOT_FULLSCREEN   = 0x00000800;

一目了然，再具体的可以看setFlag()方法和requestFeature()的具体实现，来做进一步了解。

谢谢~