實現全屏的代碼：
// 全屏顯示
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. 很明顯，第一句是關閉標題欄，第二句是關閉狀態欄。
但是，這個不夠深入，我們從源碼挨個分析。
一、requestWindowFeature()的源碼：
/**
* 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);
}由上我們知道，這個方法的作用直接等價於 getWindow().requestFeature()，是後者的方便調用的方式。其中的參數是Window類下的標記窗口特點的int標記，返回布爾值，如果爲true時，說明參數傳遞的特點是支持的，被實現的。
由此我們來看Window類下定義的窗口特點的種類：
/**
* 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;先說明Window類的概括：頂級窗口的外觀和行爲的抽象基類，這個類的實例常常作爲頂層視圖添加到窗口管理器。它提供了標準UI方法，比如背景，標題區域等。
注意：唯一實現這個抽象類的類是android.policy.PhoneWindow，而且這個類也會被重寫和通過工廠模式來創造新的實例。一般我們直接調用getWindow獲得實例。
關於Window的實例，也屬於View對象，只是這個對象在最底層，超越了我們應用的界限，我們可以通過工具看在內存中某個應用的視圖樹進一步理解。事實上，整個Android系統也可以看作由許多應用組成。這裏不再對Window進行深究。
然後說明和全屏顯示相關的標記：
/** 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：本來想介紹大部分的的，實在有點多！）
然後是requestFeature()，從字面意思上就知道獲得頂級窗口的外觀的特點。當然requestWindowFeature()同前。
二、setFlags(int flags, int mask)的源碼：
/**
* 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即標誌，但爲什麼不是和上一句相同，因爲換了對象了。前面是Window類的，後者是WindowManager.LayoutParams的。WindowManager好理解，就是窗口管理器，管Window對象的，那麼LayoutParams：
LayoutParams are used by views to tell their parents how they want to be laid out.
就是說，這個被用來告訴他們的父對象如何排布他們。比如告訴linnerLayout View如何在自己上面放置gallery對象，這裏告訴窗口管理器如何排布窗口是全屏還是可以溢出的等等。
注意：這個方法因爲設置的是頂級窗口的屬性，所以需要在一些其他設置佈局的方法前面調用，比如setContentView()。
兩個參數的意思：第一個是設置成什麼樣，第二個是設置哪裏（byte型）。
而flags具體與哪些，有分別代表什麼樣的窗口布置特點。在WindowManager類下的LayoutParams裏，這裏拿出和FLAG_FULLSCREEN有關的的源碼：
/** 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;
一目瞭然，再具體的可以看setFlag()方法和requestFeature()的具體實現，來做進一步瞭解。
謝謝~
