在Android上为了实现全屏显示，透明状态栏，沉浸模式等效果，往往需要我们掌握和系统UI显示隐藏相关的各种Flag。Android上API版本混乱，各种Flag林立。今天我们就来聊聊这些Flags。

在Android Kitkat中引入Immersive Mode

imm-states.png

相关API

Window#setFlags
View#setSystemUiVisibility (Android 3.0开始提供)

相关Flag

WindowManager.LayoutParams.FLAG_FULLSCREEN
隐藏状态栏

View.SYSTEM_UI_FLAG_VISIBLE API 14
默认标记

View.SYSTEM_UI_FLAG_LOW_PROFILE API 14
低调模式, 会隐藏不重要的状态栏图标

View.SYSTEM_UI_FLAG_LAYOUT_STABLE API 16
保持整个View稳定, 常和控制System UI悬浮, 隐藏的Flags共用, 使View不会因为System UI的变化而重新layout

View.SYSTEM_UI_FLAG_FULLSCREEN API 16
状态栏隐藏，效果同设置WindowManager.LayoutParams.FLAG_FULLSCREEN

View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN API 16
视图延伸至状态栏区域，状态栏上浮于视图之上

View.SYSTEM_UI_FLAG_HIDE_NAVIGATION API 14
隐藏导航栏

View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION API 16
视图延伸至导航栏区域，导航栏上浮于视图之上

View.SYSTEM_UI_FLAG_IMMERSIVE API 19
沉浸模式, 隐藏状态栏和导航栏, 并且在第一次会弹泡提醒, 并且在状态栏区域滑动可以呼出状态栏（这样会系统会清楚之前设置的View.SYSTEM_UI_FLAG_FULLSCREEN或View.SYSTEM_UI_FLAG_HIDE_NAVIGATION标志）。使之生效，需要和View.SYSTEM_UI_FLAG_FULLSCREEN，View.SYSTEM_UI_FLAG_HIDE_NAVIGATION中的一个或两个同时设置。

View.SYSTEM_UI_FLAG_IMMERSIVE_STIKY API 19
与上面唯一的区别是, 呼出隐藏的状态栏后不会清除之前设置的View.SYSTEM_UI_FLAG_FULLSCREEN或View.SYSTEM_UI_FLAG_HIDE_NAVIGATION标志，在一段时间后将再次隐藏系统栏）

参考Reference

https://developer.android.com/training/system-ui/immersive.html#sticky
https://www.youtube.com/embed/cBi8fjv90E4

组合搭配上面的Flag，我们可以实现沉浸模式，透明状态栏等各种你想要的样子。

使用场景

透明状态栏

style_color_colorscheme_palette1_example.png

if (Build.VERSION.SDK_INT >= 21) {
    View decorView = getWindow().getDecorView();
    int option = View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
            | View.SYSTEM_UI_FLAG_LAYOUT_STABLE;
    decorView.setSystemUiVisibility(option);
    getWindow().setStatusBarColor(Color.TRANSPARENT); //也可以设置成灰色透明的，比较符合Material Design的风格
}

全屏显示

View decorView = getWindow().getDecorView();
int option = View.SYSTEM_UI_FLAG_HIDE_NAVIGATION
        | View.SYSTEM_UI_FLAG_FULLSCREEN;
decorView.setSystemUiVisibility(option);

沉浸模式(普通Immersive Mode)

imm-states.png

 @Override
    public void onWindowFocusChanged(boolean hasFocus) {
        super.onWindowFocusChanged(hasFocus);
        if (hasFocus && Build.VERSION.SDK_INT >= 19) {
            View decorView = getWindow().getDecorView();
            decorView.setSystemUiVisibility(
                View.SYSTEM_UI_FLAG_LAYOUT_STABLE
                | View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
                | View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
                | View.SYSTEM_UI_FLAG_HIDE_NAVIGATION
                | View.SYSTEM_UI_FLAG_FULLSCREEN
                | View.SYSTEM_UI_FLAG_IMMERSIVE_STICKY);
        }
    }

沉浸模式（通过点击内容区域控制System UI的显示和隐藏）

import android.os.Handler;
import android.os.Message;
import android.support.v7.app.ActionBar;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.GestureDetector;
import android.view.LayoutInflater;
import android.view.MotionEvent;
import android.view.View;

public class MainActivity extends AppCompatActivity {

    private static final int INITIAL_DELAY = 1500;
    private View mDecorView;
    private ActionBar mActionBar;
    private View mContentView;
    private Handler handler = new Handler(){
        @Override
        public void handleMessage(Message msg) {
            hideSystemUI();
        }
    };
    private GestureDetector gestureDetector = new GestureDetector(this, new GestureDetector.SimpleOnGestureListener(){
        @Override
        public boolean onSingleTapUp(MotionEvent e) {
            if ((mDecorView.getSystemUiVisibility() & View.SYSTEM_UI_FLAG_HIDE_NAVIGATION) == 0){
                hideSystemUI();
            }else {
                showSystemUI();
            }
            return true;
        }
    });

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mDecorView = getWindow().getDecorView();
        mContentView = LayoutInflater.from(this).inflate(R.layout.activity_immersive, null);
        setContentView(mContentView);
        mDecorView.setOnSystemUiVisibilityChangeListener(new View.OnSystemUiVisibilityChangeListener() {
            @Override
            public void onSystemUiVisibilityChange(int visibility) {
                if ((visibility & View.SYSTEM_UI_FLAG_HIDE_NAVIGATION) == 0){
                    //systemUI is visible
                    mActionBar.show();
                }else {
                    //systemUI is invisible
                    mActionBar.hide();
                }
            }
        });
        mContentView.setOnTouchListener(new View.OnTouchListener() {
            @Override
            public boolean onTouch(View v, MotionEvent event) {
                gestureDetector.onTouchEvent(event);
                return true;
            }
        });
        mActionBar = getSupportActionBar();
        mActionBar.setShowHideAnimationEnabled(true);
        showSystemUI();
    }

    @Override
    public void onWindowFocusChanged(boolean hasFocus) {
        super.onWindowFocusChanged(hasFocus);
        if (hasFocus){
            delayedHide(INITIAL_DELAY);
        }else {
            handler.removeMessages(0);
            showSystemUI();
        }
    }

    private void delayedHide(int delay){
        handler.removeMessages(0);
        handler.sendEmptyMessageDelayed(0, delay);
    }

    private void hideSystemUI(){
        mDecorView.setSystemUiVisibility(View.SYSTEM_UI_FLAG_LAYOUT_STABLE
                                        |View.SYSTEM_UI_FLAG_HIDE_NAVIGATION
                                        |View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
                                        |View.SYSTEM_UI_FLAG_FULLSCREEN
                                        |View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
                                        |View.SYSTEM_UI_FLAG_IMMERSIVE);
    }

    private void showSystemUI(){
        mDecorView.setSystemUiVisibility(View.SYSTEM_UI_FLAG_LAYOUT_STABLE
        |View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
        |View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN);
    }
}