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Android

 
阅读更多

Android的版本更新的非常频繁，这给开发带来了较大的困难。每次Android的升级，SDK也会随之带来一些更新，添加一些新的东西进去。在Android开发过程中经常会遇到Call requires API level 21 (current min is 16)这样的类似问题。开发Android的时候，我们需要确定所使用的SDK版本，比如通过Android Studio开发Android的话，工程创建好后，在build.gradle配置文件（如果工程中有多个模块，这个配置文件会有多个）中，会有以下类似配置信息：

android {
    compileSdk 32

    defaultConfig {
        applicationId "app.at"
        minSdk 16
        targetSdk 32
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }
    ...
    ...
}

这里的配置信息告诉了我们开发的Android应用最低兼容的SDK版本为16。那像上面的错误提示是说我们用到了21这个SDK版本以上的API，而我们当前开发的Android应用最低兼容的SDK版本为16，这样我们开发的Android应用最低兼容的SDK版本就变为21了。

 

另外如果我们使用了比最低兼容的SDK版本（minSdk 16）还高的SDK版本的API，如上面的错误提示，我们使用了21这个SDK版本以上的API，在Android Studio中会提示如上错误信息，我们可以在对应的方法上添加@RequiresApi来消除这个错误提示，如@RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)。

 

@RequiresApi

在开发Android的时候常常会遇到这个Call requires API level 21 (current min is 16)这种类似错误提示，如下：

 

public class MyViewPager extends FrameLayout {
    public MyViewPager(@NonNull Context context) {
        super(context);
    }

    public MyViewPager(@NonNull Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }

    public MyViewPager(@NonNull Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    public MyViewPager(@NonNull Context context, @Nullable AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
    }
}
其中在重写public MyViewPager(@NonNull Context context, @Nullable AttributeSet attrs, int defStyleAttr, int defStyleRes)这个构造函数的时候，通常会调用父类的对应的构造函数，父类FrameLayout的这个构造函数是21这个SDK版本以上提供的API，就可能会出现以上错误提示。

 

只需要添加注解@RequiresApi就可以消除这个错误提示，如@RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)。

 

Android界面UI设计主要采用布局文件的方式设计，编写很少的代码。当然也可以完全代码的方式来编写整个UI。采用布局文件这种方式偏设计点，UI上面的设计可以直接拖拽，布局也一目了然，开发效率上相对来说要高。完全采用代码来编写这种方式完全面向程序员的，能够更好的理解Android程序的运行。

 

布局文件这种方式在开发时作为辅助就好，这种方式也只能在设计UI上用用，最多就是一些简单的交互。在复杂的交互场景中还是要靠代码来实现，而且一个app肯定不只是一些UI界面，还有很多数据也要考代码来完成。尤其是整个程序运行中还涉及到很多资源要去管理，如Activity的整个生命周期的管理，生命时候要创建一个Activity，什么时候要去销毁这个Activity等等。

 

完全用代码的方式编写UI的话其实也挺简单的，编写的代码量也不算多，有时候代码还要比布局文件这种方式少。也就是不太直接，感受不到采用布局文件这种方式的可见的那种视觉感受。

 

应用

 

Context

android.content.Context

每个Activity都是一个上下文，所有Activity都继承了这个抽象类。

 

包括整个Android app应用也对应一个上下文。我们调用getApplicationContext方法获取的就是整个应用程序的上下文，由Application来定义。

 

Application

 

Android程序运行时有一个代表整个应用程序的实例，它是程序运行的上下文，上面我们通过getApplicationContext方法获取的整个应用程序的上下文就是这个实例。这个实例由android.app.Application定义，默认情况下就是Application的实例。

 

我们也可以自定义这个Application，对Application进行扩展。对Application扩展后，在AndroidManifest.xml文件中指定application标签属性android:name，这里的name指定的是应用程序的类名，如android:name="app.at.App"。

 

public class App extends android.app.Application {

    public void onCreate() {
        super.onCreate();

    }

    public void onTerminate() {
        super.onTerminate();
    }
}
 

Application的启动是在主线程中进行的。当我们自定义这个Application的时候，在主线程最好不要有网络相关的操作，否则会报以下错误。

java.lang.RuntimeException: Unable to create application app.at.App: android.os.NetworkOnMainThreadException

 

如果非要这样做的话，需要授权允许这样的操作。

 

        if (android.os.Build.VERSION.SDK_INT > 9) {
            StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitNetwork().build();
            StrictMode.setThreadPolicy(policy);
        }
或者我们可以授权允许所有的操作。

 

 

        if (android.os.Build.VERSION.SDK_INT > 9) {
            StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
            StrictMode.setThreadPolicy(policy);
        }
除了permitNetwork，还有permitDiskReads,permitDiskWrites,permitUnbufferedIo,permitResourceMismatches,permitCustomSlowCalls可以授权允许操作。

主题

Android提供了大量的主题风格，如Theme.NoTitleBar、Theme.NoTitleBar.Fullscreen、Theme.NoTitleBar.OverlayActionModes、Theme.WithActionBar、Theme.Light、Theme.Light.NoTitleBar、Theme.Light.NoTitleBar.Fullscreen、Theme.Black、Theme.Black.NoTitleBar、Theme.Black.NoTitleBar.Fullscreen、Theme.Wallpaper、Theme.Wallpaper.NoTitleBar、Theme.Wallpaper.NoTitleBar.Fullscreen、Theme.Translucent、Theme.Translucent.NoTitleBar、Theme.Translucent.NoTitleBar.Fullscreen等。这些主题风格在sdk/platforms/android-32/data/res/values/themes.xml中都有定义。

状态栏 

设置状态栏颜色

调用窗口的setStatusBarColor方法可以设置状态栏颜色。比如我们有时候想将状态栏的颜色设置为透明色：setStatusBarColor(Color.TRANSPARENT);将状态栏设置为透明之后，状态栏就看不到了。其实也不是看不到，状态栏是透明了，看不到，但状态栏上面的网络信号标识、时间、电量标识还是可以看得到的，可能看得不明显。另外要注意的是，状态栏只是被设置为透明了，并不是就不见了，其实还在那里，还占用了屏幕空间。

 

半透明状态栏

设置半透明状态栏是通过窗口的FLAG_TRANSLUCENT_STATUS来标志的，参考WindowManager.LayoutParams.FLAG_TRANSLUCENT_STATUS的定义。如果需要半透明状态栏，就添加窗口的这个标志，否则的话就清除这个标志。

 

添加这个标志可以调用窗口的addFlags方法：addFlags(WindowManager.LayoutParams.FLAG_TRANSLUCENT_STATUS);清除这个标志可以调用窗口的clearFlags方法：clearFlags(WindowManager.LayoutParams.FLAG_TRANSLUCENT_STATUS);
WindowManager.LayoutParams.FLAG_TRANSLUCENT_STATUS的定义
public static final int FLAG_TRANSLUCENT_STATUS = 0x04000000;
需要注意的是，半透明状态栏带最小系统提供背景保护。并且是浮在窗口顶部的，页面直接从窗口顶部开始显示，而不是从状态栏下面开始显示。状态栏不会将页面给顶下来。
状态栏高度
private int getStatusBarHeight() {
    int height = 0;
    int resId = getResources().getIdentifier("status_bar_height",
            "dimen", "android");
    if (resId > 0) {
        height = getResources().getDimensionPixelSize(resId);
    }
    return height;
}
 

导航栏

半透明导航栏

设置半透明导航栏是通过窗口的FLAG_TRANSLUCENT_STATUS来标志的，参考WindowManager.LayoutParams.FLAG_TRANSLUCENT_STATUS的定义。如果需要半透明导航栏，就添加窗口的这个标志，否则的话就清除这个标志。

 

添加这个标志可以调用窗口的addFlags方法：addFlags(WindowManager.LayoutParams.FLAG_TRANSLUCENT_STATUS);清除这个标志可以调用窗口的clearFlags方法：clearFlags(WindowManager.LayoutParams.FLAG_TRANSLUCENT_STATUS);
WindowManager.LayoutParams.FLAG_TRANSLUCENT_STATUS的定义
public static final int FLAG_TRANSLUCENT_STATUS = 0x04000000;
需要注意的是，半透明导航栏带最小系统提供背景保护。并且是浮在窗口底部的，页面直接显示到窗口底部结束，而不是到导航栏上面显示结束。导航栏不会将页面给顶上去。

 

隐藏导航栏

WindowManager.LayoutParams params = getWindow().getAttributes();
params.systemUiVisibility =
        View.SYSTEM_UI_FLAG_HIDE_NAVIGATION | View.SYSTEM_UI_FLAG_IMMERSIVE;
getWindow().setAttributes(params);

 

 

窗口

 

 

 

屏幕

全屏
设置为全屏是通过窗口的FLAG_FULLSCREEN来标志的，参考WindowManager.LayoutParams.FLAG_FULLSCREEN的定义。如果需要全屏，就设置窗口的这个标志，否则的话就清除这个标志。
设置这个标志可以调用窗口的setFlags方法：setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);清除这个标志可以调用窗口的clearFlags方法：clearFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN);
WindowManager.LayoutParams.FLAG_FULLSCREEN的定义
public static final int FLAG_FULLSCREEN      = 0x00000400;
代码示例：
private int flags;

@Override
public void onClick(View v) {
    if ((flags & WindowManager.LayoutParams.FLAG_FULLSCREEN) == 0) {
        getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
                WindowManager.LayoutParams.FLAG_FULLSCREEN);
        flags |= WindowManager.LayoutParams.FLAG_FULLSCREEN;
    } else {
        getWindow().clearFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN);
        flags &= ~WindowManager.LayoutParams.FLAG_FULLSCREEN;
    }

}
 
 

绘图

在所有的视图中有一个onDraw方法。View中的onDraw定义如下：
    protected void onDraw(Canvas canvas) {
    }
重写onDraw方法，重绘TextView：
class TextItemView extends TextView {

    public TextItemView(Context context) {
        super(context);
    }

    protected void onDraw(Canvas canvas) {
        Paint paint = new Paint();
        super.onDraw(canvas);
        paint.setColor(Color.BLACK);
        canvas.drawLine(0, getHeight() - 1, getWidth(), getHeight() - 1, paint);
    }
}

截屏

 

视图快照

public class Snapshot {

    public static Bitmap createBitmapSnapshot(Fragment fragment, View view) {
        DisplayMetrics dm = fragment.getResources().getDisplayMetrics();
        return createBitmapSnapshot(dm, view);
    }

    public static Bitmap createBitmapSnapshot(Context context, View view) {
        DisplayMetrics dm = context.getResources().getDisplayMetrics();
        return createBitmapSnapshot(dm, view);
    }

    public static Bitmap createBitmapSnapshot(DisplayMetrics dm, View view) {
        int width = dm.widthPixels;
        int height = dm.heightPixels;
    
        int widthSpec = View.MeasureSpec.makeMeasureSpec(width, View.MeasureSpec.EXACTLY);
        int heightSpec = View.MeasureSpec.makeMeasureSpec(height, View.MeasureSpec.EXACTLY);
        view.measure(widthSpec, heightSpec);
        view.layout(0, 0, width, height);
        Bitmap bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
        Canvas canvas = new Canvas(bitmap);
        canvas.drawColor(Color.TRANSPARENT);
        canvas.setDrawFilter(new PaintFlagsDrawFilter(0, Paint.ANTI_ALIAS_FLAG | Paint.FILTER_BITMAP_FLAG));
        view.draw(canvas);
        return bitmap;
    }
}
 

文件操作

 
数据目录
用于存放Android应用所有私有文件的目录，这个目录可以通过Context的getDataDir方法获得。在开发Android应用时，最好不要直接使用这个目录，这个路径可能会随着时间的推移而改变。
缓存目录
Android应用的缓存目录，这个目录可以通过Context的getCacheDir方法获得。
文件目录
通过openFileOutput方法创建的文件存放的目录，这个目录可以通过Context的getFilesDir方法获得。
Android 应用私有文件操作
私有文件存放在/data/data/<app>目录下。
如下
/data/data/<app>/files/IMG_20220325_231218.jpg
 
Context提供了一些私有文件相关操作的方法。如openFileOutput、openFileInput等。
public abstract FileOutputStream openFileOutput(String name, @FileMode int mode) throws FileNotFoundException;
 
                            FileOutputStream os = applicationContext.openFileOutput(name, Context.MODE_PRIVATE);
                            os.write(data, toIndex + 1, data.length - (toIndex + 1));
 
public abstract FileInputStream openFileInput(String name) throws FileNotFoundException;
    private InputStream getInputStream() throws IOException {
        return context.openFileInput(path);
    }
 
 
这两个方法都需要指定一个文件名参数name，文件名不能包含路径，即不能包含“path separators”部分。openFileOutput方法还需要指定一个mode参数，用于指定操作模式。文件操作模式可以指定为MODE_PRIVATE、MODE_WORLD_READABLE、MODE_WORLD_WRITEABLE、以及MODE_APPEND，可以参考FileMode注解。
    @IntDef(flag = true, prefix = { "MODE_" }, value = {
            MODE_PRIVATE,
            MODE_WORLD_READABLE,
            MODE_WORLD_WRITEABLE,
            MODE_APPEND,
    })
    @Retention(RetentionPolicy.SOURCE)
    public @interface FileMode {}
 
 
IMG_20220325_231218.jpg: open failed: ENOENT (No such file or directory)
    java.io.FileNotFoundException: IMG_20220325_231218.jpg: open failed: ENOENT (No such file or directory)
        at libcore.io.IoBridge.open(IoBridge.java:492)
        at java.io.FileInputStream.<init>(FileInputStream.java:160)
        at java.io.FileInputStream.<init>(FileInputStream.java:115)
        at com.zhenglv.libmy.Image.getInputStream(Image.java:24)
        at com.zhenglv.libmy.Image.<init>(Image.java:18)
        at com.zhenglv.libmy.Client.readPacket(Client.java:313)
        at com.zhenglv.libmy.Client.run(Client.java:219)
     Caused by: android.system.ErrnoException: open failed: ENOENT (No such file or directory)
        at libcore.io.Linux.open(Native Method)
        at libcore.io.ForwardingOs.open(ForwardingOs.java:166)
        at libcore.io.BlockGuardOs.open(BlockGuardOs.java:254)
        at libcore.io.ForwardingOs.open(ForwardingOs.java:166)
        at android.app.ActivityThread$AndroidOs.open(ActivityThread.java:7542)
        at libcore.io.IoBridge.open(IoBridge.java:478)
        at java.io.FileInputStream.<init>(FileInputStream.java:160) 
        at java.io.FileInputStream.<init>(FileInputStream.java:115) 
        at com.zhenglv.libmy.Image.getInputStream(Image.java:24) 
        at com.zhenglv.libmy.Image.<init>(Image.java:18) 
        at com.zhenglv.libmy.Client.readPacket(Client.java:313) 
        at com.zhenglv.libmy.Client.run(Client.java:219) 
 
IMG_20220325_231218.jpg: open failed: EROFS (Read-only file system)
    java.io.FileNotFoundException: IMG_20220325_231218.jpg: open failed: EROFS (Read-only file system)
        at libcore.io.IoBridge.open(IoBridge.java:492)
        at java.io.FileOutputStream.<init>(FileOutputStream.java:236)
        at java.io.FileOutputStream.<init>(FileOutputStream.java:186)
        at com.zhenglv.libmy.Client.readPacket(Client.java:303)
        at com.zhenglv.libmy.Client.run(Client.java:217)
     Caused by: android.system.ErrnoException: open failed: EROFS (Read-only file system)
        at libcore.io.Linux.open(Native Method)
        at libcore.io.ForwardingOs.open(ForwardingOs.java:166)
        at libcore.io.BlockGuardOs.open(BlockGuardOs.java:254)
        at libcore.io.ForwardingOs.open(ForwardingOs.java:166)
        at android.app.ActivityThread$AndroidOs.open(ActivityThread.java:7542)
        at libcore.io.IoBridge.open(IoBridge.java:478)
        at java.io.FileOutputStream.<init>(FileOutputStream.java:236) 
        at java.io.FileOutputStream.<init>(FileOutputStream.java:186) 
        at com.zhenglv.libmy.Client.readPacket(Client.java:303) 
        at com.zhenglv.libmy.Client.run(Client.java:217) 

 

网络

 

Http

 

W/System.err: java.io.IOException: Cleartext HTTP traffic to 192.168.0.3 not permitted

W/System.err:     at com.android.okhttp.HttpHandler$CleartextURLFilter.checkURLPermitted(HttpHandler.java:127)

W/System.err:     at com.android.okhttp.internal.huc.HttpURLConnectionImpl.execute(HttpURLConnectionImpl.java:462)

W/System.err:     at com.android.okhttp.internal.huc.HttpURLConnectionImpl.getResponse(HttpURLConnectionImpl.java:411)

W/System.err:     at com.android.okhttp.internal.huc.HttpURLConnectionImpl.getInputStream(HttpURLConnectionImpl.java:248)

W/System.err:     at app.at.App.onStart(App.java:122)

W/System.err:     at app.at.Start$1.run(Start.java:31)

W/System.err:     at android.os.Handler.handleCallback(Handler.java:938)

W/System.err:     at android.os.Handler.dispatchMessage(Handler.java:99)

W/System.err:     at android.os.Looper.loop(Looper.java:223)

W/System.err:     at android.app.ActivityThread.main(ActivityThread.java:7656)

W/System.err:     at java.lang.reflect.Method.invoke(Native Method)

W/System.err:     at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)

W/System.err:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)

 

在配置清单AndroidManifest.xml中application标签中指定android:usesCleartextTraffic="true"。

 

或者在res/xml目录下建立一个network_security_config.xml文件：

 

<?xml version="1.0" encoding="utf-8"?>
<network-security-config>
    <base-config cleartextTrafficPermitted="true" />
</network-security-config>
然后在配置清单AndroidManifest.xml中application标签中指定android:networkSecurityConfig="@xml/network_security_config"

摄像头

Android提供了一个Camera类，注意是android.hardware.Camera，不是android.graphics.Camera。这是一个相机类，准确的说是一个摄像头类。 
打开摄像头
Camera camera = Camera.open();
这个方法打开的是一个后置摄像头。
public static Camera open() {
    int numberOfCameras = getNumberOfCameras();
    CameraInfo cameraInfo = new CameraInfo();
    for (int i = 0; i < numberOfCameras; i++) {
        getCameraInfo(i, cameraInfo);
        if (cameraInfo.facing == CameraInfo.CAMERA_FACING_BACK) {
            return new Camera(i);
        }
    }
    return null;
}
另外还有一个方法
public static Camera open(int cameraId) {
    return new Camera(cameraId);
}
 
java.lang.RuntimeException: Unable to start activity ComponentInfo{app.at/app.at.Camera}: java.lang.RuntimeException: Fail to connect to camera service
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:223)
    at android.app.ActivityThread.main(ActivityThread.java:7656)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
 Caused by: java.lang.RuntimeException: Fail to connect to camera service
    at android.hardware.Camera.<init>(Camera.java:557)
    at android.hardware.Camera.open(Camera.java:420)
    at ***.***.***.onCreate(***.java:54)
    at android.app.Activity.performCreate(Activity.java:7994)
    at android.app.Activity.performCreate(Activity.java:7978)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
    at android.os.Handler.dispatchMessage(Handler.java:106) 
    at android.os.Looper.loop(Looper.java:223) 
    at android.app.ActivityThread.main(ActivityThread.java:7656) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 

设置预览显示

在正确打开摄像头之后，需要设置预览显示，以便将摄像头投影进来的画面显示在一个预览显示上，所以这里会有一个Surface，中文理解为“面”或者“平面”。摄像头影射进来的画面就投影到这个Surface上。

 

Android通过Camera的setPreviewDisplay将摄像头影射进来的画面就投影到这个Surface上。

public final void setPreviewDisplay(SurfaceHolder holder) throws IOException {
    if (holder != null) {
        setPreviewSurface(holder.getSurface());
    } else {
        setPreviewSurface((Surface)null);
    }
}
不过这里设置并不直接是一个Surface对象，而是一个SurfaceHolder对象。Android并没有提供一个创建这样一个Surface的方法，通常也不会直接对这个Surface进行操作。SurfaceHolder可以理解为是Surface的持有者，这是一种很好的设计。对Surface的操作都通过SurfaceHolder来进行。
public interface SurfaceHolder {

    /** @deprecated this is ignored, this value is set automatically when needed. */
    @Deprecated
    public static final int SURFACE_TYPE_NORMAL = 0;
    /** @deprecated this is ignored, this value is set automatically when needed. */
    @Deprecated
    public static final int SURFACE_TYPE_HARDWARE = 1;
    /** @deprecated this is ignored, this value is set automatically when needed. */
    @Deprecated
    public static final int SURFACE_TYPE_GPU = 2;
    /** @deprecated this is ignored, this value is set automatically when needed. */
    @Deprecated
    public static final int SURFACE_TYPE_PUSH_BUFFERS = 3;

    /**
     * Exception that is thrown from {@link #lockCanvas} when called on a Surface
     * whose type is SURFACE_TYPE_PUSH_BUFFERS.
     */
    public static class BadSurfaceTypeException extends RuntimeException {
        public BadSurfaceTypeException() {
        }

        public BadSurfaceTypeException(String name) {
            super(name);
        }
    }

    /**
     * A client may implement this interface to receive information about
     * changes to the surface.  When used with a {@link SurfaceView}, the
     * Surface being held is only available between calls to
     * {@link #surfaceCreated(SurfaceHolder)} and
     * {@link #surfaceDestroyed(SurfaceHolder)}.  The Callback is set with
     * {@link SurfaceHolder#addCallback SurfaceHolder.addCallback} method.
     */
    public interface Callback {
        /**
         * This is called immediately after the surface is first created.
         * Implementations of this should start up whatever rendering code
         * they desire.  Note that only one thread can ever draw into
         * a {@link Surface}, so you should not draw into the Surface here
         * if your normal rendering will be in another thread.
         *
         * @param holder The SurfaceHolder whose surface is being created.
         */
        void surfaceCreated(@NonNull SurfaceHolder holder);

        /**
         * This is called immediately after any structural changes (format or
         * size) have been made to the surface.  You should at this point update
         * the imagery in the surface.  This method is always called at least
         * once, after {@link #surfaceCreated}.
         *
         * @param holder The SurfaceHolder whose surface has changed.
         * @param format The new {@link PixelFormat} of the surface.
         * @param width The new width of the surface.
         * @param height The new height of the surface.
         */
        void surfaceChanged(@NonNull SurfaceHolder holder, @PixelFormat.Format int format,
                @IntRange(from = 0) int width, @IntRange(from = 0) int height);

        /**
         * This is called immediately before a surface is being destroyed. After
         * returning from this call, you should no longer try to access this
         * surface.  If you have a rendering thread that directly accesses
         * the surface, you must ensure that thread is no longer touching the
         * Surface before returning from this function.
         *
         * @param holder The SurfaceHolder whose surface is being destroyed.
         */
        void surfaceDestroyed(@NonNull SurfaceHolder holder);
    }

    /**
     * Additional callbacks that can be received for {@link Callback}.
     */
    public interface Callback2 extends Callback {
        /**
         * Called when the application needs to redraw the content of its
         * surface, after it is resized or for some other reason.  By not
         * returning from here until the redraw is complete, you can ensure that
         * the user will not see your surface in a bad state (at its new
         * size before it has been correctly drawn that way).  This will
         * typically be preceeded by a call to {@link #surfaceChanged}.
         *
         * As of O, {@link #surfaceRedrawNeededAsync} may be implemented
         * to provide a non-blocking implementation. If {@link #surfaceRedrawNeededAsync}
         * is not implemented, then this will be called instead.
         *
         * @param holder The SurfaceHolder whose surface has changed.
         */
        void surfaceRedrawNeeded(@NonNull SurfaceHolder holder);

        /**
         * An alternative to surfaceRedrawNeeded where it is not required to block
         * until the redraw is complete. You should initiate the redraw, and return,
         * later invoking drawingFinished when your redraw is complete.
         *
         * This can be useful to avoid blocking your main application thread on rendering.
         *
         * As of O, if this is implemented {@link #surfaceRedrawNeeded} will not be called.
         * However it is still recommended to implement {@link #surfaceRedrawNeeded} for
         * compatibility with older versions of the platform.
         *
         * @param holder The SurfaceHolder which needs redrawing.
         * @param drawingFinished A runnable to signal completion. This may be invoked
         * from any thread.
         *
         */
        default void surfaceRedrawNeededAsync(@NonNull SurfaceHolder holder,
                @NonNull Runnable drawingFinished) {
            surfaceRedrawNeeded(holder);
            drawingFinished.run();
        }
    }

    /**
     * Add a Callback interface for this holder.  There can several Callback
     * interfaces associated with a holder.
     *
     * @param callback The new Callback interface.
     */
    public void addCallback(Callback callback);

    /**
     * Removes a previously added Callback interface from this holder.
     *
     * @param callback The Callback interface to remove.
     */
    public void removeCallback(Callback callback);

    /**
     * Use this method to find out if the surface is in the process of being
     * created from Callback methods. This is intended to be used with
     * {@link Callback#surfaceChanged}.
     *
     * @return true if the surface is in the process of being created.
     */
    public boolean isCreating();

    /**
     * Sets the surface's type.
     *
     * @deprecated this is ignored, this value is set automatically when needed.
     */
    @Deprecated
    public void setType(int type);

    /**
     * Make the surface a fixed size.  It will never change from this size.
     * When working with a {@link SurfaceView}, this must be called from the
     * same thread running the SurfaceView's window.
     *
     * @param width The surface's width.
     * @param height The surface's height.
     */
    public void setFixedSize(int width, int height);

    /**
     * Allow the surface to resized based on layout of its container (this is
     * the default).  When this is enabled, you should monitor
     * {@link Callback#surfaceChanged} for changes to the size of the surface.
     * When working with a {@link SurfaceView}, this must be called from the
     * same thread running the SurfaceView's window.
     */
    public void setSizeFromLayout();

    /**
     * Set the desired PixelFormat of the surface.  The default is OPAQUE.
     * When working with a {@link SurfaceView}, this must be called from the
     * same thread running the SurfaceView's window.
     *
     * @param format A constant from PixelFormat.
     *
     * @see android.graphics.PixelFormat
     */
    public void setFormat(int format);

    /**
     * Enable or disable option to keep the screen turned on while this
     * surface is displayed.  The default is false, allowing it to turn off.
     * This is safe to call from any thread.
     *
     * @param screenOn Set to true to force the screen to stay on, false
     * to allow it to turn off.
     */
    public void setKeepScreenOn(boolean screenOn);

    /**
     * Start editing the pixels in the surface.  The returned Canvas can be used
     * to draw into the surface's bitmap.  A null is returned if the surface has
     * not been created or otherwise cannot be edited.  You will usually need
     * to implement {@link Callback#surfaceCreated Callback.surfaceCreated}
     * to find out when the Surface is available for use.
     *
     * <p>The content of the Surface is never preserved between unlockCanvas() and
     * lockCanvas(), for this reason, every pixel within the Surface area
     * must be written. The only exception to this rule is when a dirty
     * rectangle is specified, in which case, non-dirty pixels will be
     * preserved.
     *
     * <p>If you call this repeatedly when the Surface is not ready (before
     * {@link Callback#surfaceCreated Callback.surfaceCreated} or after
     * {@link Callback#surfaceDestroyed Callback.surfaceDestroyed}), your calls
     * will be throttled to a slow rate in order to avoid consuming CPU.
     *
     * <p>If null is not returned, this function internally holds a lock until
     * the corresponding {@link #unlockCanvasAndPost} call, preventing
     * {@link SurfaceView} from creating, destroying, or modifying the surface
     * while it is being drawn.  This can be more convenient than accessing
     * the Surface directly, as you do not need to do special synchronization
     * with a drawing thread in {@link Callback#surfaceDestroyed
     * Callback.surfaceDestroyed}.
     *
     * @return Canvas Use to draw into the surface.
     */
    public Canvas lockCanvas();


    /**
     * Just like {@link #lockCanvas()} but allows specification of a dirty rectangle.
     * Every
     * pixel within that rectangle must be written; however pixels outside
     * the dirty rectangle will be preserved by the next call to lockCanvas().
     *
     * @see android.view.SurfaceHolder#lockCanvas
     *
     * @param dirty Area of the Surface that will be modified.
     * @return Canvas Use to draw into the surface.
     */
    public Canvas lockCanvas(Rect dirty);

    /**
     * <p>Just like {@link #lockCanvas()} but the returned canvas is hardware-accelerated.
     *
     * <p>See the <a href="{@docRoot}guide/topics/graphics/hardware-accel.html#unsupported">
     * unsupported drawing operations</a> for a list of what is and isn't
     * supported in a hardware-accelerated canvas.
     *
     * @return Canvas Use to draw into the surface.
     * @throws IllegalStateException If the canvas cannot be locked.
     */
    default Canvas lockHardwareCanvas() {
        throw new IllegalStateException("This SurfaceHolder doesn't support lockHardwareCanvas");
    }

    /**
     * Finish editing pixels in the surface.  After this call, the surface's
     * current pixels will be shown on the screen, but its content is lost,
     * in particular there is no guarantee that the content of the Surface
     * will remain unchanged when lockCanvas() is called again.
     *
     * @see #lockCanvas()
     *
     * @param canvas The Canvas previously returned by lockCanvas().
     */
    public void unlockCanvasAndPost(Canvas canvas);

    /**
     * Retrieve the current size of the surface.  Note: do not modify the
     * returned Rect.  This is only safe to call from the thread of
     * {@link SurfaceView}'s window, or while inside of
     * {@link #lockCanvas()}.
     *
     * @return Rect The surface's dimensions.  The left and top are always 0.
     */
    public Rect getSurfaceFrame();

    /**
     * Direct access to the surface object.  The Surface may not always be
     * available -- for example when using a {@link SurfaceView} the holder's
     * Surface is not created until the view has been attached to the window
     * manager and performed a layout in order to determine the dimensions
     * and screen position of the Surface.    You will thus usually need
     * to implement {@link Callback#surfaceCreated Callback.surfaceCreated}
     * to find out when the Surface is available for use.
     *
     * <p>Note that if you directly access the Surface from another thread,
     * it is critical that you correctly implement
     * {@link Callback#surfaceCreated Callback.surfaceCreated} and
     * {@link Callback#surfaceDestroyed Callback.surfaceDestroyed} to ensure
     * that thread only accesses the Surface while it is valid, and that the
     * Surface does not get destroyed while the thread is using it.
     *
     * <p>This method is intended to be used by frameworks which often need
     * direct access to the Surface object (usually to pass it to native code).
     *
     * @return Surface The surface.
     */
    public Surface getSurface();
}
Android中可以通过SurfaceView来得到这个SurfaceHolder。 
SurfaceView surfaceView = new SurfaceView(context);
SurfaceHolder holder = surfaceView.getHolder();
 
  

相机

可以通过PackageManager的hasSystemFeature方法检查相机在运行时是否可用，参考FEATURE_CAMERA的定义：android.hardware.camera。PackageManager还定义很多其他的Feature。
在Android模拟器环境下，hasSystemFeature(PackageManager.FEATURE_CAMERA)调用返回的是true。
 

系统相机

 
Intent intent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
startActivityForResult(intent, OpCode.Op_Take_Picture.get());
对应的，在onActivityResult方法中
if (requestCode == OpCode.Op_Take_Picture.get()) {
    Bundle extras = data.getExtras();
    Bitmap bitmap = (Bitmap) extras.get("data");

    App app = (App) getApplicationContext();
    Session session = app.getSession();
    String filePath = Android.bitmapTo(app, bitmap,
            UUID.randomUUID().toString() + "@" + session.getUserId() + ".jpg");

    Uri uri = Uri.fromFile(new File(filePath));
    onMediaUriAppend(uri);
}
需要注意的是，上面的代码虽然使用到了相机，但不需要添加相机对应的权限android.permission.CAMERA。如果加入了权限，如果在Android清单文件中加入了android.permission.CAMERA权限：
<uses-permission android:name="android.permission.CAMERA"/>
调用系统相机报如下异常：
java.lang.SecurityException: Permission Denial: starting Intent { act=android.media.action.IMAGE_CAPTURE cmp=com.android.camera2/com.android.camera.CaptureActivity } from ProcessRecord{3ade4ef 10290:app.at/u0a137} (pid=10290, uid=10137) with revoked permission android.permission.CAMERA
    at android.os.Parcel.createExceptionOrNull(Parcel.java:2373)
    at android.os.Parcel.createException(Parcel.java:2357)
    at android.os.Parcel.readException(Parcel.java:2340)
    at android.os.Parcel.readException(Parcel.java:2282)
    at android.app.IActivityTaskManager$Stub$Proxy.startActivity(IActivityTaskManager.java:3696)
    at android.app.Instrumentation.execStartActivity(Instrumentation.java:1723)
    at android.app.Activity.startActivityForResult(Activity.java:5314)
    at android.app.Activity.startActivityForResult(Activity.java:5272)
    at app.at.Post$5.onItemSelected(Post.java:571)
    at ***.***.***.onClick(***.java:90)
    at android.view.View.performClick(View.java:7448)
    at android.view.View.performClickInternal(View.java:7425)
    at android.view.View.access$3600(View.java:810)
    at android.view.View$PerformClick.run(View.java:28305)
    at android.os.Handler.handleCallback(Handler.java:938)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:223)
    at android.app.ActivityThread.main(ActivityThread.java:7656)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
 Caused by: android.os.RemoteException: Remote stack trace:
    at com.android.server.wm.ActivityStackSupervisor.checkStartAnyActivityPermission(ActivityStackSupervisor.java:1032)
    at com.android.server.wm.ActivityStarter.executeRequest(ActivityStarter.java:999)
    at com.android.server.wm.ActivityStarter.execute(ActivityStarter.java:669)
    at com.android.server.wm.ActivityTaskManagerService.startActivityAsUser(ActivityTaskManagerService.java:1100)
    at com.android.server.wm.ActivityTaskManagerService.startActivityAsUser(ActivityTaskManagerService.java:1072)

在Android清单文件中去掉android.permission.CAMERA权限，就不会出现上述情况。不过如果不去掉Android清单文件的android.permission.CAMERA权限也行，可以通过动态请求android.permission.CAMERA权限的方式：

if (ContextCompat.checkSelfPermission(Post.this,
        Manifest.permission.CAMERA)
        != PackageManager.PERMISSION_GRANTED) {
    Post.this.requestPermissions(
            new String[] {Manifest.permission.CAMERA},
            OpCode.OP_CAMERA_PERMISSION.get());
}

然后在对应的onRequestPermissionsResult方法中调用系统相机：

if (requestCode == OpCode.OP_CAMERA_PERMISSION.get()) {
    for (int i = 0; i < permissions.length; i++) {
        if (permissions[i].equals(Manifest.permission.CAMERA)) {
            if (grantResults[i] == PackageManager.PERMISSION_GRANTED) {
                Toast.makeText(this, "相机权限申请成功", Toast.LENGTH_SHORT).show();

                Intent intent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
                //if (intent.resolveActivity(context.getPackageManager()) != null) {
                startActivityForResult(intent, OpCode.Op_Take_Picture.get());
                //}
            } else {
                Toast.makeText(this, "相机权限申请失败", Toast.LENGTH_SHORT).show();
            }
        }
    }
}

 

 

 

拍照

 

生物识别

BiometricPrompt

 

 

指纹识别

 

指纹

指纹（fingerprint）

 

FingerprintManager

 

构建

 

Android Studio构建的时候会生成一个BuildConfig.java文件，内容大概如下：

public final class BuildConfig {
  public static final boolean DEBUG = Boolean.parseBoolean("true");
  public static final String APPLICATION_ID = "***";
  public static final String BUILD_TYPE = "debug";
  public static final String FLAVOR = "***";
  public static final int VERSION_CODE = 1;
  public static final String VERSION_NAME = "1.0";
}

 

 

配置build变种

参考文档https://developer.android.google.cn/studio/build/build-variants?hl=zh-cn

 

productFlavors

 

 

productFlavors {
    standard {

    }
    merchant {

    }
}
 

 

All flavors must now belong to a named flavor dimension. Learn more at https://d.android.com/r/tools/flavorDimensions-missing-error-message.html

这时我们需要配置一下变种维度

flavorDimensions 'version'

然后在每一种变种下面指定所属的变种维度

dimension "version"

 

flavorDimensions 'version'

productFlavors {
    standard {
        dimension 'version'
    }

    merchant {
        dimension 'version'
    }
}

 

如果给定的模块仅指定一个变种维度，那么Android Gradle插件会自动将该模块的所有变种分配给该维度。

 

即无需在每一种变种下面指定所属的变种维度

 


flavorDimensions 'version'

productFlavors {
    standard {
    }

    merchant {
    }
}

 

如果需要多个变种维度，可以这样

 

flavorDimensions "product", "version"

 

 

The apk for your currently selected variant cannot be signed. Please specify a signing configuration for this variant (standard-release).

 

 

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