一、页面如何添加水印？
问题一是如何给页面添加水印？针对有一定安全性要求的App，通常需要在页面中添加水印。
在我们的项目中，我们是基于Activity添加水印。
大致代码可以为
FrameLayout waterMarkView = new FrameLayout(activity);
waterMarkView.setBackground(createWaterMarkDrawable(waterMarkText));
FrameLayout contentLayout = findViewById(android.R.id.content);
contentLayout.addView(waterMarkView, -1, -1);
每个Activity都有一个ContentView，由于ContentView是一个FrameLayout，那么我们就可以在不影响原生布局的情况下，再为其添加一个水印View。
二、如何统计Activity生命周期耗时
Android的开发同学都知道，在四大组件的生命周期的方法中是不可以执行耗时操作的。那么我们要怎么监控每个Activity的生命周期方法的耗时呢？
1、Android 10以上版本
在Android 10及以上我们可以考虑依赖 ActivityLifecycleCallbacks 接口，原因是在Android10以及以上，ActivityLifecycleCallbacks接口为每个生命周期的方法都新增了onActivityPreCreated(),onActivityPostCreated()配对方法。
public interface ActivityLifecycleCallbacks {
/**
* Called as the first step of the Activity being created. This is always called before
* {@link Activity#onCreate}.
*/
default void onActivityPreCreated(@NonNull Activity activity,
@Nullable Bundle savedInstanceState) {
}
/**
* Called when the Activity calls {@link Activity#onCreate super.onCreate()}.
*/
void onActivityCreated(@NonNull Activity activity, @Nullable Bundle savedInstanceState);
/**
* Called as the last step of the Activity being created. This is always called after
* {@link Activity#onCreate}.
*/
default void onActivityPostCreated(@NonNull Activity activity,
@Nullable Bundle savedInstanceState) {
}
/**
* Called as the first step of the Activity being started. This is always called before
* {@link Activity#onStart}.
*/
default void onActivityPreStarted(@NonNull Activity activity) {
}
/**
* Called when the Activity calls {@link Activity#onStart super.onStart()}.
*/
void onActivityStarted(@NonNull Activity activity);
/**
* Called as the last step of the Activity being started. This is always called after
* {@link Activity#onStart}.
*/
default void onActivityPostStarted(@NonNull Activity activity) {
}
/**
* Called as the first step of the Activity being resumed. This is always called before
* {@link Activity#onResume}.
*/
default void onActivityPreResumed(@NonNull Activity activity) {
}
/**
* Called when the Activity calls {@link Activity#onResume super.onResume()}.
*/
void onActivityResumed(@NonNull Activity activity);
/**
* Called as the last step of the Activity being resumed. This is always called after
* {@link Activity#onResume} and {@link Activity#onPostResume}.
*/
default void onActivityPostResumed(@NonNull Activity activity) {
}
/**
* Called as the first step of the Activity being paused. This is always called before
* {@link Activity#onPause}.
*/
default void onActivityPrePaused(@NonNull Activity activity) {
}
/**
* Called when the Activity calls {@link Activity#onPause super.onPause()}.
*/
void onActivityPaused(@NonNull Activity activity);
/**
* Called as the last step of the Activity being paused. This is always called after
* {@link Activity#onPause}.
*/
default void onActivityPostPaused(@NonNull Activity activity) {
}
/**
* Called as the first step of the Activity being stopped. This is always called before
* {@link Activity#onStop}.
*/
default void onActivityPreStopped(@NonNull Activity activity) {
}
/**
* Called when the Activity calls {@link Activity#onStop super.onStop()}.
*/
void onActivityStopped(@NonNull Activity activity);
/**
* Called as the last step of the Activity being stopped. This is always called after
* {@link Activity#onStop}.
*/
default void onActivityPostStopped(@NonNull Activity activity) {
}
/**
* Called as the first step of the Activity saving its instance state. This is always
* called before {@link Activity#onSaveInstanceState}.
*/
default void onActivityPreSaveInstanceState(@NonNull Activity activity,
@NonNull Bundle outState) {
}
/**
* Called when the Activity calls
* {@link Activity#onSaveInstanceState super.onSaveInstanceState()}.
*/
void onActivitySaveInstanceState(@NonNull Activity activity, @NonNull Bundle outState);
/**
* Called as the last step of the Activity saving its instance state. This is always
* called after{@link Activity#onSaveInstanceState}.
*/
default void onActivityPostSaveInstanceState(@NonNull Activity activity,
@NonNull Bundle outState) {
}
/**
* Called as the first step of the Activity being destroyed. This is always called before
* {@link Activity#onDestroy}.
*/
default void onActivityPreDestroyed(@NonNull Activity activity) {
}
/**
* Called when the Activity calls {@link Activity#onDestroy super.onDestroy()}.
*/
void onActivityDestroyed(@NonNull Activity activity);
/**
* Called as the last step of the Activity being destroyed. This is always called after
* {@link Activity#onDestroy}.
*/
default void onActivityPostDestroyed(@NonNull Activity activity) {
}
/**
* Called when the Activity configuration was changed.
* @hide
*/
default void onActivityConfigurationChanged(@NonNull Activity activity) {
}
}
统计耗时我们可以利用onActivityPostCreated()执行的时间减去onActivityPreCreated()执行的时间戳，就可以得出该onCreate的耗时时间了。
那么为什么onActivityPostCreated() 减去 onActivityPreCreated()的时间戳就可以呢？
我们可以简单看一下Activity performCreate 代码
final void performCreate(Bundle icicle, PersistableBundle persistentState) {
if (Trace.isTagEnabled(Trace.TRACE_TAG_WINDOW_MANAGER)) {
Trace.traceBegin(Trace.TRACE_TAG_WINDOW_MANAGER, "performCreate:"
+ mComponent.getClassName());
}
dispatchActivityPreCreated(icicle);
mCanEnterPictureInPicture = true;
// initialize mIsInMultiWindowMode and mIsInPictureInPictureMode before onCreate
final int windowingMode = getResources().getConfiguration().windowConfiguration
.getWindowingMode();
mIsInMultiWindowMode = inMultiWindowMode(windowingMode);
mIsInPictureInPictureMode = windowingMode == WINDOWING_MODE_PINNED;
mShouldDockBigOverlays = getResources().getBoolean(R.bool.config_dockBigOverlayWindows);
restoreHasCurrentPermissionRequest(icicle);
if (persistentState != null) {
onCreate(icicle, persistentState);
} else {
onCreate(icicle);
}
EventLogTags.writeWmOnCreateCalled(mIdent, getComponentName().getClassName(),
"performCreate");
mActivityTransitionState.readState(icicle);
mVisibleFromClient = !mWindow.getWindowStyle().getBoolean(
com.android.internal.R.styleable.Window_windowNoDisplay, false);
mFragments.dispatchActivityCreated();
mActivityTransitionState.setEnterActivityOptions(this, getActivityOptions());
dispatchActivityPostCreated(icicle);
Trace.traceEnd(Trace.TRACE_TAG_WINDOW_MANAGER);
}
可以看到在performCreate中，先执行了dispatchActivityPreCreated()方法, 然后开始执行onCreate方法，而后执行dispatchActivityPostCreated()方法。
dispatchActivityPreCreated()
private void dispatchActivityPreCreated(@Nullable Bundle savedInstanceState) {
getApplication().dispatchActivityPreCreated(this, savedInstanceState);
Object[] callbacks = collectActivityLifecycleCallbacks();
if (callbacks != null) {
for (int i = 0; i < callbacks.length; i++) {
((Application.ActivityLifecycleCallbacks) callbacks[i]).onActivityPreCreated(this,
savedInstanceState);
}
}
}
dispatchActivityPostCreated()
private void dispatchActivityPostCreated(@Nullable Bundle savedInstanceState) {
Object[] callbacks = collectActivityLifecycleCallbacks();
if (callbacks != null) {
for (int i = 0; i < callbacks.length; i++) {
((Application.ActivityLifecycleCallbacks) callbacks[i]).onActivityPostCreated(this,
savedInstanceState);
}
}
getApplication().dispatchActivityPostCreated(this, savedInstanceState);
}
可以看到dispatchActivityPreCreated() 、dispatchActivityPostCreated()的作用就是通知Application以及Activity中注册的 ActivityLifecycleCallbacks。
以上简单介绍了一下onCreate方法，其他的生命周期方法也是同理，这里就不啰嗦了。
2、Android 9及以下版本监听生命周期方法耗时
当前Android9及以下版本在市场上占有量已经比较低了，由于低版本ActivityLifecycleCallbacks中没有可以利用的方法，所以在低版本中一般要使用Hook Instrumentation的方式。
如何Hook Instrumentation在社区也有成熟的代码，这里简单贴一下：
private static void hookInstrumentation() throws ClassNotFoundException, NoSuchMethodException, InvocationTargetException, IllegalAccessException, NoSuchFieldException {
Class<?> c = Class.forName("android.app.ActivityThread");
Method currentActivityThread = c.getDeclaredMethod("currentActivityThread");
boolean acc = currentActivityThread.isAccessible();
if (!acc) {
currentActivityThread.setAccessible(true);
}
//获取ActivityThread对象
Object o = currentActivityThread.invoke(null);
Field f = c.getDeclaredField("mInstrumentation");
acc = f.isAccessible();
if (!acc) {
f.setAccessible(true);
}
//获取Instrumentation对象
Instrumentation currentInstrumentation = (Instrumentation) f.get(o);
Instrumentation ins = new CustomInstrumentation(currentInstrumentation);
//替换
f.set(o, ins);
if (!acc) {
f.setAccessible(acc);
}
}
这里 CustomInstrumentation 是自定义的Instrumentation子类。
在CustomInstrumentation中重写 callActivityOnCreate() callActivityOnStart()等方法，就可以在方法内部添加计算耗时逻辑。
三、如何获取应用中Top Activity
在平时工作中，通常需要获取App的Top Activity 来协助定位逻辑，这里介绍几种方式：
1、通过ADB
通过ADB命令查看任意App的Top Activity：
adb shell "dumpsys window | grep mFocusedApp"
2、利用ActivityManager
利用ActivityManager获取 ActivityManager.RunningTaskInfo 对象，其内有很多数据我们可以应用，例如获取TopActivity的名称：
fun getTopActivityName() {
val activityManager: ActivityManager =
getSystemService(ACTIVITY_SERVICE) as ActivityManager
val tasks: List<ActivityManager.RunningTaskInfo> = activityManager.getRunningTasks(1)
if (tasks[0].topActivity != null) {
val text = tasks[0].topActivity?.packageName
...
}
}
我们团队内部有开发一个开发者工具，支持利用浮窗实时显示当前Top Activity的名称，以此提高大家定位Activity的效率，我们使用的是这个逻辑。
四、如何判断App是否在前台？
在我们的App中，我们通常需要监听App前后台的状态切换。例如当App切换到前台时，会立即检查长连接是否存在，不过不存在会立即去建立长连接。
这里判断App是否在前台也是通过Activity来判断。
1、依然是通过生命周期回调
方案一依然是通过 ActivityLifecycleCallbacks 接口来自行维护（不得不说这个接口非常有用），通过计数的方式在内存维护前台Activity的个数。当个数为0时，则认为App切换到了后台。
不过在实践过程中我们发现这个逻辑存在偶现的Bug，由于是偶现产生的Bug的原因我们没有实际上搞清楚，能够确定的是大概率与系统有关。基于此引入方案二。
2、通过ActivityManager
在高版本中利用 ActivityManager 获取到RunningTaskInfo对象之后，通过isVisible()方法来判断。
public void checkAppBackgroundImpl() {
if (Build.VERSION.SDK_INT > Build.VERSION_CODES.TIRAMISU) {
ActivityManager activityManager = (ActivityManager) App.getContext().getSystemService(Context.ACTIVITY_SERVICE);
List<ActivityManager.RunningTaskInfo> tasks = activityManager.getRunningTasks(1);
if (tasks.get(0) != null) {
ActivityManager.RunningTaskInfo runningTaskInfo = tasks.get(0);
boolean visible = runningTaskInfo.isVisible();
...
}
}
}
在实践过程中，这个方案还没有被反馈遇到Bug，相对来讲比较准确，确定就是需要高版本。
五、总结
本篇主要是介绍几个与Activity相关的小技巧，都是在工作中可以经常使用的，希望能够读者带来一些启发。
