LeakCanary流程介绍分析

LeakCanary，在开发阶段，可以用来检测内存泄露，项目地址：https://github.com/square/leakcanary

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具体操作：1、在Application 的onCreate()初始化，会ActivityLifecycleCallbacks监听所有的activity的onDestroy，
2、把activity生成对应key的放到集合Set retainedKeys，并生成带Key的弱引用，GC在回收的时候，如果是弱引用，会把这个回收对象放到ReferenceQueue queue，如果poll出来有对象，说明这个activity对象已回收，移除集合包含，若没有，判断集合是否包含key，若有，则就怀疑是泄漏了，需要二次确认，进行手动Runtime.getRuntime().gc()，
3、泄露对象，生成HPROF文件，分析这个快照文件有没有存在带这个key值的泄漏对象，如果没有，那么没有泄漏，
否则找出最短路径，打印给我们，就找到这个泄漏对象了。

一、初始化LeakCanary

public class ExampleApplication extends Application {
  @Override public void onCreate() {
    super.onCreate();
    LeakCanary.install(this);
  }
}

public final class LeakCanary {
    public static RefWatcher install(Application application) {
    return refWatcher(application).listenerServiceClass(DisplayLeakService.class)
        .excludedRefs(AndroidExcludedRefs.createAppDefaults().build())
        .buildAndInstall();
  }
}

refWatcher()生成AndroidRefWatcherBuilder类，.buildAnInstall()，会生成RefWatcher，并把相应的对象如（GcTrigger、AndroidWatchExecutor、AndroidHeapDumper、ServiceHeapDumpListener、AndroidExcludedRefs等）。
且Application.registerActivityLifecycleCallbacks这个方法，用来统一管理所有activity的生命周期，LeakCanary就是在onDestory()方法实现监控的.

public final class ActivityRefWatcher {
       ...
  public static void install(Application application, RefWatcher refWatcher) {
    new ActivityRefWatcher(application, refWatcher).watchActivities();
  }
  private final Application.ActivityLifecycleCallbacks lifecycleCallbacks =
      new Application.ActivityLifecycleCallbacks() {
       ...
        @Override public void onActivityDestroyed(Activity activity) {
          ActivityRefWatcher.this.onActivityDestroyed(activity);
        }
      };
  private final Application application;
  private final RefWatcher refWatcher;
  void onActivityDestroyed(Activity activity) {
    refWatcher.watch(activity);
  }
  public void watchActivities() {
    stopWatchingActivities();
    application.registerActivityLifecycleCallbacks(lifecycleCallbacks);
  }
}

二、泄露判断
在RefWatcher中watch函数中：

public final class RefWatcher {
  private final WatchExecutor watchExecutor;
  private final DebuggerControl debuggerControl;
  private final GcTrigger gcTrigger;
  private final HeapDumper heapDumper;
  private final Set retainedKeys;
  private final ReferenceQueue