Handler的runWithScissors方法是隐藏API，用于执行同步任务（即让另一个线程执行一个任务，当前线程先阻塞，等待该执行完，之后当前线程再继续执行）。源码如下：
BlockingRunnable代码如下：
ok 源码就这些。 当前线程 wait（阻塞），等待handler所关联的线程执行完任务后唤醒当前线程。可以用于执行同步任务。timeOut为超时时间。
隐藏API不方便直接调用。把这段代码复制出来测试下， 测试代码如下：
package com.example.myapplication;
import android.os.Handler;
import android.os.Looper;
import android.os.SystemClock;
import android.util.Log;
public class Test {MyHandler handler;public void testRunWithScissors() {Thread thread1 = new Thread(){@Overridepublic void run() {Log.d("zx", "thread1 start ， threadID：" + Thread.currentThread().getId());Looper.prepare();handler = new MyHandler(Looper.myLooper());Looper.loop();}};thread1.start();Thread thread2 = new Thread() {@Overridepublic void run() {Log.d("zx", "thread2 start ， threadID：" + Thread.currentThread().getId());// 线程2等待线程1执行一个任务， 执行完该任务后，线程2再做其他事。boolean res = handler.runWithScissors2(new Runnable() {@Overridepublic void run() {// 线程1中会执行这个任务Log.d("zx", "current Thread， threadID：" + Thread.currentThread().getId()+ ", task start ...");for (int i = 0; i < 5; i++) {try {Thread.sleep(100);} catch (InterruptedException e) {e.printStackTrace();}Log.i("zx", "i = " + i);}Log.i("zx", "task finish");}}, 1000);Log.d("zx", "task finish, result: " + res + ", thread2 continue ...");}};try {Thread.sleep(10); // 防止线程1还没跑起来handler为空} catch (InterruptedException e) {e.printStackTrace();}thread2.start();}public static class MyHandler extends Handler {public MyHandler(Looper looper) {super(looper);}// 同步执行任务。 意思是，阻塞当前线程，等待另一个线程执行完该任务后，当前线程再继续执行. timeout为超时时间。 返回值表示任务执行是否成功public final boolean runWithScissors2(Runnable r, long timeout) { // 不能和隐藏api的方法同名，所以换了方法名。if (r == null) {throw new IllegalArgumentException("runnable must not be null");}if (timeout < 0) {throw new IllegalArgumentException("timeout must be non-negative");}if (Looper.myLooper() == getLooper()) { // 特殊情况，没有切换线程，就在当前线程执行。r.run();return true;}BlockingRunnable br = new BlockingRunnable(r);return br.postAndWait(this, timeout);}private static final class BlockingRunnable implements Runnable {private final Runnable mTask;private boolean mDone;public BlockingRunnable(Runnable task) {mTask = task;}@Overridepublic void run() {try {mTask.run();} finally {synchronized (this) {mDone = true;notifyAll();}}}public boolean postAndWait(Handler handler, long timeout) {if (!handler.post(this)) {return false;}synchronized (this) {if (timeout > 0) {final long expirationTime = SystemClock.uptimeMillis() + timeout;while (!mDone) {long delay = expirationTime - SystemClock.uptimeMillis();if (delay <= 0) {return false; // timeout 超时了}try {wait(delay);} catch (InterruptedException ex) {}}} else {while (!mDone) {try {wait();} catch (InterruptedException ex) {}}}}return true;}}}
}然后在activity里直接测试：
打印：
ok. 结果符合预期。