Handler不仅在ANR过程中有用到，Toast中也用到了Handler。
代码如下：
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
new Thread(new Runnable() {
@Override
public void run() {
Toast.makeText(MainActivity.this,"子线程",Toast.LENGTH_SHORT).show();
}
}).start();
}
}
运行安装，Logcat里面显示了异常如下：
Process: com.exp.cpdemo, PID: 1961
java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
at android.os.Handler.<init>(Handler.java:200)
at android.os.Handler.<init>(Handler.java:114)
at android.widget.Toast$TN$2.<init>(Toast.java:336)
at android.widget.Toast$TN.<init>(Toast.java:336)
at android.widget.Toast.<init>(Toast.java:103)
at android.widget.Toast.makeText(Toast.java:256)
at com.exp.cpdemo.MainActivity$1.run(MainActivity.java:40)
at java.lang.Thread.run(Thread.java:761)
设备是Android 7.1 系统，对应源码：
/frameworks/base/core/java/android/widget/Toast.java
为什么会抛出这个异常？
从makeText方法开始看：
public static Toast makeText(Context context, CharSequence text, @Duration int duration) {
Toast result = new Toast(context);
LayoutInflater inflate = (LayoutInflater)
context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
View v = inflate.inflate(com.android.internal.R.layout.transient_notification, null);
TextView tv = (TextView)v.findViewById(com.android.internal.R.id.message);
tv.setText(text);
result.mNextView = v;
result.mDuration = duration;
return result;
}
初始化了一个Toast对象。看看Toast构造器里面的代码逻辑：
public Toast(Context context) {
mContext = context;
mTN = new TN(context.getPackageName());
mTN.mY = context.getResources().getDimensionPixelSize(
com.android.internal.R.dimen.toast_y_offset);
mTN.mGravity = context.getResources().getInteger(
com.android.internal.R.integer.config_toastDefaultGravity);
}
初始化了一个TN对象。继续看TN这个类：
private static class TN extends ITransientNotification.Stub {
......
final Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
...
}
};
TN(String packageName) {
...
}
...
}
TN是Toast的静态内部类。在生成TN对象时，成员变量mHandler赋值逻辑执行，调用了Handler的默认构造器。接着看Android 7.1 Handler的源码：
/frameworks/base/core/java/android/os/Handler.java
public Handler() {
this(null, false);
}
public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class<? extends Handler> klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());
}
}
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
果然，在Handler的两参数构造方法中，抛出了上面看到的异常。
怎么解决？
知道了原因，添加两行代码就好了，如下：
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
new Thread(new Runnable() {
@Override
public void run() {
Looper.prepare();
Toast.makeText(MainActivity.this,"子线程",Toast.LENGTH_SHORT).show();
Looper.loop();
}
}).start();
}
}
Toast显示成功了。
