子线程不显示Toast?

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.(Handler.java:200)
        at android.os.Handler.(Handler.java:114)
        at android.widget.Toast$TN$2.(Toast.java:336)
        at android.widget.Toast$TN.(Toast.java:336)
        at android.widget.Toast.(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。