子线程更新UI的问题

代码如下：

public class MainActivity extends Activity {

    private Button button;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //第一种情况
        button = (Button) this.findViewById(R.id.button);
        button.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View arg0) {
                Thread thread = new Thread() {

                    @Override
                    public void run() {
                        Toast.makeText(MainActivity.this, "Click!", 1000)
                                .show();
                    }
                };

                thread.start();
            }
        });

        //第二种情况：
        Thread thread = new Thread() {
            @Override
            public void run() {
                button = (Button) MainActivity.this.findViewById(R.id.button);
                button.setOnClickListener(new OnClickListener() {

                    @Override
                    public void onClick(View v) {
                        Toast.makeText(MainActivity.this, "Click!", 1000)
                                .show();
                    }
                });

            }
        };

        thread.start();
        
        //第三种情况
        Thread thread1 = new Thread() {
            @Override
            public void run() {
                Toast.makeText(MainActivity.this, "------- Click --------- ",
                        1000).show();

            }
        };

        thread1.start();

    }

第一种情况会出现问题：报的错误：

01-07 11:59:10.404: E/AndroidRuntime(4114): FATAL EXCEPTION: Thread-114
01-07 11:59:10.404: E/AndroidRuntime(4114): java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
01-07 11:59:10.404: E/AndroidRuntime(4114):  at android.os.Handler.<init>(Handler.java:121)
01-07 11:59:10.404: E/AndroidRuntime(4114):  at android.widget.Toast$TN.<init>(Toast.java:317)
01-07 11:59:10.404: E/AndroidRuntime(4114):  at android.widget.Toast.<init>(Toast.java:91)
01-07 11:59:10.404: E/AndroidRuntime(4114):  at android.widget.Toast.makeText(Toast.java:233)
01-07 11:59:10.404: E/AndroidRuntime(4114):  at com.example.subthreadtest.MainActivity$1$1.run(MainActivity.java:30)

 

第二种没有问题：Toast能显示

第三种也会有问题：

01-07 12:03:12.809: E/AndroidRuntime(4664): FATAL EXCEPTION: Thread-129
01-07 12:03:12.809: E/AndroidRuntime(4664): java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
01-07 12:03:12.809: E/AndroidRuntime(4664):  at android.os.Handler.<init>(Handler.java:121)
01-07 12:03:12.809: E/AndroidRuntime(4664):  at android.widget.Toast$TN.<init>(Toast.java:317)
01-07 12:03:12.809: E/AndroidRuntime(4664):  at android.widget.Toast.<init>(Toast.java:91)
01-07 12:03:12.809: E/AndroidRuntime(4664):  at android.widget.Toast.makeText(Toast.java:233)
01-07 12:03:12.809: E/AndroidRuntime(4664):  at com.example.subthreadtest.MainActivity$1.run(MainActivity.java:63)

有错误的情况在Toast前后分别加上：Looper.prepare()和Looper.loop();即可。

可以这样解释：一、三两种情况其实是在子线程中更新了UI，第二种却不是：虽然button在子线程中，但是他是用的主线程的引用。