设置进度条类型的可见性时，应用程序会崩溃。

Question

我正在构建一个打开when视图的应用程序，当它打开网页时，我想显示一个旋转类型的进度条，但是当我使用setvisibility()改变可见性时，应用程序就会崩溃。

这是我的活动代码：

public class MainActivity extends Activity {

    WebView website;
    ProgressBar pro;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        pro= (ProgressBar) findViewById(R.id.progress);
        setContentView(R.layout.activity_main);

        website = (WebView) findViewById(R.id.website);
        website.setWebViewClient(new WebViewClient());

        WebSettings webSettings = website.getSettings();
        webSettings.setJavaScriptEnabled(true);
        website.loadUrl("http://www.google.com");
        website.setWebChromeClient(new WebChromeClient() {

            @Override
            public void onProgressChanged(WebView view, int newProgress) {
                //change your progress bar
                if(pro.getVisibility()==view.VISIBLE){
                   pro.setVisibility(view.INVISIBLE);
                }
                else {
                    pro.setVisibility(view.VISIBLE);
                }
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if ((keyCode ==KeyEvent.KEYCODE_BACK) && website.canGoBack()) {
            website.goBack();
            return true;
        }
        else{
            finish();
        }
        return super.onKeyDown(keyCode, event);
    }

}

原木猫

3:08:57 PM Gradle构建在9秒3:22:44 PM Gradle构建在35秒内完成3:24:41 PM Gradle构建在7秒内完成3:24:44 PM会话“app”运行3:26:47 PM Gradle构建在6秒内完成3:26:49 PM会话“app”：运行3:30:31 PM Gradle构建在13秒内完成3:30:34 PM会话“app”运行3:38:54 PM Gradle构建在9秒3:39:02完成3:39:02 PM会话“app”：运行3:39:04 PM Gradle构建在10秒内完成3:39:06 PM会话“app”：调试器连接了3:57:21 PM Gradle构建在11秒内完成了3:57:24 PM会话“app”：运行11:16:08 AM Gradle构建在37秒11:16:11 AM会话'app'：运行11:52:39 AM Gradle在11秒11:55:00 AM Gradle构建在28秒11:58:45中完成。AM Gradle构建在18秒11:59:04 AM Gradle构建在6秒11:59:09 AM会话“app”中完成:运行3:51:43 PM Gradle构建在25秒内完成3:53:31 PM Gradle构建在40秒内完成3:53:33 PM会话“app”：运行3:56:04 PM Gradle构建在2秒内完成3:56:07 PM会话“应用”：运行3:56:51 PM Gradle构建在2秒内完成4:04:03 PM Gradle构建在6秒4:04:07完成PM会话“app”：运行4:04:54 PM Gradle构建在4秒4:04:56 PM会话“app”：运行4:05:34 PM Gradle构建在3秒内完成4:05:37 PM会话“app”：运行4:08:51 PM Gradle构建在32秒4:09:00 PM会话“app”：运行4:13:42 PM Gradle构建在36秒4:13:44 PM会话“app”：运行4:50:31 PM Gradle构建在14秒4:50:36完成。PM会话'app'：运行5:04:19 PM Gradle构建在10秒内完成5:04:22 PM会话:运行

Answer 1

你应该打电话给：

setContentView(R.layout.activity_main);

在打电话之前：

pro = (ProgressBar) findViewById(R.id.progress);

因为在查找子视图之前必须设置一个视图。