WebView加载Firebase PDF返回后重启空白，求修复方案

Let's break down what's causing the blank screen and how to fix it step by step:

Key Issues in Your Original Code: #

1. Missing PDF URL persistence: If your activity gets recreated (e.g., screen rotation), you lose the Pdf_data value, leading to an empty URL load.
2. Duplicate WebView configuration: Calling ShowContent() in both onCreate and onResume re-sets up WebView clients/settings multiple times, causing conflicts.
3. Unnecessary postDelayed: The 400ms delay can lead to duplicate load requests when the activity resumes, disrupting the WebView state.
4. Lack of error handling: You can't diagnose why loads fail without an error callback.
5. Potential cache conflicts: Stale cached content might be displayed instead of the fresh PDF.

Corrected Code with Fixes: #

First, update your member variables and add state saving:

public class ShowPdfActivity extends AppCompatActivity {
    private WebView webView;
    private ProgressDialog progressDialog;
    private String Pdf_data;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_show_pdf);
        webView = findViewById(R.id.webView);

        // Restore PDF URL if activity was recreated
        if (savedInstanceState != null) {
            Pdf_data = savedInstanceState.getString("PDF_URL");
        } else {
            // Get PDF URL from intent (adjust the key to match your intent extra)
            Pdf_data = getIntent().getStringExtra("PDF_DATA");
        }

        // Initialize WebView settings once
        initWebView();
        // Load the PDF content
        loadPdfContent();
    }

    // Save PDF URL to survive activity recreation
    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putString("PDF_URL", Pdf_data);
    }

Next, add the WebView initialization method (called once in onCreate):

private void initWebView() {
        webView.setWebChromeClient(new WebChromeClient());
        WebSettings webSettings = webView.getSettings();
        
        webSettings.setJavaScriptEnabled(true);
        webSettings.setBuiltInZoomControls(true);
        webSettings.setCacheMode(WebSettings.LOAD_NO_CACHE); // Prevent stale cache
        webSettings.setDomStorageEnabled(true); // Improve viewer compatibility

        webView.setWebViewClient(new WebViewClient() {
            @Override
            public void onPageStarted(WebView view, String url, Bitmap favicon) {
                super.onPageStarted(view, url, favicon);
                showProgressDialog();
            }

            @Override
            public void onPageFinished(WebView view, String url) {
                super.onPageFinished(view, url);
                hideProgressDialog();
            }

            @Override
            public void onReceivedError(WebView view, int errorCode, String description, String failingUrl) {
                super.onReceivedError(view, errorCode, description, failingUrl);
                hideProgressDialog();
                Toast.makeText(ShowPdfActivity.this, "Failed to load PDF: " + description, Toast.LENGTH_SHORT).show();
            }
        });
    }

Add helper methods for progress dialog and PDF loading:

private void showProgressDialog() {
        if (progressDialog == null) {
            progressDialog = new ProgressDialog(this);
            progressDialog.setTitle("Please Wait");
            progressDialog.setMessage("Previewing your Content ..");
            progressDialog.setIcon(R.mipmap.ic_launcher_round);
            progressDialog.setCanceledOnTouchOutside(false);
        }
        progressDialog.show();
    }

    private void hideProgressDialog() {
        if (progressDialog != null && progressDialog.isShowing()) {
            progressDialog.dismiss();
        }
    }

    private void loadPdfContent() {
        if (TextUtils.isEmpty(Pdf_data)) {
            Toast.makeText(this, "No PDF URL available", Toast.LENGTH_SHORT).show();
            return;
        }

        String encodedUrl;
        try {
            encodedUrl = URLEncoder.encode(Pdf_data, "UTF-8");
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
            Toast.makeText(this, "Error encoding PDF URL", Toast.LENGTH_SHORT).show();
            return;
        }

        // Use proper & instead of HTML entity &
        String viewerUrl = "http://drive.google.com/viewerng/viewer?embedded=true&url=" + encodedUrl;
        webView.loadUrl(viewerUrl);
    }

Update lifecycle methods to handle resume and cleanup properly:

@Override
    protected void onResume() {
        super.onResume();
        // Reload PDF to ensure fresh content when returning to the activity
        loadPdfContent();
    }

    private void destroyWebView() {
        hideProgressDialog(); // Clean up progress dialog first

        if (webView != null) {
            webView.stopLoading();
            webView.clearHistory();
            webView.clearCache(true);
            webView.loadUrl("about:blank");
            webView.pauseTimers();
            webView.removeAllViews();
            webView.destroy();
            webView = null;
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        destroyWebView();
    }
}

Key Fixes Explained: #

• URL Persistence: Saving Pdf_data in onSaveInstanceState ensures we don't lose the URL if the activity is recreated.
• One-Time Initialization: initWebView() sets up WebView settings and clients once, avoiding duplicate configurations.
• Cache Control: LOAD_NO_CACHE prevents stale cached content from being displayed.
• Error Handling: onReceivedError helps diagnose loading failures and informs the user.
• Clean Progress Dialog Management: Ensures no memory leaks and proper dismissal.
• Direct Load Without Delay: Removes the unnecessary postDelayed to avoid duplicate load requests.

If the PDF URL from Firebase is short-lived (expires after some time), you'll need to fetch a fresh URL from Firebase in onResume instead of reusing the old one. Otherwise, the above fixes should resolve the blank screen issue.

内容的提问来源于stack exchange，提问作者Zed