安卓bug - Android中，intent初始化為什麼不能寫在方法外面？

Question

初學者。今天寫一個活動之間跳轉的程式碼，用到了顯式的intent，但是我在類別中定義了一個intent的成員變量，在方法中使用時候，程式卻崩潰了，這是為什麼呢？ {代碼...} 如上代碼，但是錯誤訊息提示是： Caused by: ...

phpcn_u1582 · Answer

這麼做之前, 先要了解清楚Activity与Context之间的关系: 虽然Activity继承了Context, 但是它却不是真正的实现类, 真正的实现可能是ContextWrapper#getBaseContext()回傳物件所對應的類別.

Activity是ContextWrapper的子类, 所以我们先找到并打开ContextWrapper.java源碼, 關鍵代碼如下:

public class ContextWrapper extends Context {
    public ContextWrapper(Context base) {
        mBase = base;
    }
    
    /**
     * Set the base context for this ContextWrapper.  All calls will then be
     * delegated to the base context.  Throws
     * IllegalStateException if a base context has already been set.
     * 
     * @param base The new base context for this wrapper.
     */
    protected void attachBaseContext(Context base) {
        if (mBase != null) {
            throw new IllegalStateException("Base context already set");
        }
        mBase = base;
    }
    
    ... ...
    
    /**
     * @return the base context as set by the constructor or setBaseContext
     */
    public Context getBaseContext() {
        return mBase;
    }

    @Override
    public AssetManager getAssets() {
        return mBase.getAssets();
    }

    @Override
    public Resources getResources() {
        return mBase.getResources();
    }

    ... ...

ContextWrapper里基于Context的调用都是直接使用mBase来间接实现调用的. 那么这个mBase是什么时候被赋值的呢? 找到并打开ActivityThread.java, 就能找到它被賦值的程式碼部分, 關鍵程式碼如下:

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
    // System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")");

    ... ...

    // -------------------------------------------------------------------
    // 创建Activity实例
    // -------------------------------------------------------------------
    Activity activity = null;
    try {
        java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
        activity = mInstrumentation.newActivity(
                cl, component.getClassName(), r.intent);
        StrictMode.incrementExpectedActivityCount(activity.getClass());
        r.intent.setExtrasClassLoader(cl);
        r.intent.prepareToEnterProcess();
        if (r.state != null) {
            r.state.setClassLoader(cl);
        }
    } catch (Exception e) {
        if (!mInstrumentation.onException(activity, e)) {
            throw new RuntimeException(
                "Unable to instantiate activity " + component
                + ": " + e.toString(), e);
        }
    }

    try {
        Application app = r.packageInfo.makeApplication(false, mInstrumentation);

        if (localLOGV) Slog.v(TAG, "Performing launch of " + r);
        if (localLOGV) Slog.v(
                TAG, r + ": app=" + app
                + ", appName=" + app.getPackageName()
                + ", pkg=" + r.packageInfo.getPackageName()
                + ", comp=" + r.intent.getComponent().toShortString()
                + ", dir=" + r.packageInfo.getAppDir());

        if (activity != null) {
            Context appContext = createBaseContextForActivity(r, activity);
            CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
            Configuration config = new Configuration(mCompatConfiguration);
            if (r.overrideConfig != null) {
                config.updateFrom(r.overrideConfig);
            }
            if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
                    + r.activityInfo.name + " with config " + config);
            Window window = null;
            if (r.mPendingRemoveWindow != null && r.mPreserveWindow) {
                window = r.mPendingRemoveWindow;
                r.mPendingRemoveWindow = null;
                r.mPendingRemoveWindowManager = null;
            }
            
            // -------------------------------------------------------------------
            // 设置 appContext 为Activity 的 BaseContext
            // -------------------------------------------------------------------
            activity.attach(appContext, this, getInstrumentation(), r.token,
                    r.ident, app, r.intent, r.activityInfo, title, r.parent,
                    r.embeddedID, r.lastNonConfigurationInstances, config,
                    r.referrer, r.voiceInteractor, window);

以上, 可知: 實例化Activity时, ContextWrapper#getBaseContext()返回的是null, 因此, 不能在构造函数或者构造成员变量时直接调用与Context相關的任何函數和類別的實例化. 如果需要, 則在其生命週期函數中去調用.

伊谢尔伦 · Answer

你的類別繼承了Activity類別,那麼它就有生命週期,所有邏輯都在這幾個生命週期裡面進行,換而言之,你的邏輯代碼都要寫在那幾個生命週期的方法裡面.一般來說,都是重寫onCreate的方法,在那裡面寫頁面跳轉.你的方法的標識符命名也不規範

天蓬老师 · Answer

要重寫onCreate()方法啊