Android中Activity的startActivity和Context的startActivity有什么不同

在使用中的不同

1.在Activity中跳转到其他的Activity时,两种使用方法是一样的:

this.startActivity(intent);
context.startActivity(intent);

2.从非 Activity (例如从其他Context中)启动Activity则必须给intent设置Flag:FLAG_ACTIVITY_NEW_TASK:

intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK) ; 
mContext.startActivity(intent);

探究一下为什么会有这方面的差异


首先看下Activity和context的继承关系:

要知道Activity和context中的StartActivity有什么区别,我们看下它们分别是怎么实现startActivity函数的:

1.Context是抽象类,它的startActivity函数是抽象方法:

public abstract void startActivity(@RequiresPermission Intent intent);

2.ContextWrapper类只是调用了Context的实现:

@Override
public void startActivity(Intent intent) {
    mBase.startActivity(intent);
}

3.ContextThemeWrapper中没有实现此方法

4.Activity中:

@Override
    public void startActivity(Intent intent, @Nullable Bundle options) {
        if (options != null) {
            startActivityForResult(intent, -1, options);
        } else {
            // Note we want to go through this call for compatibility with
            // applications that may have overridden the method.
            startActivityForResult(intent, -1);
        }
    }

由此可见,在Activity中无论是使用哪一种startActivity方法都会调用到Activity自身的方法,所以是一样的。

然而在其他的Context子类,例如ContextImpl.java中的实现,会检查有没有设置Flag:FLAG_ACTIVITY_NEW_TASK,否则会报错:

@Override
    public void startActivity(Intent intent, Bundle options) {
        warnIfCallingFromSystemProcess();

        // Calling start activity from outside an activity without FLAG_ACTIVITY_NEW_TASK is
        // generally not allowed, except if the caller specifies the task id the activity should
        // be launched in. A bug was existed between N and O-MR1 which allowed this to work. We
        // maintain this for backwards compatibility.
        final int targetSdkVersion = getApplicationInfo().targetSdkVersion;

        if ((intent.getFlags() & Intent.FLAG_ACTIVITY_NEW_TASK) == 0
                && (targetSdkVersion < Build.VERSION_CODES.N
                        || targetSdkVersion >= Build.VERSION_CODES.P)
                && (options == null
                        || ActivityOptions.fromBundle(options).getLaunchTaskId() == -1)) {
            throw new AndroidRuntimeException(
                    "Calling startActivity() from outside of an Activity "
                            + " context requires the FLAG_ACTIVITY_NEW_TASK flag."
                            + " Is this really what you want?");
        }
        mMainThread.getInstrumentation().execStartActivity(
                getOuterContext(), mMainThread.getApplicationThread(), null,
                (Activity) null, intent, -1, options);
    }

这也就是为什么Activity的startActivity和Context的startActivity会有上面那些使用上的不同的原因了。

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