1. Display invocation of Intent and implicit invocation 2. Matching rules for implicit invocation 3. Usage scenarios of implicit invocation 4. Advantages of implicit invocation

Intents fall into two types:

1. Intent displays the call;
2. Intent an implicit call;

IntentFilter: IntentFilter: IntentFilter: IntentFilter: IntentFilter: IntentFilter: IntentFilter: IntentFilter: IntentFilter: IntentFilter Implicit call correlation is covered in a new section.

Reduced coupling: In the era of componentization, components can be invoked implicitly without dependency. More secure: there is no need to specify component information and only components that match the specification will be invoked. More flexible: with a wider range of application scenarios, we can call many services provided by the system, such as making phone calls, sending SMS messages, etc.

We all know that implicit calls match components to start them, but how do they match? Let’s look at IntentFilter in more detail.

IntentFilter supports action, category, and Data filtering information.

Let’s start with a simple filtering rule

<activity android:name="SecondActivity"> <intent-filter> <action android:name="android.intent.action.Second"/> <category  android:name="android.intent.category.DEFAULT"/> <data android:mimeType="image/*"/> </intent-filter> </activity>Copy the code

This is a simple Intent that can be launched using the following rules.

Intent intent = new Intent();  
intent.setAction("android.intent.action.Second");  
intent.addCategory("android.intent.category.DEFAULT");  
intent.setDataAndType(Uri.parse("file://abc"),"image/png");
startActivity(intent);   Copy the code

This lets you implicitly launch SecondActivity.

Now that we have a basic understanding of IntentFilter, let’s look at its matching rules in more detail.

So a category is a string, and the system defines a bunch of categories, and you can also customize them, and they match differently than an action, so if there are several categories in an intent, Each category must be defined in the intent filter. That is, an intent may not have any category. If it does, each category must be the same as any category defined in the Intent filter. Why we didn’t add android to the intent filter. The intent. The category. The DEFAULT this category would be an error? Reason is the system call startActivity or startActivityForResult will DEFAULT in intent with android. Intent. The category. The DEFAULT in this category.

MimeType refers to the media type, such as image/ JPEG, text/plain, and video/*. It can refer to different media formats such as pictures, texts, and videos.

After understanding the structure of data, the filtering rules of data will be introduced

• Case 1: The data rule is incomplete. As shown below.

<intent-filter>
      <data android:mimeType="image/*"/>
</intent-filter>Copy the code

The mimeType attribute in an intent must be “image/*” in order for a match to succeed. In this case, although no URI is specified, the default schema for the URL part of the intent is content and file. The intent does not specify a URI, but the schema of the URI part of the intent must be content or file.

• Case 2: Multiple sets of data rules are defined, and each data has a full property defined, both URI and mimeType. As shown below.

<intent-filter>
    <data android:mimeType="video/mpeg" android:scheme="http" ... />
    <data android:mimeType="audio/mpeg" android:scheme="http" ... />
</intent-filter>
Copy the code

To match the intent filter, we also need to define a set of data rules in the intent to match successfully. In addition, to specify the full data for an intent, you must call setDataAndType. You cannot call setData first and then setType, because the two methods clear each other’s values. Finally, when we launch an Activity implicitly, we can make a judgment call to see if any Activity matches our intent to avoid unnecessary crashes. There are two ways to judge: one is to use the PackageManager resolveActivity method, the other is to use the Intent resolveActivity method, if they can not find a matching Activity will return null, according to its return value we avoid the above problem. First, let’s look at the source of the Intent’s resolveActivity method

public ComponentName resolveActivity(PackageManager pm) { if (mComponent ! = null) { return mComponent; } ResolveInfo info = pm.resolveActivity( this, PackageManager.MATCH_DEFAULT_ONLY); if (info ! = null) { return new ComponentName( info.activityInfo.applicationInfo.packageName, info.activityInfo.name); } return null; }Copy the code

From which we can see that its internal is also directly called PackageManager resolveActivity method to determine whether the match is successful. Let’s look at the PackageManager resolveActivity method again

public abstract ResolveInfo resolveActivity(Intent intent, int flags);Copy the code

The first parameter is the intent we parse. The second parameter uses the MATCH_DEFAULT_ONLY flag bit in the source code. This flag bit matches only activities that declare the category in the Intent filter. The point of using it is that startActivity must succeed as long as this method does not return NULL. Finally, an example:

Intent intent = new Intent(); intent.setAction("com.dev.test"); intent.setType("image/*"); ComponentName componentName = intent.resolveActivity(getPackageManager()); If (componentName == null) {// If null, toast.maketext (mainactivity.this, "Matching failed ", toast.length_short).show(); } else {// if not null, startActivity startActivity(intent); }Copy the code