Add Chapter1 iOS notes on various topics

Chapter1 - iOS/1.29.md

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+# JavascriptCore
+
+1. JSCore is a JavaScript wrapper implemented in C/C++ based on WebKit that makes interaction between JavaScript and native code simpler.
+
+- JSContext  
+  JSContext represents an instance of a JavaScript execution environment. All JavaScript execution occurs inside a context. JSContext is also used to manage the lifecycle of JavaScript objects in the VM.
+
+- JSValue  
+  JSValue is used to receive the return results from JSContext execution. A JSValue can be any JavaScript type (primitive, object, function, etc.).
+
+- JSManagedValue  
+  JSManagedValue is a wrapper around JSValue that helps resolve retain-cycle issues between JS and Objective-C. The most common use is to safely reference a JSValue stored on the heap. If a JSValue is stored on the heap incorrectly, it can easily create retain cycles that prevent JSContext from being released properly.
+
+- JSExport  
+  A protocol used to expose native objects to JS; the exposed object can point to itself and to other objects.
+
+- JSVirtualMachine  
+  Manages the JS object space and needed resources.
+
+2. Native calling JS
+
+- Load JS code
+      (JSValue *)evaluateScript:(NSString *)script;
+
+- Call JS function
+      JSValue *callback = self.context[@"sayHi"];
+      [callback callWithArguments:@[@"Hangzhou Xiao Liu"]];
+
+3. JS calling Native
+
+- Implement via a block. Then call the method directly from JS. Note: inside the block do not directly use externally defined JSContext or JSValue; pass them as parameters or obtain them via + (JSContext *)currentContext; otherwise it may cause retain cycles and memory won't be freed correctly.
+      self.context[@"showMessage"] = ^(NSString *message){
+              UIAlertController *alertCtr = [UIAlertController alertControllerWithTitle:@"Notice" message:message preferredStyle:UIAlertControllerStyleAlert];
+              UIAlertAction *cancel = [UIAlertAction actionWithTitle:@"OK" style:UIAlertActionStyleCancel handler:nil];
+              [alertCtr addAction:cancel];
+              // Note: the block executes on a background thread; update UI on main thread.
+              dispatch_async(dispatch_get_main_queue(), ^{
+                  [weakSelf presentViewController:alertCtr animated:YES completion:nil];
+              });
+          };
+
+- Implement via the JSExport protocol; JS will call methods on an injected Objective-C object, so the methods must be declared in the protocol and implemented on the injected object. Inject the native object when the web view finishes loading.
+      // Protocol declaration
+      
+      @protocol JSInject <JSExport>
+      - (void)showMessage:(NSString *)message;
+      @end
+      
+      // Implementation
+      
+      - (void)showMessage:(NSString *)message{
+            UIAlertController *alertCtr = [UIAlertController alertControllerWithTitle:@"Notice" message:message preferredStyle:UIAlertControllerStyleAlert];
+              UIAlertAction *cancel = [UIAlertAction actionWithTitle:@"OK" style:UIAlertActionStyleCancel handler:nil];
+              [alertCtr addAction:cancel];
+              // Note: the method runs on a background thread; update UI on main thread.
+              dispatch_async(dispatch_get_main_queue(), ^{
+                  [weakSelf presentViewController:alertCtr animated:YES completion:nil];
+              });
+      }
+      
+      // Injection
+      
+      - (void)webViewDidFinishLoad:(UIWebView *)webView
+      {
+          // Get the JSContext from the web view.
+          self.context = [webView valueForKeyPath:@"documentView.webView.mainFrame.javaScriptContext"];
+      
+          // Inject the Objective-C object needed by JS
+          self.context[@"Bridge"] = [JSInject new];
+      }
+  
+
+Example
+
+JS calls Native
+
+    // Native object to expose (with some properties and methods)
+    #import <Foundation/Foundation.h>
+    #import <JavaScriptCore/JavaScriptCore.h>
+    
+    @protocol PersonInjectExport<JSExport>
+    
+    @property (nonatomic, strong) NSString *name;
+    
+    @property (nonatomic, strong) NSString *hobby;
+    
+    - (id)sayHi;
+    
+    @end
+    
+    
+    @interface PersonInject : NSObject<PersonInjectExport>
+    
+    @property (nonatomic, strong) NSString *name;
+    
+    @property (nonatomic, strong) NSString *hobby;
+    
+    - (id)sayHi;
+    
+    @end
+    
+    
+    // view controller
+    
+    - (void)webViewDidFinishLoad:(UIWebView *)webView{
+        self.jsContext = [webView valueForKeyPath:@"documentView.webView.mainFrame.javaScriptContext"];
+        PersonInject *person = [[PersonInject alloc] init];
+        person.name = @"Hangzhou Xiao Liu";
+        person.hobby = @"Coding, Movie, Music, Table tennis, Fit";
+        self.jsContext[@"lbp"] = person;
+    }
+    
+    // JS
+    <body>
+    
+      Hi. Hello everyone, I am
+      <p id="name">***</p>
+    
+      <button id="show">Then do an introduction</button>
+    
+    </body>
+    <script>
+      var u = navigator.userAgent
+      var isAndroid = u.indexOf('Android') > -1 || u.indexOf('Linux') > -1
+      var isiOS = u.match(/\(i[^;]+;( U;)? CPU.+Mac OS X/) // iOS device  
+    
+      document.getElementById("show").onclick = function () {
+        if (isiOS) {
+          document.getElementById("name").innerHTML = lbp.name;
+          setTimeout(() => {
+            alert(lbp.sayHi());
+          }, 1000);
+        }
+      }
+    </script>
+
+
+
+Native calls JS
+
+    // Native
+    - (void)callJS{
+        JSValue *functionName = self.jsContext[@"sum"];
+        NSInteger sum = [[functionName callWithArguments:@[@"2",@"18"]] toInt32];
+        
+        UIAlertController *alertVC = [UIAlertController alertControllerWithTitle:@"Calculation from JS" message:[NSString stringWithFormat:@"%zd",sum] preferredStyle:UIAlertControllerStyleAlert];
+        UIAlertAction *okAction = [UIAlertAction actionWithTitle:@"OK" style:UIAlertActionStyleCancel handler:nil];
+        [alertVC addAction:okAction];
+        [self presentViewController:alertVC animated:YES completion:nil];
+    }
+    
+    // JS
+    function sum(a ,b){
+    	return parseInt(a) + parseInt(b);
+    }